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An approach to classification and hi-tech applications of room-temperature ionic liquids (RTILs): A review
Accepted Manuscript An approach to classification and hi-tech applications of roomtemperature ionic liquids (RTILs): A review
Fatima Javed, Faheem Ullah, Muhammad Razlan Zakaria, Hazizan Md. Akill PII: DOI: Reference:
S0167-7322(18)31908-1 doi:10.1016/j.molliq.2018.09.005 MOLLIQ 9611
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
10 April 2018 28 August 2018 1 September 2018
Please cite this article as: Fatima Javed, Faheem Ullah, Muhammad Razlan Zakaria, Hazizan Md. Akill , An approach to classification and hi-tech applications of roomtemperature ionic liquids (RTILs): A review. Molliq (2018), doi:10.1016/ j.molliq.2018.09.005
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ACCEPTED MANUSCRIPT An Approach to Classification and Hi-Tech Applications of Room-Temperature Ionic Liquids (RTILs): A Review
Department of Chemistry, Shaheed Benazir Bhutto Women University, Peshawar 25000,
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains
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Malaysia, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia
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*Corresponding Author:
Tel: +6045996161
Fax: +6045996907
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Email: [email protected]
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Khyber Pakhtunkhwa, Pakistan
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Fatima Javeda,b, Faheem Ullahb, Muhammad Razlan Zakariab, Hazizan Md. Akil1*b
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Graphical
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Highlights
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o Classification of RTILs on the basis of structural and induced ionic moieties o Investigating the intrinsic properties of RTILs in comparison to typical solvents
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o Advanced chemistry and cutting-edge applications of RTILs in hi-tech interactions
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Abstract
A new background to organize diverse synthetic and natural resources to fabricate
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functional materials in a safe environment by using Room Temperature Ionic Liquids (RTILs) is highly stressed. A new approach to classify various RTILs on the basis of induced structural moieties is explored. RTILs are reviewed as reaction media to process functional materials, green composites, cellulose dissolution, energy production, additives, cleaner, chromatography, hetero/ biocatalysis, bio/ electrochemical sensors and interaction with bio-membranes to attract current academic and industrial development. RTILs are expected to substitute conventional
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ACCEPTED MANUSCRIPT solvents as RTILs have challenged both our experimental and intellectual abilities to explore further. Keywords; RTILs and Green composites; electrochemical sensor; solar-cooling system; catalysis; biocatalysis; biosensors.
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1. Introduction
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The academic and industrial prospects of emerging green chemicals highlighted as Room Temperature Ionic Liquids (RTILs) are striking to substitute hazardous and contaminating
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organic strippers. Switching from a typical organic solvent to an ionic liquid can lead to novel and unusual chemical reactivity. RTILs have the potential to change the idea of synthetic and
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applied material science research for 21st century scientists. The significance of RTILs is
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emphasized by the incremental rate of publications regarding their academic and industrial
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considerations but their chemistry is still unexplored. The official definition of RTILs uses the boiling point of water as a reference. Thus, RTILs
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are ionic compounds which are liquids below 100 oC. More specifically, salts that are liquid at
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room temperature are known as Room Temperature Ionic Liquids (RTILs). At structural and molecular level description, they are completely different from all other solvents [1]. RTILs are
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termed as electrically neutral salts as the constituent positive ions match equivalently with the negative ions in the liquid. They do not contain any molecular level presentation like other solvents which are known for their polar or apolar nature based on the interaction between the solvent molecules [2]. RTILs are known for their purity, creating an opportunity for researchers to address and commercialize the newly discovered low melting salts. RTILs exhibit exclusive thermophysical properties such as high electrical conductivity, highly thermal stability, immiscibility, non-aqueous nature, low nucleophilicity, lower melting point, viscosity, density, 3
ACCEPTED MANUSCRIPT refractive index and a large electrochemical space [3]. Such properties are the basis of RTILs for hi-tech targeted applications where a common solvent fails to work well. Moreover these properties can be tailored to perform in a much better route. RTILs being non-volatile are recognized as “green solvents” to replace the volatile solvents commonly uses in the organic
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industry, which are marked for a variety of environmental concerns. Another attraction of RTILs
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is based on their ability to be reused and recycled without evaporating, allowing them to be
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considered as potential candidates for high-vacuum systems to eliminate many toxin problems
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[4].
Switching from an organic solvent to RTILs result in significant improvement in catalytic
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ability, kinetic stability and enantio-selectivity but the internal responses for such changes is still
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unknown. Due to this reason a comprehensive knowledge of the classification, application, their mode of interactions along with the processining chemistry should be caution during their
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selection as a replaceable solvent [5, 6]. Almost 399 RTILs have been synthesized and processed
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without investigating their multi-solvation interactions, thermokinetic and enhanced catalytic conditions. There is an urgent need to overview the efficient classification, application, synthetic
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and processing schemes of RTILs, to attract and ease the acadamic and industrial research for
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hi-tech targeted interactions.
We are introducing two types of RTILs. Firstly, the organoaluminates, which have the donor and acceptor potential and their acidic and basic nature can be achieved by varying the composition of Aluminium as shown in Figure1 (a). Unlike acidic organoaluminates, the basic haloaluminates tend to stop the solvation and solvolysis of metallic ions, and provide a large electrochemical space for chloro, bromo and iodo based-RTILs [7]. Such type of controlled Lewis acid based RTILs are striking candidates in various catalytic reactions (Ziegler- Natta type 4
ACCEPTED MANUSCRIPT catalytic reactions) with controlled stereo-selectivity, chemo-selectivity and moisture sensitivity. Also the molar friction (x) of Aluminium compound is crucial to control the viscosity (υ),
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melting point (Mp) and facilitate the reaction at acidic or basic conditions as shown in Table 1.
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Table 1
Mp (oC) -80 2 -60
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υ (p) 0.16 0.20 1.59
pH acidic neutral basic
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Mole friction (x) 0.66 0.50 0.36
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Effect of Aluminium compound on the melting point and viscosity of chloroaluminates
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Fig. 1 (a) Effect of Aluminium compound on the act of 1-Ethyl-3-methylimidazoliumchloride / Aluminium Chloride RTILs.
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Secondly, water and air stable RTILs, which can be achieved by the substitution of halide anion by another weakly coordinating anions of the 1,3- dialkylimidazolium cation. Such type of RTILs are preferably unsymmetrical, the anions of which affect the melting point, preferably used for the immobilization of transition-metal catalyst precursors in biphase catalysis. They are immiscible with some organic solvents, e.g. alkanes, hence, can be used in two-phase systems. This give rise to the possibility of a multiphase reaction procedure with easy isolation and recovery of homogeneous catalyst. Polarity and hydrophilicity and or lipophilicity can be readily
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ACCEPTED MANUSCRIPT adjusted by a suitable choice of cation/anion and RTILs have been referred to as ‘designer solvents’. The investigated effect of anion on the melting point of imidazolium based RTILs is shown in Table 2. Table 2
N R
Me
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Melting point (oC) 125 87 65 38 7 6 -9 -3 -14 16
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Anion (X ) Cl Cl Cl NO3 AlCl4 BF4 CF3SO3 (CF3SO3)2N CF3CO2 CF3SO3
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Alkyl group (R) Me Et n-Bu Et Et Et Et Et Et n-Bu
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N
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Effect of anion on melting point of Imidazolium based RTILs
To date, RTILs are intensively considered for potential applications in biomedical,
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biocatalysis, biospeapration, green biocomposites, biosensors, fluid thermodynamics, catalysis,
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organic synthesis, electrochemical windows, separation, purification, gas sensors, ion sensors, biomass processing and producing functional energy. Therfore, it is important to realize the importance of induced moieites into RTILs, the possible combinations of cations, anions and their effective processing for selective applications [8, 9]. Few thousand of RTILs are roughly estimated to be available in the market in near future, therefore an efficient classification of RTILs is compulsory to understand the synthetic flexibility and tunibility by using a single or a mixture of ionic liquids with induced hydrophilicity, hydrophobicity, sensitivity and selectivity
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ACCEPTED MANUSCRIPT [10]. Some of concluding remarks must be addressed to explore further regarding RTILs processing and utilization for a specific application as demonstrated by Taige et al., who stated that pyridinium and imidazolium based RTILs have the potential to produce binary liquids with low viscosity and higher thermal and ionic conductivities [11]. Annat et al., demonstrated that
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the crystallization is completely suppressed by using a mixture of phosphomium and pyridinium
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based ionic liquids [12]. Currently, Stolte et al., introduced a new class known as tunable aryl
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alkyl ionic liquids (TAAILs), which are preferable with better substitution pattern where the electronic interactions between the core and aromatic substituents can easily tune the
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physicochemical properties as compared to typical imidazolium based ionic liquids [13].
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The green aspects of preparing RTILs with 99% yield under mild conditions with no by-
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product formation and easily separation just by filtration are the basis of RTILs in various reactions [14]. The synthesis of RTILs including Metathesis (a reaction coupled with the
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characteristic exchange of ions between two compounds usually with the precipitation of an
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insoluble product), acid-based neutralization and the direct combination of halide based salts with metallic halide were investigated [15]. Several imidazolium, pyridinium, alkyl-ammonium
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halides are available commercially and their synthesis is also reported by reacting amine with
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halogenoalkanes [16, 17]. In 1992 the first ionic liquids [emim][BF4] with a melting point of 12 °C, was synthesized via the metathesis of [emim]I with Ag[BF4] in methanol [18]. Metathesis reactions for the first time were very common but they contaminated the ionic liquids with some halide ions that may have reacted with solute materials [19]. Another method based on acid base neutralization and the direct combination of a halide salt with a metal halide are discussed systematically to expoler the art of RTILs processing [20].
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ACCEPTED MANUSCRIPT Overall, this review aims to highlight the importance of RTILs to recognize and replace the harmful raw materials and generated wastes. Additionally, how to choose a specific ionic liquid for a specific use, many recent applications of RTILs with detailed analysis and the development of novel synthetic procedures have been investigated to charm the academic and
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1.1
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industrial knowledge of RTILs.
This review aims to explore the most recent literature about room temperature ionic liquids
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and attempts to address the new classification with detailed processing and multiple applications.
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The field is still very young, challenging and interdisciplinary but at the same time also very applied. The authors expect a number of vitally important developments to come from exploring
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the RTILs to ease the synthesis, processing and implementation in challenging technologies. The
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arena of ionic liquid is evolving swiftly with a large number of research directions, thus it is not possible to overview the complete literature of ionic liquids in this review. Therefore, the
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thermophysical properties of ionic liquids are not included in this work. A new concept of RTILs
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classification is described which might be helpful for researchers to select a desirable RTIL as a first choice. Further, the most recent applications and processing technologies RTILs are
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explored which might be helpful to understand the chemistry and impact of RTIL for hi-tech targeted interactions where a common solvent fails to overcome the objectives. The processing technologies are assured to produce novel RTILs which must be applied in dissolution processes, ion-selective sensors, energy generation, catalysis, bioseparation, additives and electro-analytical methods. Consequently, the development of new RTILs and relevant application is quite certain to allow RTILs in challenging tasks.
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ACCEPTED MANUSCRIPT 1.2
Characteristic features of RTILs The cations control the physical properties (viscosity, melting point, vapour pressure,
density) and the anions reflect the chemical properties and thus the reactivity of RTILs [21]. Based on the thermoochemical properties, RTILs are termed as “Designer Solvents” for
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applications where high conductivity, thermal and chemical stability are needed with a variety of
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solvation potentials. The characteristic properties of RTILs ( based on the dissolution of a variety of organic and inorganic materials, extraction procedures, chemical separation and synthesis)
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depend upon the strategic interactions, structure, charge, immiscibility and distinctive combination of the constituent ions being virtuous, non- coordinating, non-volatile and highly
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polar [22].
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The variation in thermophysical properties is attributed to the modification and splitting of chain-length in alkyl groups integrated to the cations [23]. For instance, the solubility
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vanished as the alkyl chain increased from C1 to C9 as reported in 1-alkyl-3-methyl-
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imadazolium hexafluorophosphate [Cnmim] [PF6]. Additionally, exchanging the PF6− anion by BF4− in 1-alkyl-3-alkylimadazolium dramatically increased the solubility of the ionic liquid in
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water while replacement with the bis(perfluoromethyl-sulfonyl)imide anion (CF3SO2)2N− (also
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known as bistriflate imide (Tf2N−) ) result in reduced water solubility [24]. Most common singly charged RTILs are known to have a diverse combination of organic/ inorganic anions based on nitrate or methanesulfonate and organic heterocyclic cations such as dialkylimidazolium. Properties of a representative RTIL are reported in Table 3, while the general comparison of RTILs with other organic solvents are shown in Table 4 [25].
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ACCEPTED MANUSCRIPT Table 3 Specific properties of a representative RTIL
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APH4/CGH3 Ultra-pure grade Anion C15H20F18NP 587.27 24 oC 250 oC Immiscible Unknown Miscible 1.59 173.81 carbocation Very high
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Short name Quality Type of RTILs Formula Molecular weight Melting point Decomposition temperature Miscibility with water Miscibility with toluene Miscibility with acetonitrile Density (g.cm-3) Viscosity (mm2/s) Stabilizing Electrochemical stability
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Room Temperature Ionic Liquid Detailed data 1-Butyl,1-Methyl-Pyrolidinium Tris(pentafluoroethyl)Trifluorophosphate
RTILs Few thousand Multifunctional Tunable Tunable Negligible Nonflammable Strongly solvating Polarity depends designer solvents 2-100 times of organic solvent Economic imperative 22–40,000 0.8–3.3 1.5–2.2
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Features Total solvents Applicability Catalytic capability Chirality Vapour pressure Flammability Solvation Polarity Tuneability Cost Recyclability Viscosity/cP Density/g cm3 Refractive index
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Table 4 Comparison between RTILs and Organic Solvents
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Organic solvents Millions Unifunctional Rare Rare Follow C-Clapeyron equation Usually flammable Weakly solvating Conventional polarity Limited range Cheaper Green imperative 0.2–100 0.6–1.7 1.3–1.6
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Classification of RTILs Figure 1 (b), present the most recent selection of integral cations and anions for distinctive
RTILs. Such a variety of asymmetric organic cations and inorganic anions do not present any major difficulty in preparing RTILs of high purity, without time dependent decay and
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evaporation. Cations based on the imidazolium or pyridinium ring with one or more alkyl groups
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and anions based on hexafluorophosphate halide ions, tetrafluoroborate, tetrachloroaluminate, bis
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(perfluoromethyl-sulfonyl) imide and quaternary ammonium salts are recently most studied [26]. It is important to mention here, the alkyl group present in the cation chain of RTILs reflect the
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solubility and viscosity in any fluid. Mutual electrostatic or ionic interactions compare to other
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conventional solvent interactions (dipole–dipole, van der Waals, hydrogen bonding) reflect the miscible nature of RTILs with many polar solvents [27]. Furthermore H-bonding, hydrophilicity,
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lipophilicity, miscibility and stability can be modified by a suitable choice of anion, cation and
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chain length of the attached alkyl group. Anionic substitution and symmetry have been applied to
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reduce the ion packing and coulombic attraction thus decreasing the melting points [25].
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ACCEPTED MANUSCRIPT Cations
Anions Amide Methanide Halide
Phosphate, Sulphonate, Borate, Acetate
5-membered 6-membered
Inorganic
Imidazolium
phosphonium Hexafluorophosphate +
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Tetrafluoroborate
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Sulphonium
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Imidazolium Functionalitie s
Cholinium
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IIodide
Tosylate BrBromide
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N-alkylIsoquinolinium
Dicyanamide
Dialkylphosphate
Acetate
ClChloride
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Thiazolium
Bis (trifluromethanesul fonyl) amide
Alkylsulphate
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Pyrazinium
Triazolium
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Pyridinium
Pyridazinium
Oxazolium
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NH4 ammonium Pyrolidinium
Trifluromethanesulfonate
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Pyrimidinium
Benzotriazolium Fig.1 (b) Characteristic cations and anions in RTILs. Classification of RTILs is based on their diverse chemical structures. The inherited properties of RTILs to behave more like surfactants, crystals, ionic and molecular liquids further add to the classification respectively [28]. Classification of RTILs is more challenging by suggesting the presence of cationic, anionic or induced functionalities. Before extending the classification, RTILs can be divided into two main types,based on proton donating ( protic) and 12
ACCEPTED MANUSCRIPT non-donating (aprotic) catagories, where uni and dicationic functionalities decide the nature of RTILs [29]. The newly investigated classification of RTILs based on various properties is shown in Figure 2. 2.1
Protic and Aprotic RTILs
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The most easier and inexpensive RTILs with a high degree of purity are produced by a
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simple transfer of proton between equimolar bronsted acid and base pairs are called protic
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RTILS [30]. Thus protic RTILs are regarded as pure mixture consist of ions with excellent ionic behavior in comparison to common salts.[31]. In protic RTILs, the specific sites for hydrogen
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bonding is created as a result for proton transfer, which show characteristic hydrogen bonding as
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compare to organic solvents. Thus, due to extreme hydrogen bonding capabilities such RTILs exhibit extreme conductivity, stability, catalytic and thermal efficacy.
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Unlike protic RTILs, Aprotic RTILs lack the specified feature but exist in a wide range of cation
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and anion groups with or without hydrogen bonding capabilities. Further, aprotic RTILs are synthesized with multistep synthesis due to activation and then formation of covalent bonds
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between the specified ions [32]. Aprotic RTILs are categorized with better electrochemical and
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thermal features as compared to protic RTILs due to stronger covalent bonding as shown in Waldon plots with excellent ionic performance [33]. Chiral and Achiral RTILs
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2.2
Molecular chirality is one of the interest in classification of RTILs for potential application in organic, inorganic and physical chemistry. In RTILs, chirality is generally applied to the constituent cationic moieties. Thus, chiral molecule is defined with a non-superimposable mirror image. The two mirror images are called as optical isomers or enantiomers. In 1999, Sudden et al., synthesized first chiral ionic liquid 1-butyl-3- methylimidazolium ([BMIM])
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ACCEPTED MANUSCRIPT lactate III by anion exchange between [BMIM][Cl] and sodium (S)-2- hydroxypropionate. In 2005,the preparation of oxazolinium salts from (S)-valine methyl ester and propionic acid have been reported as chiral ionic liquids [16]. Similarly, a molecule with a superimposable mirror image is called as achiral molecule. RTILs characterized with achiral cationic molecules do
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have a plane of symmetry or a center of symmetry. Various achiral ester based ionic liquid,
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achiral amide and imidazolium bromide RTILs have been synthesized and investigated for
Magnetic RTILs
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2.3
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specific applications.
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Such type of RTILs exhibit magnetic properties without any added magnetic particles. These properties are induced by the selective cations and anions. Normally magnetic RTILs
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consist of Transition metal and Lanthanide complex anionic moieties. They exhibit characteristic
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physicochemical properties as multi-responsive materials. For the first time Hayashi et al., reported the first magnatic RTIL as [C4mim][FeCl4] and to date the anion has been
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functionalized by paramagnetic group and further investigated to increase the catalytic and
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solvent effect by replacing FeCl3 in Grignard and Friedal craft reactions [34]. Branco et al., reported the synthesis of novel Magnetic Room Temperature Ionic Liquids based on the
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combination of 1-ethyl-3-imidazolium cation and iron(III) and chromium(III) EDTA complexe anions [35]. The novel electrochromic and magnetic ionic liquids have been prepared by a simple combination of cobalt (III), chromium (III) and iron (III) ethylenediamine tetraacetic complexes as anions and the cations 1-ethyl-3-methylimidazolium [EMIM], 1-butyl-3methylimidazolium [BMIM], 1-octyl-3- methylimidazolium [OMIM], tri-octylmethylammonium [ALIQUAT] and trihexyltetradecylphosphonium respectively. Such novel magnetic RTILs based
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ACCEPTED MANUSCRIPT on chromium(III) and iron(III) EDTA complexes have been reported as alternative, robust and efficient MRI contrast agents for many diagnostic applications. 2.4
Polymeric RTILs A specified class of RTILs has been identified as polymerizable ionic liquid where the
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constituent anion has the ability to self polymerize. Such type of cationic or anionic
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polymerizable moieties are able to formulate homopolymers and block copolymer analogues
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[36]. These polymeric RTILs have the characteristic properties of both ionic liquid as well as typical polymers and are explored as well defined polyelectrolytes. Such type of ionic liquids are
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potential candidate to be used in polymer self assembling, polymer dispersion and reinforcement
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[37-39]. The standard techniques including atom-transfer, free-radical polymerization are reported for synthesis of polymeric RTILs with inclusive literature [40, 41]. Highly organized
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polymeric RTILs nanoparticles have also been reported by Yuan et al., [42]. For a better
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nanostructure and physicochemical dependency, polymeric RTILs can be adjusted by simple ion exchanges. Also the stimuli dependent behavior are studied by induced sensing moieties in the
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desired system for specified applications [43]. Up to date a few report to state the art of
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dynamics, structure and computational analysis for polymer blends in polymeric RTILs have been addressed and many applications are still coming to design nano ane meso-structural
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materials for demanding technologies [44].
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Fig.2 Classification of RTILs on the basis of structural and induced chemical moieties.
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2.5 Chelating RTILs
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Angell et al., suggested a new type of RTILs containing concentrated salt and respective solvent as solvated or chelating ionic liquid [45]. The point of concern is that ionic liquid are
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non-cordinating but their constituent ions consist of ligands make them as coordinating for a variety of applications so that electctron transfer between anion and cation can take plae. Some of coordinating complexes based on mercury and Ferroceium cations are reported as chelating ionic liquids. Still this field has to grow with new ideas and are expected to replace the vast variety of available ligands with upgraded potentials [46, 47]. In 1999 Rees et al., reported the first chelating RTIL consist of Barium bisalt-oxide and reported the structure resembling crown ether complexes [48]. Such scheme was extended by using acetylacetones, glyme, 16
ACCEPTED MANUSCRIPT oligoethercarboxylate (TOTO) species, for coordination with small and hard cations. Their cations were of special interest as strongly Lewis- acid chelating moieties with flexible characteistics for multi-directional chelating features [49, 50]. Several reports confirm the chelating abilities of such ionic liquids by investigating Ag+, Li+, Na+ and K+ cations with
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concentration and nature dependent properties [51]. In order to investigate the radius size,
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flexibility and coordinating strength, various modern characterization is still required for better
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implementation of chelating RTILs to replace the existing toxic and unfriendly chelating agents with better recognition features. Fluorous RTILs
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2.6
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In chemistry, Fluorinated groups are introduced to replace the existing organic groups in order to change the geometry and structure of the compounds. The same pattern is investigated
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by various researchers to replace the exixting group by inducing Fluorinated moieties to get more
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organized structures for specified applications. Several characterization techniques including XRD and NMR have confirmed the analytically organized and tricontinious structure of
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Fluorinated RTILs due to highly polar Fluorinated sphere in comparison to apolar hydrocarbon
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domains [52, 53]. As reported, the Fluorocarbon and hydrocarbon are slightly miscible with each other, so their segregation also hinder their performance. Thus, such ionic liquid can find
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potential application but still no sufficient data is available to state the properties that facilitate the Fluorinated RTILs with better structural and geometrical performance. The reports on Fluorinated RTILs are not sufficient but their electronegative and stronger solvophobic properties are the interesting features for future technology applications [52].
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ACCEPTED MANUSCRIPT 2.7
Dicationic RTILs Dicationic RTILs which resemble Gemini and surfactants are characterized with well
organized structures, dynamics and solvation properties [54-56]. For the first time Davis et al., synthesized dicationic RTIL with both aprotic and protic centres. Dicationic RTILs synthesis is a
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tough job due to transfer of two electrons which is difficult to achieve or may be explosive but
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not impossible. Further many efforts reported the synthesis of various magnetic and
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polymerizable dicationic RTILs for high-tech targeted applications [57, 58]. In dicationic RTILs, it has been reported that shorter alkyl or spacer groups donot effect the bulk properties which
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refer to advanced technology applications where surface dependent properties are highly desired
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[54, 59]. Still there is very little literature available to state art of dicationic RTILs synthesis, reactivity, dynamics and physicochemical research. Similarly dianionic RTILs also will be a
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topic of interest in future as there is no report available about dianionic RTILs regarding
Applications of RTILs
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synthesis characterization and applications.
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Industries, environment, green technology and academic research are the most attractive concerning RTILs in term of forthcoming applications. The author explores the most significant
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achievements in the field of RTILs as a solvent and or combination of identified cations and anions for a desired approach where simple chemical specie failed to response. RTILs nowadays are specialized for biomass dissolution to prepare nano-cellulose based polymer composite, electrochemical-sensors, bio-sensors, gas-sensors, ion selective sensors and newly voltametric strategies. RTILs in terms of its catalytic ability and formulation of bio-electrochemical sensors fascinated the scientist to concentrate. RTILs, due to their immiscible and non-volatile nature,
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ACCEPTED MANUSCRIPT are insoluble in supercritical and other organic solvents, so they can act as a catalyst in phase transfer catalysis for separation technology like gas capturing (sulphur-dioxide, carbon-dioxide and hydrogen). The storing, transferring and capturing of solar energy to electrical energy, biomass to useful and cleaner products, nuclear technology generation, implementation of green
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chemistry approaches and alternative energy resources are some of the best applications related
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to RTILs [60]. Figure 3, presents the application of RTILs, which seem to be quite suitable to
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replace the current synthetic organic solvents in different industries with fruitful results for a
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variety of targeted applications.
Fig. 3. Hi- tech targeted applications of RTILs as Designer Solvents.
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ACCEPTED MANUSCRIPT 3.1 Water splitting One of the great challenges of future technologies is the sustainability of energy production and storage. Thus, strong materials are required which have a solid impact on the processing of energy technologies. In this regard, RTILs are presented with strong electrostatic
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forces among the constituent ions and characteristic tunable properties (low-volatility, low
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flammability, higher thermal, ionic and electrochemical stability) as innovative materials for
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efficient processing and optimization of energy technologies. RTILs are investigated with unique features for electrochemical energy generation through water splitting also known as generation
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of hydrogen (as a fuel) from water. In water splitting technology, RTILs have been applied in
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two ways as to produce electro-catalyst and in the form of hydrated RTILs (solvent/ RTIL) mixture. Previously, simultaneous oxidation and reduction of water has been carried out by
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applying 450 mV potential with a significant loss of energy as compared to obtainable energy
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from generated hydrogen [61]. So electro-catalysis of water needs effective materials to carry out the energy evolution reaction (EER) at ambient conditions as photosynthesis convey EER
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through oxo-Manganese clusters as catalytic center. Accordingly, many metal oxides were
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investigated to produce higher current at lower potential and to direct EER at ambient conditions. RTILs have the potential to act as an electrolyte and also as a solvent where the water molecules
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act as solute during water splitting. Accordingly, RTIL ireases the free energy of water molecules by breaking the hydrogen bonding in water up to some extent where a little energy will be required for water splitting. This process with dissolution of water or hydrogen breakage is determined as endothermic process. Few related RTILs have verified the process of water splitting at 1.25 mV at 150 oC. Protic RTILs have also shown significant contribution in water splitting as rorted by Izgorodintl., [62]. Accordingly Manganese oxide was used for water
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ACCEPTED MANUSCRIPT oxidation in protic-RTIL and buffer-water mixture to produce hydrogen peroxide from water at low potential (150-250 mV) as shown in Figure 4 (A) [63]. The investigations revealed the importance of RTILs as electrocatalyst for water splitting and to explore further to optimize the structure and reaction conditions of ionic liquid. A lot of research work is required to explore
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most efficient RTILs for easy water splitting as it will also facilitate the reversible metal- air
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batteries involving water oxidation principles which is still a major problem in incompetence of
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these batteries.
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Fig. 4 (A) A representative flow reactor using RTIL for water splitting process
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3.2 Innovative polymer electrolytes (IPEs) Presently, RTILs are considered as strategic materials for electrochemical challenges [64]. RTILs are characterized with inherent properties to enhance the stability, performance, reusability, speed, security and thermal stability of hi-tech electrochemical devices suc as batteries, solar cells, fuel cells, actuators and transistors, if proper encapsulation is maintained due to leakage of RTILs [64]. In order to remove this discripency of RTIL-leaking, new strategies are in progress to develop pseudo or solid electrolytes such as polymeric electrolytes 21
ACCEPTED MANUSCRIPT which have the superior advantages of improved stability, mechanical sterength, safety, carefully processing and easy administration [65]. Accordingly, there are two classes of polymer electrolytes as solid polymer electrolytes (SPEs) and gel polymer electrlytes (GPEs). SPEs consist of polymer and salt (polyethylene and Lithium salt), where the conductivity depends
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upon both cation and anion moieties. Similarly, GPEs consist of polymer, salt and additional
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solvent (poly (vinylidene fluoride), Lithium salt and organic solvent) and the enhanced
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conductivity of GPEs is attributed to the presence of molecular organic solvent which facilitate the transportation of ions [66]. Further, typical SPEs are recently replaced by poly(ionic liquids)
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because there is no need to add any Lithium salt and the pendant groups in poly(ionic liquids)
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facilitate the transportation of ions and thus the ionic conductivity. In this regard, several polycation and polyanion [poly(ionic liquids)], have been tested with improved electrochemical
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characterization as compared to conventional SPEs. Additionally, in GPEs the solvent has been
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replaced by using suitable ionic liquids which attributed several advantages to GPEs like high thermal stability, durability, non-volatile and non-flammibility and GPEs are completely
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replaced by ion gels [67]. Ion gels has advantages over GPEs, for instance, it can be prepared by
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using different polymer (uncharged polymers or siloxane) matrices in the presence of ionic liquid. Consequently, ionic liquids form the basis of modern SPEs and GPEs as Innovative
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polymer electrolytes (IPEs) for electrochemical applications. Therefore, ion gels ar a new type of gels comparable to hydrogels, where the liquid phase is replaced by ionic liquid, and this solvent imparts characteristic features as enhanced electrochemical-stability, superior-conductivity, high thermal-stability, re-usability and operational temperature up to 400 oC. There are several methods reported to produce ion gels as solvent casting, swelling of the polymer in ionic liquid and in-situ copolymerization. In solvent casting method, the ionic liquid is dissolved and the
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ACCEPTED MANUSCRIPT solvent is evaporated after producing or casting films, but the problem usually exist is synthesis of sticky films due to high concentration of ionic liquid. Here, in-situ copolymerization method has the advantage to fabricate ion gels by using a diffusional crosslinker to produce homogenous and three dimensional crosslinked networks. In-situ copolymerization has another advantage
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over solvent casting technique, where the ionic liquid itself act as reaction media with 99%
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product purity. Swelling method is also reported with excellent properties of synthesized ion gels
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but the effect of swelling ratio impartially effect the properties of ionic liquid which is still a challenging task. Recently, Shaplov et.al., reported a new concept of synthesizing ion gels by
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radical copolymerization of ionic monomer, namely N-methylpyrrolidinium bis(fluorosulfonyl)
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imide) with poly (ethylene glycol) (di) methacrylates in the presence of the dissolved nitrile butadiene rubber, ionic liquid and lithium salt, [65]. The suggested approach allows for the
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concurrent collaborative of high ionic conductivity (1.3 X 10-4 S/cm at 25 oC) and excellent
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mechanical properties (tensile strength up to 80 kPa, elongation up 60%) to a single polymer material. Such ionic semi-IPNs exhibited wide electrochemical stability window (4.9 V) and
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acceptable time-stable interfacial properties in contact with metallic lithium. Preliminary battery
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tests have shown that Li/LiFePO4 solid-state cells are capable to deliver a 77 mA h/g average specific capacity at 40 oC during 75 charge/discharge cycles as shown in Figure 4 (B). The
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detailed investigations reveal that ion gels are very up-and-coming nowadays and they are being adapted to a great number of approaches such as lithium batteries, supercapacitors, dyesensitized solar cells, electrochromic devices or fuel cells, new types of batteries (metal/air, Li/ S, Na) and new devices in emerging technologies such as optoelectronics, field effect transistors, artificial muscles and bioelectronics.
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Wood Industry
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3.3
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Fig. 4 (B) (a) ions gel based polymer electrolyte membranes for batteries and fuel cell applications (b) Cyclic presentation of the battery with a charge rate C/50 [68].
Cellulose, hemicellulose, lignin and other functional materials are primarily obtained
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from wood with an estimated percentage of 40-55% cellulose, 30% hemicelluloses and 6-35%
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lignin [69]. All these functional materials are reported with distinct physical, chemical properties and structures. The most abundant and valuable biopolymer is called cellulose which exhibits
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many inter and intramolecular H-bonding which are regarded as the key parameters for its dissolution in any medium [70]. Our interest is to explore wood processing especially for cellulose dissolution by RTILs to meet the requirements of green technology (in terms of nontoxic chemicals, low waste, less energy use and reusability of RTILs). Therefore, for cellulose processing, the criteria for RTILs should be summarised as having low melting point, being stable, nontoxic, odourless, high recovery and low-cost production of highly functional fibres so that they can be used as a good source for the development of cellulose reinforced 24
ACCEPTED MANUSCRIPT polymer composites, electrochemical and energy tools. The patents need to address the structural properties, mechanism, finest conditions and product quality along with their thermodynamics and electrokinetic. Table 5, provides a detailed summary of RTILs for cellulose processing at optimum conditions which need much improvement in terms of reaction conditions and
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Table 5 Cellulose Dissolution Using Different Ionic Liquids
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efficiency [71].
Raw material
Dissolution
Condition
Ref.
[Mmim][(MeO)2PO2]
Avicel
10wt%
100 ͦ C, 1h
[72]
[Cyanomim][Br]
Cellulose
3.4 wt%
[Emim][F]
Avicel
[Emim][Cl]
Cellulose
[Emim][Br]
Avicel
[Emim][OAc]
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Ionic liquid
80-90 ͦ C, 20min [73] 100 ͦ C, 1h
[74]
15.8 wt%
85 ͦ C, 1h
[75]
1-2 wt%
100 ͦ C
[76]
Cellulose
85 ͦ C
[77]
MCC
10 wt%
65 ͦ C, 30min
[78]
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2 wt%
13.5 wt%
[Emim][(EtO)2PO2]
Avicel
12-14 wt%
100 ͦ C, 1h
[79]
[Emim][(EtOSO3]
Cellulose
0.5 wt%
110 ͦ C
[70]
[Emim][BF4]
Avicel
insoluble
90 ͦ C
[80]
[Emim][Tos]
Avicel
1 wt%
100 ͦ C, 1h
[81]
[Emim][Ntf2]
Avicel
Very low
110 ͦ C
[82]
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[Emim][(MeO)2PO2]
The table reveals the effect of various cations ( Imidazolium, Pyrrolidinium, Pyrrolidinium, choline) and anions (Amides, Thiocyanate, Sulphates, Phosphates, Sulfonates, Halogens and Formats) on cellulose processing, reaction conditions and efficiency for enhanced
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ACCEPTED MANUSCRIPT cellulose recovery. The effect of size is also crucial in cellulose processing as it has been reported that smaller cations result in better recovery of cellulose [83]. Accordingly, 1 allyl-3methylimidazolium cation, [amim]+, shows more capability to dissolve cellulose as compared to 1-butyl- 3-methylimidazolium [bmim]+ due to its reduced size [76]. According to Zhao et al, [76]
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the ability of H-bonding with cellulose decreases as the size of the cations increase. Similarly the
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cellulose dissolution was also reported to decrease due to the presence of hydroxyl ending-
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groups in RTILs because OH- groups are regarded to readily cooperate with acetate or Cl- anions as compared to H-bonding in cellulose. The dissolution conditions like viscosity, temperature,
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and conductivity are vital for cellulose dissolution in RTILs. Haisong et al., [84] reported the
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effects of a higher temperature on the viscosity and conductivity with the dissolution being more affected at higher temperatures as compared to lower temperatures.
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The precipitation of cellulose by using a proper anti-solvent like water, methanol,
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ethanol, and chloroform is also explored. The low viscosities of RTILs were addressed to facilitate the cellulose dissolution at low temperatures to decrease the thermal degradation of
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cellulose [83]. For an extended period, viscosities of RTILs went on increasing and the rate of
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reaction went on decreasing due to the presence of crystalline residual cellulose fibrils. Furthermore, during dissolution, the viscosity of the solution increased. The precipitation of
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dissolved cellulose and the idea of anti-solvent is that upon adding water to the system, ionic ions attracted by the water molecules and thus shielded by water. The result was that the direct interaction of cellulose with RTIL ions was stopped. Another challenge regarding cellulose dissolution by RTILs refers to decreased crystallinity where it was reported by Gupta et al., that a decrease of 30-75 % in crystallinity was observed when using normal ionic liquids [85]. To overcome this problem, the development of RTILs to retain the crystallinity of cellulose is
26
ACCEPTED MANUSCRIPT another challenging task for researchers. Another important parameter was the selection of cations and anions for the development of enzyme-friendly RTILs. Such a system will positively respond to cellulose in term of enzymatic hydrolysis. Figure 4 (C), presents cellulose processing
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for a variety of cellulose reinforced composites and energy processing applications [86].
Hydrosilylation
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Fig. 4 (C) Dissolution of cellulose using RTILs for nanocellulose reinforced polymer composites and energy concerns [71].
Organosilicon compounds are synthesised on an industrial scale using a hydrosilylation reaction where Si-H bonds are added across unsaturated bonds as shown in Figure 5 (a).
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Fig. 5 (a) Hydrosilylation Reaction (b) Hydrosilylation Process in the presence of RTILs
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Hydrosilylation was applied for the first time by Degussa et al., [87]. Polyethersiloxanes used as
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a surface-active material was prepared by this reaction where such ionic liquids were applied as a source of catalyst heterogenisation. In this process the catalyst (H2[PtCl6] (ionic) and [(m-
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Cl)2{PtCl-(cyclohexene)}paer7
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pa] were allowed to dissolve in 1-butyl-4-methylpyridinium tetrafluoroborate, 1-butyl-3methylpyridinium chloride, and 1,2,3-trimethylimidazolium methylsulfate to prevent leaching
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and for product insolubility as shown in Figure 5 (b) [88]. 1,3-dialkylimidazolium failed to give the chosen polyethersiloxanes, as the 2-H position of the cation was too reactive while 1,4dialkylimidazolium gave a better product with a high conversion in a very short time. The factors affecting the reaction were the hydrophilicity/hydrophobicity of the substrates, the byproducts, the combination ratio of catalysts to ionic liquids and most importantly, the nature of the catalyst used [89]. The molecular catalyst, [(m-Cl)2{PtCl(cyclohexene)}2], leached and the leaching strength was increased by the increase of the catalyst concentration. In contrast, in 28
ACCEPTED MANUSCRIPT H2[PtCl6] as an ionic catalyst, the observed leaching was negligible (below 1 ppm), and also independent of catalyst concentration. The hydrophobicity increased with the increase of the alkyl chain length and decreased with silane functionalization. Using highly hydrophobic polyethers, the RTILs phase separation occurred most simply in the organosilicon products due
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to their higher hydrophobic nature.
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3.5 Energy Production
RTILs leads in the energy sector for capturing, transforming and exchanging solar energy
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into chemical and electrical energy. Furthermore RTILs form the basis to convert biomass or vestige resources into domestic fuels, to develop newly nuclear skills and to progress functional
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material. These materials find key applications in biochemical and industrial processing [90].
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3.5.1 Solar Photo-Conversion
RTILs with inherited properties including low melting points, conductivity, thermal
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stability and low volatility have been reported as electrolytes for dye-sensitized solar cells
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(DSSCs) [91]. In such a system, organometallic or organic dye was allowed to adsorb on a semiconductor-like TiO2, and applied to an electrode. Accordingly different groups were excited
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by photons in the respective dye, which then gave an electron to TiO2. Similarly charge carrier
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normally iodide ion reduced the oxidized dye within the electrolytic system. At the same time, electrons and the oxidized species tended to associate at the counter electrode, resulting in circuit completion [92].
3.5.2 Solar Thermal Conversion Solar thermal energy aims to use solar energy in terms of thermal energy stored in heat transfer fluids contained in special vessels. Through this process, heat was conducted in a systematic way and converted into electricity by thermoelectric conversions. Such fluid was 29
ACCEPTED MANUSCRIPT stored in tightly insulated vessels to produce electricity when needed [93]. The aspect of such heated fluids must bear a wide range of temperatures ranging from 25- 250 oC. Fortunately, several ILs were stable in this range and formed the basis for suitable applications. Furthermore, RTILs were addressed to facilitate the electrical installation on a large-scale in order to provide
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heating and cooling facilities for in and outdoor activities in Mediterranean states [94]. Two
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altered designs, with extremely technological solutions have developed and evaluated in order to
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maximize the energy caught from the sun (by linear parabolic solar collectors) as shown in Figure 6 (a). Both solutions were primarily aimed to produce heat that will be used in advanced
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two-stage ammonia chillers in order to produce refrigeration (cold water≈ 5°C) for air
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conditioning and hot water (50 °C) for sanitary requirements. 3.5.3 Biofuel Production
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Biofuels, especially biodiesel, represent a potentially supportable transportation energy [95].
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The energy available from such biomass lies in their lignocellulosic structure. Such carbohydrate polymers are known for their firm H-bonding in cellulose and hemicellulose derivatives along
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with lignin to resist many biochemical and environmental outbreaks. In order to convert them
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directly for the purpose of energy production, a very specialised type of chemical treatment was necessary for a desired technology, which was only achieved by the use of RTILs [95]. RTILs
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with specified anions were inherited with strong H-bonding acceptance features, such as chlorides and carboxylates, which could dissolve cellulose along with lignin by disturbing their linkages of intramolecular H-bonding. The softened cellulose could be systematically recovered from the solution by the addition of anti-solvents such as water [96]. Furthermore, enzymes easily attacked the disordered assembly of cellulose to convert them into fermentable sugars. In addition to cellulose, there were other natural biopolymers such as wool, wood keratin and silk to
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ACCEPTED MANUSCRIPT process hybrid biopolymer composites with engineering materials. Another efficient fuel known as biodiesel, are nowadays a focal point on behalf of easy processing and good compatibility with the existing setup. Applications of RTILs have also been reported in conversion of triglycerides to biodiesel and glycerol (fatty acid methyl esters) through standardized catalysis
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[97].
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3.6 Paint Additives
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RTILs present a new class of additives in paint technology for improved and systematic drying, finishing and transparency [98]. The colour durability, stability and resistance to rub
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produced by using RTILs as secondary dispersing agents for durable painting, coating and dye
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technology seem to mean that RTILs are likely to replace the existing volatile organic substances in paints and glazes in near future of paint technology [99]. Figure 6 (b), present a comparison
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of painted surfaces with and without using ionic liquids.
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Fig. 6. (a) Solar cooling system, (b) Painted surfaces with and without added ionic liquids.
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3.7 Catalysis
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RTILs are known for homogenous catalysis due to the combined features of their anions and cations. Parshall, for the first time in 1972, described the Pt based hydroformylation of ethane in
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tetraethyl ammonium trichlorostannate with a melting point of 78 °C [18]. Later on, a number of
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groups established RTILs as a novel media for catalytic hydroformylation, olefin dimerization,
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shown in Table 6.
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oligomrisation, hydrogenation and polymerisation. A detailed summary of such reactions is
Table 6 Survey of RTILs based catalytic hydrogenation, C-C, C=O and coupling reactions Reaction Description
Ionic Liquid Scale Hydrogenation Stereo selective hydrogenation [bmim]Cl/ 30 g of aromatic compounds AlCl3 Hydrogenation of 1-pentene [bmim]BrF, PF6, SbF6 with isomerisation to 2-pentene using rhodium catalyst 32
4ml
Advantages Excellent yields and selectivities, efficient under mild conditions Liquid–liquid biphasic protocol realised
Ref. [100]
[101]
ACCEPTED MANUSCRIPT Catalytic cracking of polyethylene Catalytic cracking of high/low [emim]Cl/ AlCl3, 10 g Easy separation and density polyethylene to give [Nbutylpyridine]Cl/ reusable ionic liquid light alkanes AlCl3, Dissolution of kerogen and heavy oil. Heterocyclic [bmim]Cl/AlCl3, PF6 1ml No hazardous workup stages like distillation of azide, catalyst redesign, easy catalyst recycling Asymmetric ring opening reaction Ring opening reactions of [bmim]PF6/SbF6/ 1ml No hazardous workup epoxides catalysed by Cr BF4/OTf stages like distillation of azide, catalyst re-design, easy catalyst recycling Friedal craft alkylation and [bmin] PF6 0.5 g acylation Diels-alder reaction Cycloaddition-reaction [emim]Cl/AlCl3 2.0 g Very good reaction rate catalysed by acid catalyst with high exo and endo selectivity and solubility Dimerization, Oligomerization Polymerization Ethylene polymerisation by [emim]Cl/AlCl3 6.8 g Best product with easy Ziegler-Natta catalyst catalyst recycling Alkylation Alkylation of iso-butane with 2- [bmim]Cl/AlCl3 7 ml Good alkylate with no butene formation of red oil Heck reaction of aryl halides Tetraalkyl ammonium 1.5 g Easy catalyst recycling with butyl acrylate and Phosphonium halides Suzuki cross coupling of aryl [bmim]BF4 0.3 g halide with aryl boronic acid Carbonylation of aryl halides [bmim]BF4/ PF6 2 ml Easy product separation Witting reaction C=C formation [bmim]BF4 2 ml Selective extraction of alkenes
[103]
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Cleavage of compounds
[102]
[104]
[104]
[105]
[106]
[107] [108]
[109] [110] [111]
The table presents a series of RTILs as solvents for homogenous catalysis with unique features like dissolution, stability, reusability for various reactants, precursors and metal complexation. RTILs were addressed to maintain the active sites of catalyst in terms of chemo and regioselectivity [112]. Normally, for heterogeneous catalysis, the catalyst is bound to a specific region with a reduced number of active sites for reaction. Additionally, the separation of catalyst, 33
ACCEPTED MANUSCRIPT unreacted monomers and other by-products is considered a big problem in heterogeneous catalysis. In order to avoid such problems, RTILs gave a platform for both homogenous and heterogeneous catalyses [3]. In light of the above discussion, RTILs present a new class of solvent for catalysis, but the fundamentals of RTILs in terms of kinetics, electrokinetic,
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thermophysical and stability need to be addressed . The author summarises RTILs catalysis in
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three steps.
2. Reactions proceeding with C–O and C–C cleavage.
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1. Reactions proceeding with hydrogenation followed by rearrangement.
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3. Coupling reactions of heteroatom with C and/or C–C. These reactions form the basis for
Air Products
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3.8
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RTILs catalysis and further multistep catalysis are considered future challenges.
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RTILs introduce a new class of air products with exponential performance as compared to competing procedures running for the physical adsorption of different gases on some solid
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surfaces in order to get rid of these hazardous gases [113]. The main problem with these gases is
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their storage and transportation to a safe place at a higher pressure instead of at room atmospheric pressure [114, 115]. It is important to mention here that Complexed Gas
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Technology (CGT) termed RTILs as Guard- I sub atmospheric systems due to their easy handling, adsorption and transportation. RTILs present a green chemistry approach for the storage and transportation of such hazards under the Lewis acid-based concept for a variety of gases. Basic-nature RTILs such as [Cnmim][BF4], are used to store the Lewis acidic gases (such as fluoride and boron) and acidic-nature RTILs (such as [Cnmim][Cu2Cl3] or [Cnmim][Cu2Br3]) are used to store the Lewis basic gases (phosphine, PH3, or arsine, AsH3). In a simple vacuum transfer of highly purified gases, RTILs open a window for electronic industry in terms of 34
ACCEPTED MANUSCRIPT offering a pure and fresh supply of desired gases in special containers as shown in Figure 7 (a) [116]. 3.8.1 Green Cleaners Technologies RTILs as antistatic cleaning agents are used to clean very sensitive and high value surfaces.
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Generally the brushes are moistened by aqueous solutions or sodium chloride solutions as
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wetting agents but the effect of replacing these solutions with RTILs was increased exponentially
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as shown in Figures 7 (b). Beside RTILs find extraordinary applications in biomedicine, coating, implantation, food, biosensors, thermal pumps and the most recently developed green
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applications like DSSCs, phase-changing tools to accumulate universal energy, adsorption/
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desorption and solar cooling [117].
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3.9
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Fig.7 (a) Air Products based on AsH3 and PH3 (CGT) Cylinder Commercial Aids [115]. (b) Cleaning Technology Based on Nozzle Spray Containing NaCl(aq) and IL each after 10 h of operation (c) Purification by Brush bristles coated with IL Film [98]. Liquid Chromatography (LC)
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3.9.1 TLC
Normally silica gel is used for analysis based on HPLC, but silica surface is considered to
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contain silanol groups, so they raise a problem in separation analysis in terms of the broadening
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of the peaks. To avoid this problem, amine based additives are commonly used to block this
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acidified surface peaks broadening [20, 118]. It was reported by Kaliszan et al., that ILs are capable of cutting the effects of silanol on the retention of the drugs during TLC analysis [119].
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Amine additives (ammonia, dimethyloctylamine, triethylamine) failed in suppressing the effects
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of free silanols, but the application of ILs solved the problem of suppressing such effects in a more precise way. ILs applied to such analysis were BF4 containing 1-ethyl-3-methyl, 1-methyl3-hexyl, 1-ethyl-3-methyl, 1-hexyl-3-hepty, 1-ethyl-3-hexylloxymethylimidazolium compounds. Consequently, retention of the analyte and separation of the peptides were highly optimised. It was observed that retention and efficiency was also affected by the presence of different alkyl groups present on the imidazolium cations. ILs were also addressed as additives to distinct nucleotides and other bioactive species. [120].
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ACCEPTED MANUSCRIPT 3.9.2 LC Stationary Phases Joshi et al., examined the imidazolium-based ILs as stationary phases and observed a strong exchange of anions, nucleotides, amines and phase interaction [121]. According to Liu et al., the stationary phases based on silane can be utilized at the maximum extent, thereby achieving quantitative
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and reproducible bonding. Along with sucessive recycling [122]. Another type of stationary phase
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consist of ctylbenzimidazolium-modified silica was prepared by covalent attachment of 1-
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octylbenzimidazole to γ-chloropropyl silica. The proposed material was suggested as potential mixed-mode stationary phase for separation of polycyclic aromatic hydrocarbons, mono-
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substituted derivatives of benzene, anilines, and phenols [123]. Another class of ionic liquid
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based Novel octadecyl stationary phases differ in distribution of imidazolium group have been prepared and comparatively evaluated via linear solvation energy relationships (LSER) model.
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The results were based on the comparative studies of both the stationary phases in term of the
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retention time and difference in selectivity by eluting alkylbenzenes, alkylnaphthalenes, condensed-ring and phenylene polynuclear aromatic hydrocarbons by using LSER [124]. Wang
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et al., merged ionic liquid in the porous polymer monoliths to afford stationary phases with
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enhanced chromatographic performance for small molecules in reversed‐phase HPLC. The formulations were based on the separations of various small molecules including aromatic
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hydrocarbons, isomers where the results suggested that poor resolution and low efficiency of monoliths were significantly enhanced by the addition of ionic liquids into the polymer monoliths [125]. Another strategy based on butylimidazolium bromide surface-confined IL stationary phase was synthesized and investigated for separation of five peptides (Gly-Tyr, ValTyr-Val, leucine-enkephalin, methionine-enkephalin, and angiotensin-II) [126]. An interesting aspect of RTILs for future applications lies in counter-current chromatography (CCC), where
37
ACCEPTED MANUSCRIPT they are responsible for both liquid containing stationary and mobile phases. Berthod et al., reported the use of [bmim][PF6] in a ratio of 40:20:40 w/w water–acetonitrile, in order to investigate the distribution constants in different phases [127]. 3.9.3 Micellar Capillary Electrophoresis (MCE)
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Micellar capillary electrophoresis (MCE) is known for its ability to separate both charged
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and neutral analytes. As the name indicates, a suitable surfactant is needed to onset micellization
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for effectiveness and selectivity. For the first time, ILs in micellar electrophoresis were introduced in order to separate both chiral and achiral complexes [128]. Sodium containing
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various surfactants was tested to differentiate various derivatives of aryl, alkyl, phenols and
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ketones. Among various ionic liquids, [bmim][BF4] seemed to work very efficiently in a variety of ways. It was also noticed that the concentration of ILs affected peak determination and
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resolution. It was established that, along with modifiers, RTILs can also be applied as surfactants
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as the micellization performance alkylimidazolium based ionic liquid was examined. Further [bmim]-dodecanesulfonate (BAS) was applied in microprocessor chips and established high-
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electro osmotic flow, also expanding the separation of proteins and nucleotides as shown in
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Figure 8 [129].
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[20, 130]. 3.10
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Fig. 8 Polyphenol Separation Based on (A) [emim]BF4, (B) [bmim] BF4 and (C) [emim] PF6
Biocatalysis
One of the fundamental aspects of RTILs is to maintain the stability and catalytic activity of enzymes suspended in ionic which remain in any other organic polar solvent [131]. RTILs are considered as the favourite solvent for enzymatic catalysis in non-aqueous media. Many biological intermediates including nucleotides, peptides and sugars were investigated for applied 39
ACCEPTED MANUSCRIPT research in biomaterials, pharmaceuticals, chemicals and materials such as hydrogel technology. Biocatalysis, isolation of specific molecules, reuse and separation of RTILs are considered as the main facts for such applications [132]. The lower melting point, lessened vapour pressure, comparable polarity to organics, greater enzymatic stability and reactivity, higher reaction
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kinetics and their reusability reflects the greener approach and controlling pH, dissociation
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shown in Table 7, presents RTILs’ efficiency for biocatalysis .
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Table 7 RTILs Catalysed Enzymatic Reactions Enzymes/ Microorganism
Reactions
Thermolysin α-chymotrypsin
Peptide synthesis Transesterification
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Reduction of ketones 10:1 Biphasic (Ionic Liquid: H2O) Oxidation of guaiacol 25% Ionic Liquid in water Oxidation of 25% Ionic Liquid in water syninggaldazine Lipases Transesterification Biphasic (Ionic Liquid: H2O) Perhydrolysis 25% Ionic Liquid in water Transesterification Biphasic (Ionic Liquid: H2O) Polyester synthesis 25% Ionic Liquid in water
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Formate dehydrogenase Baker’s yeast Peroxidases Laccases C
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B-Galactosidase
PCL
Comments
Proteases 25 % ionic liquid in H2O Biphasic (Ionic Liquid: H2O) Esterease Transesterification Biphasic (Ionic Liquid: H2O) Glycosidase N-Acetyllactosamine Biphasic (Ionic Liquid: H2O) synthesis Oxidoreductases Regeneration of NADH Biphasic (Ionic Liquid: H2O)
BSE
Cal-B
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constant, low are some of the bases for the application of RTILs in biocatalysis. A brief report, as
Ref
[131] [131] [133] [134]
[135] [136] [137]
[138] [131] [139] [140]
3.11 Electrochemical Industry High ionic conductivity, lower volatility and vapour pressure represent the aspects of RTILs that are applicable in electrochemical devices, fuel cells, batteries, solar cells and applied 40
ACCEPTED MANUSCRIPT cells. Electrochemistry deals with RTILs’ interfacial features, extraction, voltammetric analysis and transportation of metallic ammonium ions across membrane [141]. Many efforts have been made to design such an electrolyte system with significant stability and precision to efficiently replace the present liquid organic based electrolytic solutions particularly based on lithium salts
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in RTILs. However their practical availability and advanced chemistry is under discussion as the
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structural interface of RTILs/electrodes is still unknown. It has also been reported that RTILs are
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unstable at reduced or low voltages, e.g., when in contact with highly negative electrodes, such as lithium metal and/or graphite anodes [142]. Serious attention is required to investigate the
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vital reactions happening at the RTIL-solid electrode site. Both experimental (for instance
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impedance spectroscopy) and computational modelling is necessary to explore basic electrochemistry with new techniques and implementations may be appropriate to shed some
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light on the basic electrochemistry of ionic liquids. The above discussions only suggest
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investigating advanced chemistry, as modern electrochemical approaches frequently deal with the application of RTILs in various fields. The reported properties, including very high
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conductivity (4.5V in comparison with 1.2V) in aqueous electrolytes, electro, hydro and thermal
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stability, and in flammability are the main studied features which form the basis for RTILs as supreme electrolytes in electrochemical strategies for batteries, fuel cells, capacitors, actuators,
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photovoltaic, electrochemical and optical sensors. Takaya et al., [143] investigated ammonium salt with a methoxyethyl group in combination with tetrafluoroborate (BF4-) and bis(trifluoromethyl sulfonyl)imide [TFSI; (CF3SO2)2N−] anions. The oxidation-reduction potential, some thermophysical properties like conductivity investigation practically considered them for electrochemical capacitors. Consequently, they held very big potential (6.0 V); highly ionic conductivity (4.8 mS cm−1 at 25 ◦C) made them an ideal candidate for electrochemical
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ACCEPTED MANUSCRIPT sensors. The standard oxidation and reduction potential was investigated and the limiting current density was defined. Such systems showed an improved c stability compared to phenyl based ILs due to their non-electron conjugated system. Therefore, were considered better electrolytes for a high operating voltage electric double layer capacitor (EDLC). A typical voltammogram for ILs
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is shown in Figure 9, with respective parameters shown in Table 8.
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Fig. 9 Cyclic Voltammogram of the ILs Based on DEME Cation and EMI-BF4 at 25 ◦C [143].
Table 8 Physical Properties of the Investigated RTILs [143] Ionic Liquid
Density at 20 Molar. Conc ◦ C / (g cm−3) (mol dm−3)
DEME-BF4 DEME-TFSI EMI-BF4
1.18 1.42 1.24
5.02 3.33 6.26 42
Tm (◦C)
Tf (◦C)
Tg (◦C)
Td (◦C)
9 n.d 15
-35 n.d -51
n.d -91 -87
318 383 391
ACCEPTED MANUSCRIPT 3.11.1 Ion Selective Sensors Reference electrodes based on RTILs are implemented currently due to a variety of electrochemical advantages related to RTILs. In order to produce very high sensitivity for ion detection, RTILs are reported to be used with Ag/AgCl microelectrodes in polyvinyl chloride
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membranes [dmim][Cl], producing a solid electrolyte [144]. Kakiuchi et al., [145] also
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contributed to report the thermodynamics of solid-state reference electrodes containing RTILs.
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Potentiometry by ion selective electrodes is reported to be a very fast, accurate and comparatively cheap method to sense many species at very low detection limits and high
graphite
powder,
N-octylpyridinium
hexafluorophosphate
[OPy][PF6],
and
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containing
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selectivity. Kachoosangi and Faridbod., [146, 147] reported potentiometric ion sensors
[bmim][PF6]. It was concluded that such a combination responded to various ions due to the
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partition of ions between the electrode membrane and the aqueous phase due to the lack of
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ionophores in the assembly of electrodes. Furthermore, Solid-state ion selective electrodes are under investigation to encourage polymerisation for a variety of monomers. One such reported
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polymer, known as PEDOT, a conducting polymer, was electro-polymerised with the help of
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RTILs containing large organic anions. The experiment resulted in an improved potentiometric reaction in 10−5 to 10−1M KCl(aq) [148]. Therefore, RTILs, due to their ionic nature and highly
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plasticising ability, attracted the researchers to formulate different membranes for ion selective electrodes. Coll et al., also reported on the above mentioned membranes for specific ion selection in the electrodes by using a hydrophobic ionic liquid [bmim][PF6]. Furthermore, [bdmim][Tf2N] was used in order to induce higher ionic selectivity by plasticising the PVC and PMMA membranes. This resulted in a stable and highly selective response to hydrophobic anions and cations and surfactant composition analysis was achieved for such electrodes [147].
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ACCEPTED MANUSCRIPT 3.11.2 Voltammetric Sensors Voltammetry is one of the electro analytical methods where RTILs are used for a high degree of detection and selectivity. [bmim][PF6], [bmim][Tf2N] and [bmim][BF4] were reported to detect a very low level of chlorides (ppb) in comparison to typical square wave, linear sweep
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and cathodic stripping voltammetry [149, 150]. Wang et al., reported the imidazolium-based
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polyelectrolyte system to analyse the flow-injection in different electrochemical analysers by
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using the voltammetric approach [151]. All the electrodes (working, reference and auxiliary) were employed side-by-side within the flow passage and shielded with a thin film of RTILs. It
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has been noted that without disturbance from water-air bubbles, the electrochemical species are
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diffused from the channel via a thin film on the surface of the working electrode with negligible time loss. Kanakobu et al., reported the higher electocatalytic activity of a carbon paste electrode
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by using [omim][PF6] and [OPy][PF6] using the redox analysis, where an increase in the
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electocatalytic activity of the electrode was noted due to the addition of RTIL [152]. 3.11.3 Gas Sensors
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RTILs, characterised by inherent conductivity, negligible vapour pressure and broad ionic
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organisation, do not require any supporting electrolyte solution to develop gas sensors for O2, CO2, and NH3 [153, 154]. Platinum (Pt), glassy carbon, and gold (Au) or) electrodes containing
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RTILs are reported to detect the superoxide radical (O2•−) produced by the in situ reduction of O2 at a very low level (ppb) [155]. Accordingly, O2 sensors built on PVC membranes containing [emim][BF4] showed a high sensitivity, widespread detection range, and outstanding reusability and reproducibility. It has been reported that with the high concentration of CO2 in the sample, cyclic voltammetry gave an enlarged cathodic peak current due to the production of (O2•−) and the decreased peak current with reverse scan of oxidation. This indicates that the generated (O2•−)
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ACCEPTED MANUSCRIPT radical reacted irreversibly with CO2 to form a peroxydicarbonate ion, C2O6
2−
. Another kinetic
investigation based on ([N6, 2, 2, 2][Tf2N] and [emim][Tf2N] addressed an amperometric way to detect CO2 [156]. The reaction between the (O2•−) radical and CO2 was seen to progress in a similar mechanism in both cases, which reflects the gas sensor applications related to RTILs.
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Figure 10 shows a conceptual diagram sketching the sensing of CO2, NH3 and other gases as
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reported by A. Inaba et al., [157].
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Fig. 10 A proposed gas sensor
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3.11.4 Gelation of RTILs and their Use in Biosensors Development of biosensors containing RTILs is a predominantly worthy of consideration. The maximum strength of enzymes in RTILs in comparison to mostly organic solvents facilitated and improved the stability of different enzymes and proteins even at a higher temperature [158]. Dagade et al., investigated the hemin catalysis by initiating through an electron acceptor in ionic liquid solution [159]. It was concluded that H-bonding and electrostatic interactions are responsible for a high kinetic barrier for the unfolding of enzymes and the 45
ACCEPTED MANUSCRIPT catalytic activity of hemin increased by increasing the volume of the ionic liquid. Further, the thermal stability, higher activity and the structure disruption of enzymes immobilised in IL-based matrices were investigated. It was concluded that the choice of the RTILs for specific application matter as some of them inactivate enzymes. Another effort to produce a biosensor based on the
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in-situ functionalization of SCNTs in [bmim][PF6] in order to fabricate a glucose sensor showed The CNT- ILs based
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the capability to covalently bind the glucose oxidase (GOD) [160].
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biosensors were tested with analytical skill for biological molecules such as Lactic acid (LA), ascorbic acid (AA), dopamine (DA) and (NADH) in order to further increase the sensitivity and
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thus the conductivity of the biosensors [161].
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Nitric oxide (NO) was generated in both normal and abnormal tissues and was able to react very rapidly with O2, haemoglobin enzymes, proteins and other bio-oxidative molecules in
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vitro and also in vivo. It is important from the medical and biochemical points of view.
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Biosensors based on ([hmim][PF6]) showed an efficient sensing of NO by the electrochemical– chemical (EC) oxidation mechanism within the 100nM to 100mM range [162]. Lenzo et al.,
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tested [emim][NTf2]-based ammonia sensors based on the electro-oxidation of hydroquinone
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[163]. Such a sensor was without any absorbed water, thus with no moisture and humidity it may find application in hi-tech analysis where ammonia and related gases can be detected in an
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extremely low range.
RTILs are thus the catching material in sensor technology. RTILs-based sensors are considered a hot favorite research field with inherited sensitivity, selectivity, reusability, separation and an extremely low detection limit. RTILs based sensors act as functioned as a nonvolatile electrolyte and do not need a membrane for sample separation. More importantly,
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ACCEPTED MANUSCRIPT RTILs-based sensors may find applications to work in extreme environments and in incineration engineering at extraordinary temperatures. 3.12 Application of RTILs in Pharmacology, Bio-medicine and Bio-nanotechnology The remarkable interaction of RTILs with biomolecules such as phospholipid bilayers,
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nucleic acids, peptides and proteins for targeted applications have been described by Benedetto
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et al.,[164-166]. Accordingly, the role of RTILs in pharmaceutical, drug delivery, preservation of
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nucleic acids, molecular- biomedicine and bio-nanotechnology are promising as next generations technologies. The fundamental of RTILs is dependent of their selectivity, sensitivity and bio-
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availability at physiological conditions. Currently, RTILs have been applied for a variety of
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targeted applications, regardless their side effects by producing toxic species, radicals and ions from the decomposition and side reactions of anions like [HSO4]-, [BF4]-, [PF6]- and
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[PF6]- respectively. On the other hand, the variety of complex interactions between the
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biomoleculeas and RTILs are also important but unknown to result in complex behaviors and
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interfaces at physiological conditions.
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3.12.1 Interaction of RTILs with phospholipid bilayer The effect of RTIL on salt concentration [NaCl, MgCl2, KCl] was investigated in hydrated
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environment and the results were established by various experiments and simulations as shown in Figure 11 [167]. The results suggested that the cations of respective RTILs can enter the bilayer system with the ability to interact with the carbonyl and phosphonium anions. Further, tri and divalent cations were observed with higher interactions with the bilayer molecules as compared to monovalent cations, with a steady increase in transition temperature and bending ragidity of the overall system for desired applications. Consequently, the interfacial tension
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ACCEPTED MANUSCRIPT vanishes between water-phospholipid bilayer system, so there is no effect of any salt addition to the system in presence of hydrated RTILs. One of the drawback was observed to destabilize the
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phospholipid bilayer by using long chain hydrophobic cation or anions respectively [168, 169].
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Fig. 11 Interaction of RTIL and phospholipid bilayer controlling the effect of salt.
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3.12.2 Interaction of RTILs with nucleic acids (RNA/ DNA)
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The negatively charged character of the nucleic acids including RNA/ DNA play a key
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role to interact with a variety of cations associated with diferent RTILs [170]. Thus, a variety of reactions, stability and different ways of tunning the interactions between nucleic acids and
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RTILs can be originated for high tech- targeted applications like bio-nanobiotechnology [165]. Another striking aspect of RTILs aim to solubilize the RNA/ DNA for separation, processing and
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time and temperature-dependent stability applications [171]. Advantages related to RTIL-nucleic acid interaction are necessary to produce hydrated solutions to extend the lifetime of RNA/ DNA at standard conditions, to preserve the nucleic acid for extended time, to save and reduce the expenditure of medical treatment and finally to reserve and revive the fossils [172]. As RNA is less stable than DNA, so the emerging research is focusing on the solubility and processing of DNA for desired applications as described by Arguably et al., [173]. Accordingly, the results
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ACCEPTED MANUSCRIPT suggeste that DNA is soluble in (20% hydrated) choline based ionic liquids. The exceptional stability of DNA was obderved for six months at room temperature confirmwed by retaining of the UV- visible and Fluorescence spectral peaks. Further, the the stabilization of DNA structure and the absence of denaturation peak up to 900 C was confirmed by Fluorescence spectroscopy.
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Finally, the electrophoric measurements showed that the composition and size of the DNA were
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unchanged for six months kept at room temperature. The study contributed a lot to the
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modification, processing, stabilization and functionalize the DNA for bi- nanotechnology applications to arise.
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3.12.3 Interaction of RTILs with proteins and amino acids
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The better solubility enzymatic-action, and thermo-chemical aspects of proteins in RTILs perhaps exceeds expectations. The interaction of protein (amino acid) and RTILs have been
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focused to explore the phase performance, solubility, biphase- and ternary phase separation, bio-
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products, bio-synthesis and bio-catalysis [174]. Similarly, another study report on the stabilization of amino acid in ammonium based RTILs by investigating the change in free energy
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to transform one mole of amino acid from water to RTIL solution [175]. Furhter, the protein-
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RTIL interaction is presented in term of Hofmeister series by measuring the ability of solute to salt-in and salt-out proteins [176]. The Hofmeister series also contribute to calculate the
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electrophoric mobility, biological and enzymatic activity of different proteins andd amino acids in solution. Consequently, the destabilizing effect of cation was observed with increasing hydrophobic chain length, which favorably interact with hydrophobic amino acid chain groups. The effect of RTIL on the structure of protein was investigated by using solvation dynamics measurements fluorescence correlation spectroscopy respectively [177, 178]. Some of proteins have been reported with inverse enzymatic activity while majority of proteins like lypases have
49
ACCEPTED MANUSCRIPT been reported with increased enzymatic activity after interacting with RTILs [179]. Regarding interaction of RTILs with proteins, another report describes the formation of amyloid fibres, i.e. fibrous aggregates of proteins and peptides that might indeed represent the thermodynamically stable form of these systems as shown in Figure 12 [180]. Concerning the vast bio-
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nanotechnology and pharmacological interest, still there is no report to address the mechanism
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and effects of such interactions. So, a significant research is still needed to explore through in
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vivo experiments, computer modeling and simulations establish the fundamental of interactions
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between RTILs and various proteins with different chain lengths at physiological conditions.
Fig. 12 Interaction of RTILs and proteins for nano-biotechnology applications. 5.0 Summary RTILs are proved to be the useful reaction media and catalyst in various processing and synthetic routes. The author aimed to visualize the future, advance chemistry and unseen applications of RTILs. Understanding the internal properties and analytical chemistry of RTILs 50
ACCEPTED MANUSCRIPT prior to relating it is considered the key criteria for RTILs application. The built-in functionalities of the desired RTILs have the potential to expand and modernize industrial setup. RTILs can be synthesized by combining a variety of organic cation and asymmetrical anions in a simple reaction. The processing schemes are confident enough to produce novel RTILs which
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must be applied in dissolution processes, ion-selective sensors, energy generation, catalysis,
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bioseparation, additives and electro-analytical methods. Consequently, the development of new
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synthesis and relevant application is quite certain to allow RTILs in challenging tasks. RTILs have challenged both our experimental and our intellectual abilities. Detailed descriptions of how
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ionic liquids interact with solute species to change their reactivities have begun to emerge, yet
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much remains to be discovered.
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Acknowledgment
The author is highly grateful to the School of Materials and Mineral resources engineering,
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USM, for sponsoring this research under the project Grant FRGS-203/ PBAHAN/ 6071337.
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Conflict of interest
2. 3. 4. 5. 6.
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Fatima Javed, Faheem Ullah, Muhammad Razlan Zakaria, Hazizan Md. Akill PII: DOI: Reference:
S0167-7322(18)31908-1 doi:10.1016/j.molliq.2018.09.005 MOLLIQ 9611
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
10 April 2018 28 August 2018 1 September 2018
Please cite this article as: Fatima Javed, Faheem Ullah, Muhammad Razlan Zakaria, Hazizan Md. Akill , An approach to classification and hi-tech applications of roomtemperature ionic liquids (RTILs): A review. Molliq (2018), doi:10.1016/ j.molliq.2018.09.005
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ACCEPTED MANUSCRIPT An Approach to Classification and Hi-Tech Applications of Room-Temperature Ionic Liquids (RTILs): A Review
Department of Chemistry, Shaheed Benazir Bhutto Women University, Peshawar 25000,
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains
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Malaysia, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia
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*Corresponding Author:
Tel: +6045996161
Fax: +6045996907
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Email: [email protected]
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Khyber Pakhtunkhwa, Pakistan
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Fatima Javeda,b, Faheem Ullahb, Muhammad Razlan Zakariab, Hazizan Md. Akil1*b
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Graphical
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Highlights
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o Classification of RTILs on the basis of structural and induced ionic moieties o Investigating the intrinsic properties of RTILs in comparison to typical solvents
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o Advanced chemistry and cutting-edge applications of RTILs in hi-tech interactions
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Abstract
A new background to organize diverse synthetic and natural resources to fabricate
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functional materials in a safe environment by using Room Temperature Ionic Liquids (RTILs) is highly stressed. A new approach to classify various RTILs on the basis of induced structural moieties is explored. RTILs are reviewed as reaction media to process functional materials, green composites, cellulose dissolution, energy production, additives, cleaner, chromatography, hetero/ biocatalysis, bio/ electrochemical sensors and interaction with bio-membranes to attract current academic and industrial development. RTILs are expected to substitute conventional
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ACCEPTED MANUSCRIPT solvents as RTILs have challenged both our experimental and intellectual abilities to explore further. Keywords; RTILs and Green composites; electrochemical sensor; solar-cooling system; catalysis; biocatalysis; biosensors.
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1. Introduction
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The academic and industrial prospects of emerging green chemicals highlighted as Room Temperature Ionic Liquids (RTILs) are striking to substitute hazardous and contaminating
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organic strippers. Switching from a typical organic solvent to an ionic liquid can lead to novel and unusual chemical reactivity. RTILs have the potential to change the idea of synthetic and
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applied material science research for 21st century scientists. The significance of RTILs is
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emphasized by the incremental rate of publications regarding their academic and industrial
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considerations but their chemistry is still unexplored. The official definition of RTILs uses the boiling point of water as a reference. Thus, RTILs
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are ionic compounds which are liquids below 100 oC. More specifically, salts that are liquid at
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room temperature are known as Room Temperature Ionic Liquids (RTILs). At structural and molecular level description, they are completely different from all other solvents [1]. RTILs are
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termed as electrically neutral salts as the constituent positive ions match equivalently with the negative ions in the liquid. They do not contain any molecular level presentation like other solvents which are known for their polar or apolar nature based on the interaction between the solvent molecules [2]. RTILs are known for their purity, creating an opportunity for researchers to address and commercialize the newly discovered low melting salts. RTILs exhibit exclusive thermophysical properties such as high electrical conductivity, highly thermal stability, immiscibility, non-aqueous nature, low nucleophilicity, lower melting point, viscosity, density, 3
ACCEPTED MANUSCRIPT refractive index and a large electrochemical space [3]. Such properties are the basis of RTILs for hi-tech targeted applications where a common solvent fails to work well. Moreover these properties can be tailored to perform in a much better route. RTILs being non-volatile are recognized as “green solvents” to replace the volatile solvents commonly uses in the organic
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industry, which are marked for a variety of environmental concerns. Another attraction of RTILs
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is based on their ability to be reused and recycled without evaporating, allowing them to be
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considered as potential candidates for high-vacuum systems to eliminate many toxin problems
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[4].
Switching from an organic solvent to RTILs result in significant improvement in catalytic
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ability, kinetic stability and enantio-selectivity but the internal responses for such changes is still
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unknown. Due to this reason a comprehensive knowledge of the classification, application, their mode of interactions along with the processining chemistry should be caution during their
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selection as a replaceable solvent [5, 6]. Almost 399 RTILs have been synthesized and processed
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without investigating their multi-solvation interactions, thermokinetic and enhanced catalytic conditions. There is an urgent need to overview the efficient classification, application, synthetic
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and processing schemes of RTILs, to attract and ease the acadamic and industrial research for
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hi-tech targeted interactions.
We are introducing two types of RTILs. Firstly, the organoaluminates, which have the donor and acceptor potential and their acidic and basic nature can be achieved by varying the composition of Aluminium as shown in Figure1 (a). Unlike acidic organoaluminates, the basic haloaluminates tend to stop the solvation and solvolysis of metallic ions, and provide a large electrochemical space for chloro, bromo and iodo based-RTILs [7]. Such type of controlled Lewis acid based RTILs are striking candidates in various catalytic reactions (Ziegler- Natta type 4
ACCEPTED MANUSCRIPT catalytic reactions) with controlled stereo-selectivity, chemo-selectivity and moisture sensitivity. Also the molar friction (x) of Aluminium compound is crucial to control the viscosity (υ),
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melting point (Mp) and facilitate the reaction at acidic or basic conditions as shown in Table 1.
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Table 1
Mp (oC) -80 2 -60
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υ (p) 0.16 0.20 1.59
pH acidic neutral basic
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Mole friction (x) 0.66 0.50 0.36
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Effect of Aluminium compound on the melting point and viscosity of chloroaluminates
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Fig. 1 (a) Effect of Aluminium compound on the act of 1-Ethyl-3-methylimidazoliumchloride / Aluminium Chloride RTILs.
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Secondly, water and air stable RTILs, which can be achieved by the substitution of halide anion by another weakly coordinating anions of the 1,3- dialkylimidazolium cation. Such type of RTILs are preferably unsymmetrical, the anions of which affect the melting point, preferably used for the immobilization of transition-metal catalyst precursors in biphase catalysis. They are immiscible with some organic solvents, e.g. alkanes, hence, can be used in two-phase systems. This give rise to the possibility of a multiphase reaction procedure with easy isolation and recovery of homogeneous catalyst. Polarity and hydrophilicity and or lipophilicity can be readily
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ACCEPTED MANUSCRIPT adjusted by a suitable choice of cation/anion and RTILs have been referred to as ‘designer solvents’. The investigated effect of anion on the melting point of imidazolium based RTILs is shown in Table 2. Table 2
N R
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Melting point (oC) 125 87 65 38 7 6 -9 -3 -14 16
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Anion (X ) Cl Cl Cl NO3 AlCl4 BF4 CF3SO3 (CF3SO3)2N CF3CO2 CF3SO3
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Alkyl group (R) Me Et n-Bu Et Et Et Et Et Et n-Bu
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Effect of anion on melting point of Imidazolium based RTILs
To date, RTILs are intensively considered for potential applications in biomedical,
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biocatalysis, biospeapration, green biocomposites, biosensors, fluid thermodynamics, catalysis,
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organic synthesis, electrochemical windows, separation, purification, gas sensors, ion sensors, biomass processing and producing functional energy. Therfore, it is important to realize the importance of induced moieites into RTILs, the possible combinations of cations, anions and their effective processing for selective applications [8, 9]. Few thousand of RTILs are roughly estimated to be available in the market in near future, therefore an efficient classification of RTILs is compulsory to understand the synthetic flexibility and tunibility by using a single or a mixture of ionic liquids with induced hydrophilicity, hydrophobicity, sensitivity and selectivity
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ACCEPTED MANUSCRIPT [10]. Some of concluding remarks must be addressed to explore further regarding RTILs processing and utilization for a specific application as demonstrated by Taige et al., who stated that pyridinium and imidazolium based RTILs have the potential to produce binary liquids with low viscosity and higher thermal and ionic conductivities [11]. Annat et al., demonstrated that
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the crystallization is completely suppressed by using a mixture of phosphomium and pyridinium
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based ionic liquids [12]. Currently, Stolte et al., introduced a new class known as tunable aryl
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alkyl ionic liquids (TAAILs), which are preferable with better substitution pattern where the electronic interactions between the core and aromatic substituents can easily tune the
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physicochemical properties as compared to typical imidazolium based ionic liquids [13].
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The green aspects of preparing RTILs with 99% yield under mild conditions with no by-
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product formation and easily separation just by filtration are the basis of RTILs in various reactions [14]. The synthesis of RTILs including Metathesis (a reaction coupled with the
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characteristic exchange of ions between two compounds usually with the precipitation of an
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insoluble product), acid-based neutralization and the direct combination of halide based salts with metallic halide were investigated [15]. Several imidazolium, pyridinium, alkyl-ammonium
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halides are available commercially and their synthesis is also reported by reacting amine with
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halogenoalkanes [16, 17]. In 1992 the first ionic liquids [emim][BF4] with a melting point of 12 °C, was synthesized via the metathesis of [emim]I with Ag[BF4] in methanol [18]. Metathesis reactions for the first time were very common but they contaminated the ionic liquids with some halide ions that may have reacted with solute materials [19]. Another method based on acid base neutralization and the direct combination of a halide salt with a metal halide are discussed systematically to expoler the art of RTILs processing [20].
7
ACCEPTED MANUSCRIPT Overall, this review aims to highlight the importance of RTILs to recognize and replace the harmful raw materials and generated wastes. Additionally, how to choose a specific ionic liquid for a specific use, many recent applications of RTILs with detailed analysis and the development of novel synthetic procedures have been investigated to charm the academic and
Scope of the Article
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1.1
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industrial knowledge of RTILs.
This review aims to explore the most recent literature about room temperature ionic liquids
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and attempts to address the new classification with detailed processing and multiple applications.
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The field is still very young, challenging and interdisciplinary but at the same time also very applied. The authors expect a number of vitally important developments to come from exploring
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the RTILs to ease the synthesis, processing and implementation in challenging technologies. The
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arena of ionic liquid is evolving swiftly with a large number of research directions, thus it is not possible to overview the complete literature of ionic liquids in this review. Therefore, the
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thermophysical properties of ionic liquids are not included in this work. A new concept of RTILs
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classification is described which might be helpful for researchers to select a desirable RTIL as a first choice. Further, the most recent applications and processing technologies RTILs are
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explored which might be helpful to understand the chemistry and impact of RTIL for hi-tech targeted interactions where a common solvent fails to overcome the objectives. The processing technologies are assured to produce novel RTILs which must be applied in dissolution processes, ion-selective sensors, energy generation, catalysis, bioseparation, additives and electro-analytical methods. Consequently, the development of new RTILs and relevant application is quite certain to allow RTILs in challenging tasks.
8
ACCEPTED MANUSCRIPT 1.2
Characteristic features of RTILs The cations control the physical properties (viscosity, melting point, vapour pressure,
density) and the anions reflect the chemical properties and thus the reactivity of RTILs [21]. Based on the thermoochemical properties, RTILs are termed as “Designer Solvents” for
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applications where high conductivity, thermal and chemical stability are needed with a variety of
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solvation potentials. The characteristic properties of RTILs ( based on the dissolution of a variety of organic and inorganic materials, extraction procedures, chemical separation and synthesis)
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depend upon the strategic interactions, structure, charge, immiscibility and distinctive combination of the constituent ions being virtuous, non- coordinating, non-volatile and highly
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polar [22].
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The variation in thermophysical properties is attributed to the modification and splitting of chain-length in alkyl groups integrated to the cations [23]. For instance, the solubility
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vanished as the alkyl chain increased from C1 to C9 as reported in 1-alkyl-3-methyl-
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imadazolium hexafluorophosphate [Cnmim] [PF6]. Additionally, exchanging the PF6− anion by BF4− in 1-alkyl-3-alkylimadazolium dramatically increased the solubility of the ionic liquid in
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water while replacement with the bis(perfluoromethyl-sulfonyl)imide anion (CF3SO2)2N− (also
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known as bistriflate imide (Tf2N−) ) result in reduced water solubility [24]. Most common singly charged RTILs are known to have a diverse combination of organic/ inorganic anions based on nitrate or methanesulfonate and organic heterocyclic cations such as dialkylimidazolium. Properties of a representative RTIL are reported in Table 3, while the general comparison of RTILs with other organic solvents are shown in Table 4 [25].
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ACCEPTED MANUSCRIPT Table 3 Specific properties of a representative RTIL
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APH4/CGH3 Ultra-pure grade Anion C15H20F18NP 587.27 24 oC 250 oC Immiscible Unknown Miscible 1.59 173.81 carbocation Very high
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Short name Quality Type of RTILs Formula Molecular weight Melting point Decomposition temperature Miscibility with water Miscibility with toluene Miscibility with acetonitrile Density (g.cm-3) Viscosity (mm2/s) Stabilizing Electrochemical stability
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Room Temperature Ionic Liquid Detailed data 1-Butyl,1-Methyl-Pyrolidinium Tris(pentafluoroethyl)Trifluorophosphate
RTILs Few thousand Multifunctional Tunable Tunable Negligible Nonflammable Strongly solvating Polarity depends designer solvents 2-100 times of organic solvent Economic imperative 22–40,000 0.8–3.3 1.5–2.2
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Features Total solvents Applicability Catalytic capability Chirality Vapour pressure Flammability Solvation Polarity Tuneability Cost Recyclability Viscosity/cP Density/g cm3 Refractive index
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Table 4 Comparison between RTILs and Organic Solvents
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Organic solvents Millions Unifunctional Rare Rare Follow C-Clapeyron equation Usually flammable Weakly solvating Conventional polarity Limited range Cheaper Green imperative 0.2–100 0.6–1.7 1.3–1.6
ACCEPTED MANUSCRIPT 3
Classification of RTILs Figure 1 (b), present the most recent selection of integral cations and anions for distinctive
RTILs. Such a variety of asymmetric organic cations and inorganic anions do not present any major difficulty in preparing RTILs of high purity, without time dependent decay and
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evaporation. Cations based on the imidazolium or pyridinium ring with one or more alkyl groups
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and anions based on hexafluorophosphate halide ions, tetrafluoroborate, tetrachloroaluminate, bis
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(perfluoromethyl-sulfonyl) imide and quaternary ammonium salts are recently most studied [26]. It is important to mention here, the alkyl group present in the cation chain of RTILs reflect the
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solubility and viscosity in any fluid. Mutual electrostatic or ionic interactions compare to other
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conventional solvent interactions (dipole–dipole, van der Waals, hydrogen bonding) reflect the miscible nature of RTILs with many polar solvents [27]. Furthermore H-bonding, hydrophilicity,
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lipophilicity, miscibility and stability can be modified by a suitable choice of anion, cation and
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chain length of the attached alkyl group. Anionic substitution and symmetry have been applied to
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reduce the ion packing and coulombic attraction thus decreasing the melting points [25].
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ACCEPTED MANUSCRIPT Cations
Anions Amide Methanide Halide
Phosphate, Sulphonate, Borate, Acetate
5-membered 6-membered
Inorganic
Imidazolium
phosphonium Hexafluorophosphate +
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Tetrafluoroborate
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Sulphonium
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Imidazolium Functionalitie s
Cholinium
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IIodide
Tosylate BrBromide
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N-alkylIsoquinolinium
Dicyanamide
Dialkylphosphate
Acetate
ClChloride
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Thiazolium
Bis (trifluromethanesul fonyl) amide
Alkylsulphate
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Pyrazinium
Triazolium
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Pyridinium
Pyridazinium
Oxazolium
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NH4 ammonium Pyrolidinium
Trifluromethanesulfonate
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Pyrimidinium
Benzotriazolium Fig.1 (b) Characteristic cations and anions in RTILs. Classification of RTILs is based on their diverse chemical structures. The inherited properties of RTILs to behave more like surfactants, crystals, ionic and molecular liquids further add to the classification respectively [28]. Classification of RTILs is more challenging by suggesting the presence of cationic, anionic or induced functionalities. Before extending the classification, RTILs can be divided into two main types,based on proton donating ( protic) and 12
ACCEPTED MANUSCRIPT non-donating (aprotic) catagories, where uni and dicationic functionalities decide the nature of RTILs [29]. The newly investigated classification of RTILs based on various properties is shown in Figure 2. 2.1
Protic and Aprotic RTILs
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The most easier and inexpensive RTILs with a high degree of purity are produced by a
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simple transfer of proton between equimolar bronsted acid and base pairs are called protic
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RTILS [30]. Thus protic RTILs are regarded as pure mixture consist of ions with excellent ionic behavior in comparison to common salts.[31]. In protic RTILs, the specific sites for hydrogen
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bonding is created as a result for proton transfer, which show characteristic hydrogen bonding as
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compare to organic solvents. Thus, due to extreme hydrogen bonding capabilities such RTILs exhibit extreme conductivity, stability, catalytic and thermal efficacy.
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Unlike protic RTILs, Aprotic RTILs lack the specified feature but exist in a wide range of cation
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and anion groups with or without hydrogen bonding capabilities. Further, aprotic RTILs are synthesized with multistep synthesis due to activation and then formation of covalent bonds
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between the specified ions [32]. Aprotic RTILs are categorized with better electrochemical and
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thermal features as compared to protic RTILs due to stronger covalent bonding as shown in Waldon plots with excellent ionic performance [33]. Chiral and Achiral RTILs
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2.2
Molecular chirality is one of the interest in classification of RTILs for potential application in organic, inorganic and physical chemistry. In RTILs, chirality is generally applied to the constituent cationic moieties. Thus, chiral molecule is defined with a non-superimposable mirror image. The two mirror images are called as optical isomers or enantiomers. In 1999, Sudden et al., synthesized first chiral ionic liquid 1-butyl-3- methylimidazolium ([BMIM])
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ACCEPTED MANUSCRIPT lactate III by anion exchange between [BMIM][Cl] and sodium (S)-2- hydroxypropionate. In 2005,the preparation of oxazolinium salts from (S)-valine methyl ester and propionic acid have been reported as chiral ionic liquids [16]. Similarly, a molecule with a superimposable mirror image is called as achiral molecule. RTILs characterized with achiral cationic molecules do
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have a plane of symmetry or a center of symmetry. Various achiral ester based ionic liquid,
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achiral amide and imidazolium bromide RTILs have been synthesized and investigated for
Magnetic RTILs
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2.3
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specific applications.
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Such type of RTILs exhibit magnetic properties without any added magnetic particles. These properties are induced by the selective cations and anions. Normally magnetic RTILs
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consist of Transition metal and Lanthanide complex anionic moieties. They exhibit characteristic
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physicochemical properties as multi-responsive materials. For the first time Hayashi et al., reported the first magnatic RTIL as [C4mim][FeCl4] and to date the anion has been
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functionalized by paramagnetic group and further investigated to increase the catalytic and
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solvent effect by replacing FeCl3 in Grignard and Friedal craft reactions [34]. Branco et al., reported the synthesis of novel Magnetic Room Temperature Ionic Liquids based on the
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combination of 1-ethyl-3-imidazolium cation and iron(III) and chromium(III) EDTA complexe anions [35]. The novel electrochromic and magnetic ionic liquids have been prepared by a simple combination of cobalt (III), chromium (III) and iron (III) ethylenediamine tetraacetic complexes as anions and the cations 1-ethyl-3-methylimidazolium [EMIM], 1-butyl-3methylimidazolium [BMIM], 1-octyl-3- methylimidazolium [OMIM], tri-octylmethylammonium [ALIQUAT] and trihexyltetradecylphosphonium respectively. Such novel magnetic RTILs based
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ACCEPTED MANUSCRIPT on chromium(III) and iron(III) EDTA complexes have been reported as alternative, robust and efficient MRI contrast agents for many diagnostic applications. 2.4
Polymeric RTILs A specified class of RTILs has been identified as polymerizable ionic liquid where the
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constituent anion has the ability to self polymerize. Such type of cationic or anionic
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polymerizable moieties are able to formulate homopolymers and block copolymer analogues
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[36]. These polymeric RTILs have the characteristic properties of both ionic liquid as well as typical polymers and are explored as well defined polyelectrolytes. Such type of ionic liquids are
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potential candidate to be used in polymer self assembling, polymer dispersion and reinforcement
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[37-39]. The standard techniques including atom-transfer, free-radical polymerization are reported for synthesis of polymeric RTILs with inclusive literature [40, 41]. Highly organized
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polymeric RTILs nanoparticles have also been reported by Yuan et al., [42]. For a better
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nanostructure and physicochemical dependency, polymeric RTILs can be adjusted by simple ion exchanges. Also the stimuli dependent behavior are studied by induced sensing moieties in the
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desired system for specified applications [43]. Up to date a few report to state the art of
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dynamics, structure and computational analysis for polymer blends in polymeric RTILs have been addressed and many applications are still coming to design nano ane meso-structural
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materials for demanding technologies [44].
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Fig.2 Classification of RTILs on the basis of structural and induced chemical moieties.
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2.5 Chelating RTILs
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Angell et al., suggested a new type of RTILs containing concentrated salt and respective solvent as solvated or chelating ionic liquid [45]. The point of concern is that ionic liquid are
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non-cordinating but their constituent ions consist of ligands make them as coordinating for a variety of applications so that electctron transfer between anion and cation can take plae. Some of coordinating complexes based on mercury and Ferroceium cations are reported as chelating ionic liquids. Still this field has to grow with new ideas and are expected to replace the vast variety of available ligands with upgraded potentials [46, 47]. In 1999 Rees et al., reported the first chelating RTIL consist of Barium bisalt-oxide and reported the structure resembling crown ether complexes [48]. Such scheme was extended by using acetylacetones, glyme, 16
ACCEPTED MANUSCRIPT oligoethercarboxylate (TOTO) species, for coordination with small and hard cations. Their cations were of special interest as strongly Lewis- acid chelating moieties with flexible characteistics for multi-directional chelating features [49, 50]. Several reports confirm the chelating abilities of such ionic liquids by investigating Ag+, Li+, Na+ and K+ cations with
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concentration and nature dependent properties [51]. In order to investigate the radius size,
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flexibility and coordinating strength, various modern characterization is still required for better
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implementation of chelating RTILs to replace the existing toxic and unfriendly chelating agents with better recognition features. Fluorous RTILs
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2.6
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In chemistry, Fluorinated groups are introduced to replace the existing organic groups in order to change the geometry and structure of the compounds. The same pattern is investigated
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by various researchers to replace the exixting group by inducing Fluorinated moieties to get more
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organized structures for specified applications. Several characterization techniques including XRD and NMR have confirmed the analytically organized and tricontinious structure of
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Fluorinated RTILs due to highly polar Fluorinated sphere in comparison to apolar hydrocarbon
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domains [52, 53]. As reported, the Fluorocarbon and hydrocarbon are slightly miscible with each other, so their segregation also hinder their performance. Thus, such ionic liquid can find
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potential application but still no sufficient data is available to state the properties that facilitate the Fluorinated RTILs with better structural and geometrical performance. The reports on Fluorinated RTILs are not sufficient but their electronegative and stronger solvophobic properties are the interesting features for future technology applications [52].
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ACCEPTED MANUSCRIPT 2.7
Dicationic RTILs Dicationic RTILs which resemble Gemini and surfactants are characterized with well
organized structures, dynamics and solvation properties [54-56]. For the first time Davis et al., synthesized dicationic RTIL with both aprotic and protic centres. Dicationic RTILs synthesis is a
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tough job due to transfer of two electrons which is difficult to achieve or may be explosive but
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not impossible. Further many efforts reported the synthesis of various magnetic and
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polymerizable dicationic RTILs for high-tech targeted applications [57, 58]. In dicationic RTILs, it has been reported that shorter alkyl or spacer groups donot effect the bulk properties which
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refer to advanced technology applications where surface dependent properties are highly desired
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[54, 59]. Still there is very little literature available to state art of dicationic RTILs synthesis, reactivity, dynamics and physicochemical research. Similarly dianionic RTILs also will be a
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topic of interest in future as there is no report available about dianionic RTILs regarding
Applications of RTILs
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3.0
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synthesis characterization and applications.
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Industries, environment, green technology and academic research are the most attractive concerning RTILs in term of forthcoming applications. The author explores the most significant
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achievements in the field of RTILs as a solvent and or combination of identified cations and anions for a desired approach where simple chemical specie failed to response. RTILs nowadays are specialized for biomass dissolution to prepare nano-cellulose based polymer composite, electrochemical-sensors, bio-sensors, gas-sensors, ion selective sensors and newly voltametric strategies. RTILs in terms of its catalytic ability and formulation of bio-electrochemical sensors fascinated the scientist to concentrate. RTILs, due to their immiscible and non-volatile nature,
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ACCEPTED MANUSCRIPT are insoluble in supercritical and other organic solvents, so they can act as a catalyst in phase transfer catalysis for separation technology like gas capturing (sulphur-dioxide, carbon-dioxide and hydrogen). The storing, transferring and capturing of solar energy to electrical energy, biomass to useful and cleaner products, nuclear technology generation, implementation of green
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chemistry approaches and alternative energy resources are some of the best applications related
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to RTILs [60]. Figure 3, presents the application of RTILs, which seem to be quite suitable to
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replace the current synthetic organic solvents in different industries with fruitful results for a
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variety of targeted applications.
Fig. 3. Hi- tech targeted applications of RTILs as Designer Solvents.
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ACCEPTED MANUSCRIPT 3.1 Water splitting One of the great challenges of future technologies is the sustainability of energy production and storage. Thus, strong materials are required which have a solid impact on the processing of energy technologies. In this regard, RTILs are presented with strong electrostatic
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forces among the constituent ions and characteristic tunable properties (low-volatility, low
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flammability, higher thermal, ionic and electrochemical stability) as innovative materials for
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efficient processing and optimization of energy technologies. RTILs are investigated with unique features for electrochemical energy generation through water splitting also known as generation
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of hydrogen (as a fuel) from water. In water splitting technology, RTILs have been applied in
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two ways as to produce electro-catalyst and in the form of hydrated RTILs (solvent/ RTIL) mixture. Previously, simultaneous oxidation and reduction of water has been carried out by
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applying 450 mV potential with a significant loss of energy as compared to obtainable energy
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from generated hydrogen [61]. So electro-catalysis of water needs effective materials to carry out the energy evolution reaction (EER) at ambient conditions as photosynthesis convey EER
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through oxo-Manganese clusters as catalytic center. Accordingly, many metal oxides were
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investigated to produce higher current at lower potential and to direct EER at ambient conditions. RTILs have the potential to act as an electrolyte and also as a solvent where the water molecules
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act as solute during water splitting. Accordingly, RTIL ireases the free energy of water molecules by breaking the hydrogen bonding in water up to some extent where a little energy will be required for water splitting. This process with dissolution of water or hydrogen breakage is determined as endothermic process. Few related RTILs have verified the process of water splitting at 1.25 mV at 150 oC. Protic RTILs have also shown significant contribution in water splitting as rorted by Izgorodintl., [62]. Accordingly Manganese oxide was used for water
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ACCEPTED MANUSCRIPT oxidation in protic-RTIL and buffer-water mixture to produce hydrogen peroxide from water at low potential (150-250 mV) as shown in Figure 4 (A) [63]. The investigations revealed the importance of RTILs as electrocatalyst for water splitting and to explore further to optimize the structure and reaction conditions of ionic liquid. A lot of research work is required to explore
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most efficient RTILs for easy water splitting as it will also facilitate the reversible metal- air
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batteries involving water oxidation principles which is still a major problem in incompetence of
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these batteries.
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Fig. 4 (A) A representative flow reactor using RTIL for water splitting process
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3.2 Innovative polymer electrolytes (IPEs) Presently, RTILs are considered as strategic materials for electrochemical challenges [64]. RTILs are characterized with inherent properties to enhance the stability, performance, reusability, speed, security and thermal stability of hi-tech electrochemical devices suc as batteries, solar cells, fuel cells, actuators and transistors, if proper encapsulation is maintained due to leakage of RTILs [64]. In order to remove this discripency of RTIL-leaking, new strategies are in progress to develop pseudo or solid electrolytes such as polymeric electrolytes 21
ACCEPTED MANUSCRIPT which have the superior advantages of improved stability, mechanical sterength, safety, carefully processing and easy administration [65]. Accordingly, there are two classes of polymer electrolytes as solid polymer electrolytes (SPEs) and gel polymer electrlytes (GPEs). SPEs consist of polymer and salt (polyethylene and Lithium salt), where the conductivity depends
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upon both cation and anion moieties. Similarly, GPEs consist of polymer, salt and additional
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solvent (poly (vinylidene fluoride), Lithium salt and organic solvent) and the enhanced
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conductivity of GPEs is attributed to the presence of molecular organic solvent which facilitate the transportation of ions [66]. Further, typical SPEs are recently replaced by poly(ionic liquids)
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because there is no need to add any Lithium salt and the pendant groups in poly(ionic liquids)
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facilitate the transportation of ions and thus the ionic conductivity. In this regard, several polycation and polyanion [poly(ionic liquids)], have been tested with improved electrochemical
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characterization as compared to conventional SPEs. Additionally, in GPEs the solvent has been
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replaced by using suitable ionic liquids which attributed several advantages to GPEs like high thermal stability, durability, non-volatile and non-flammibility and GPEs are completely
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replaced by ion gels [67]. Ion gels has advantages over GPEs, for instance, it can be prepared by
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using different polymer (uncharged polymers or siloxane) matrices in the presence of ionic liquid. Consequently, ionic liquids form the basis of modern SPEs and GPEs as Innovative
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polymer electrolytes (IPEs) for electrochemical applications. Therefore, ion gels ar a new type of gels comparable to hydrogels, where the liquid phase is replaced by ionic liquid, and this solvent imparts characteristic features as enhanced electrochemical-stability, superior-conductivity, high thermal-stability, re-usability and operational temperature up to 400 oC. There are several methods reported to produce ion gels as solvent casting, swelling of the polymer in ionic liquid and in-situ copolymerization. In solvent casting method, the ionic liquid is dissolved and the
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ACCEPTED MANUSCRIPT solvent is evaporated after producing or casting films, but the problem usually exist is synthesis of sticky films due to high concentration of ionic liquid. Here, in-situ copolymerization method has the advantage to fabricate ion gels by using a diffusional crosslinker to produce homogenous and three dimensional crosslinked networks. In-situ copolymerization has another advantage
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over solvent casting technique, where the ionic liquid itself act as reaction media with 99%
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product purity. Swelling method is also reported with excellent properties of synthesized ion gels
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but the effect of swelling ratio impartially effect the properties of ionic liquid which is still a challenging task. Recently, Shaplov et.al., reported a new concept of synthesizing ion gels by
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radical copolymerization of ionic monomer, namely N-methylpyrrolidinium bis(fluorosulfonyl)
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imide) with poly (ethylene glycol) (di) methacrylates in the presence of the dissolved nitrile butadiene rubber, ionic liquid and lithium salt, [65]. The suggested approach allows for the
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concurrent collaborative of high ionic conductivity (1.3 X 10-4 S/cm at 25 oC) and excellent
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mechanical properties (tensile strength up to 80 kPa, elongation up 60%) to a single polymer material. Such ionic semi-IPNs exhibited wide electrochemical stability window (4.9 V) and
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acceptable time-stable interfacial properties in contact with metallic lithium. Preliminary battery
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tests have shown that Li/LiFePO4 solid-state cells are capable to deliver a 77 mA h/g average specific capacity at 40 oC during 75 charge/discharge cycles as shown in Figure 4 (B). The
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detailed investigations reveal that ion gels are very up-and-coming nowadays and they are being adapted to a great number of approaches such as lithium batteries, supercapacitors, dyesensitized solar cells, electrochromic devices or fuel cells, new types of batteries (metal/air, Li/ S, Na) and new devices in emerging technologies such as optoelectronics, field effect transistors, artificial muscles and bioelectronics.
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Wood Industry
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3.3
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Fig. 4 (B) (a) ions gel based polymer electrolyte membranes for batteries and fuel cell applications (b) Cyclic presentation of the battery with a charge rate C/50 [68].
Cellulose, hemicellulose, lignin and other functional materials are primarily obtained
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from wood with an estimated percentage of 40-55% cellulose, 30% hemicelluloses and 6-35%
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lignin [69]. All these functional materials are reported with distinct physical, chemical properties and structures. The most abundant and valuable biopolymer is called cellulose which exhibits
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many inter and intramolecular H-bonding which are regarded as the key parameters for its dissolution in any medium [70]. Our interest is to explore wood processing especially for cellulose dissolution by RTILs to meet the requirements of green technology (in terms of nontoxic chemicals, low waste, less energy use and reusability of RTILs). Therefore, for cellulose processing, the criteria for RTILs should be summarised as having low melting point, being stable, nontoxic, odourless, high recovery and low-cost production of highly functional fibres so that they can be used as a good source for the development of cellulose reinforced 24
ACCEPTED MANUSCRIPT polymer composites, electrochemical and energy tools. The patents need to address the structural properties, mechanism, finest conditions and product quality along with their thermodynamics and electrokinetic. Table 5, provides a detailed summary of RTILs for cellulose processing at optimum conditions which need much improvement in terms of reaction conditions and
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Table 5 Cellulose Dissolution Using Different Ionic Liquids
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efficiency [71].
Raw material
Dissolution
Condition
Ref.
[Mmim][(MeO)2PO2]
Avicel
10wt%
100 ͦ C, 1h
[72]
[Cyanomim][Br]
Cellulose
3.4 wt%
[Emim][F]
Avicel
[Emim][Cl]
Cellulose
[Emim][Br]
Avicel
[Emim][OAc]
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Ionic liquid
80-90 ͦ C, 20min [73] 100 ͦ C, 1h
[74]
15.8 wt%
85 ͦ C, 1h
[75]
1-2 wt%
100 ͦ C
[76]
Cellulose
85 ͦ C
[77]
MCC
10 wt%
65 ͦ C, 30min
[78]
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2 wt%
13.5 wt%
[Emim][(EtO)2PO2]
Avicel
12-14 wt%
100 ͦ C, 1h
[79]
[Emim][(EtOSO3]
Cellulose
0.5 wt%
110 ͦ C
[70]
[Emim][BF4]
Avicel
insoluble
90 ͦ C
[80]
[Emim][Tos]
Avicel
1 wt%
100 ͦ C, 1h
[81]
[Emim][Ntf2]
Avicel
Very low
110 ͦ C
[82]
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[Emim][(MeO)2PO2]
The table reveals the effect of various cations ( Imidazolium, Pyrrolidinium, Pyrrolidinium, choline) and anions (Amides, Thiocyanate, Sulphates, Phosphates, Sulfonates, Halogens and Formats) on cellulose processing, reaction conditions and efficiency for enhanced
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ACCEPTED MANUSCRIPT cellulose recovery. The effect of size is also crucial in cellulose processing as it has been reported that smaller cations result in better recovery of cellulose [83]. Accordingly, 1 allyl-3methylimidazolium cation, [amim]+, shows more capability to dissolve cellulose as compared to 1-butyl- 3-methylimidazolium [bmim]+ due to its reduced size [76]. According to Zhao et al, [76]
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the ability of H-bonding with cellulose decreases as the size of the cations increase. Similarly the
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cellulose dissolution was also reported to decrease due to the presence of hydroxyl ending-
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groups in RTILs because OH- groups are regarded to readily cooperate with acetate or Cl- anions as compared to H-bonding in cellulose. The dissolution conditions like viscosity, temperature,
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and conductivity are vital for cellulose dissolution in RTILs. Haisong et al., [84] reported the
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effects of a higher temperature on the viscosity and conductivity with the dissolution being more affected at higher temperatures as compared to lower temperatures.
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The precipitation of cellulose by using a proper anti-solvent like water, methanol,
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ethanol, and chloroform is also explored. The low viscosities of RTILs were addressed to facilitate the cellulose dissolution at low temperatures to decrease the thermal degradation of
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cellulose [83]. For an extended period, viscosities of RTILs went on increasing and the rate of
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reaction went on decreasing due to the presence of crystalline residual cellulose fibrils. Furthermore, during dissolution, the viscosity of the solution increased. The precipitation of
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dissolved cellulose and the idea of anti-solvent is that upon adding water to the system, ionic ions attracted by the water molecules and thus shielded by water. The result was that the direct interaction of cellulose with RTIL ions was stopped. Another challenge regarding cellulose dissolution by RTILs refers to decreased crystallinity where it was reported by Gupta et al., that a decrease of 30-75 % in crystallinity was observed when using normal ionic liquids [85]. To overcome this problem, the development of RTILs to retain the crystallinity of cellulose is
26
ACCEPTED MANUSCRIPT another challenging task for researchers. Another important parameter was the selection of cations and anions for the development of enzyme-friendly RTILs. Such a system will positively respond to cellulose in term of enzymatic hydrolysis. Figure 4 (C), presents cellulose processing
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for a variety of cellulose reinforced composites and energy processing applications [86].
Hydrosilylation
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Fig. 4 (C) Dissolution of cellulose using RTILs for nanocellulose reinforced polymer composites and energy concerns [71].
Organosilicon compounds are synthesised on an industrial scale using a hydrosilylation reaction where Si-H bonds are added across unsaturated bonds as shown in Figure 5 (a).
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Fig. 5 (a) Hydrosilylation Reaction (b) Hydrosilylation Process in the presence of RTILs
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Hydrosilylation was applied for the first time by Degussa et al., [87]. Polyethersiloxanes used as
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a surface-active material was prepared by this reaction where such ionic liquids were applied as a source of catalyst heterogenisation. In this process the catalyst (H2[PtCl6] (ionic) and [(m-
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Cl)2{PtCl-(cyclohexene)}paer7
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pa] were allowed to dissolve in 1-butyl-4-methylpyridinium tetrafluoroborate, 1-butyl-3methylpyridinium chloride, and 1,2,3-trimethylimidazolium methylsulfate to prevent leaching
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and for product insolubility as shown in Figure 5 (b) [88]. 1,3-dialkylimidazolium failed to give the chosen polyethersiloxanes, as the 2-H position of the cation was too reactive while 1,4dialkylimidazolium gave a better product with a high conversion in a very short time. The factors affecting the reaction were the hydrophilicity/hydrophobicity of the substrates, the byproducts, the combination ratio of catalysts to ionic liquids and most importantly, the nature of the catalyst used [89]. The molecular catalyst, [(m-Cl)2{PtCl(cyclohexene)}2], leached and the leaching strength was increased by the increase of the catalyst concentration. In contrast, in 28
ACCEPTED MANUSCRIPT H2[PtCl6] as an ionic catalyst, the observed leaching was negligible (below 1 ppm), and also independent of catalyst concentration. The hydrophobicity increased with the increase of the alkyl chain length and decreased with silane functionalization. Using highly hydrophobic polyethers, the RTILs phase separation occurred most simply in the organosilicon products due
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3.5 Energy Production
RTILs leads in the energy sector for capturing, transforming and exchanging solar energy
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into chemical and electrical energy. Furthermore RTILs form the basis to convert biomass or vestige resources into domestic fuels, to develop newly nuclear skills and to progress functional
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material. These materials find key applications in biochemical and industrial processing [90].
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3.5.1 Solar Photo-Conversion
RTILs with inherited properties including low melting points, conductivity, thermal
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stability and low volatility have been reported as electrolytes for dye-sensitized solar cells
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(DSSCs) [91]. In such a system, organometallic or organic dye was allowed to adsorb on a semiconductor-like TiO2, and applied to an electrode. Accordingly different groups were excited
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by photons in the respective dye, which then gave an electron to TiO2. Similarly charge carrier
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normally iodide ion reduced the oxidized dye within the electrolytic system. At the same time, electrons and the oxidized species tended to associate at the counter electrode, resulting in circuit completion [92].
3.5.2 Solar Thermal Conversion Solar thermal energy aims to use solar energy in terms of thermal energy stored in heat transfer fluids contained in special vessels. Through this process, heat was conducted in a systematic way and converted into electricity by thermoelectric conversions. Such fluid was 29
ACCEPTED MANUSCRIPT stored in tightly insulated vessels to produce electricity when needed [93]. The aspect of such heated fluids must bear a wide range of temperatures ranging from 25- 250 oC. Fortunately, several ILs were stable in this range and formed the basis for suitable applications. Furthermore, RTILs were addressed to facilitate the electrical installation on a large-scale in order to provide
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heating and cooling facilities for in and outdoor activities in Mediterranean states [94]. Two
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altered designs, with extremely technological solutions have developed and evaluated in order to
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maximize the energy caught from the sun (by linear parabolic solar collectors) as shown in Figure 6 (a). Both solutions were primarily aimed to produce heat that will be used in advanced
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two-stage ammonia chillers in order to produce refrigeration (cold water≈ 5°C) for air
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conditioning and hot water (50 °C) for sanitary requirements. 3.5.3 Biofuel Production
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Biofuels, especially biodiesel, represent a potentially supportable transportation energy [95].
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The energy available from such biomass lies in their lignocellulosic structure. Such carbohydrate polymers are known for their firm H-bonding in cellulose and hemicellulose derivatives along
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with lignin to resist many biochemical and environmental outbreaks. In order to convert them
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directly for the purpose of energy production, a very specialised type of chemical treatment was necessary for a desired technology, which was only achieved by the use of RTILs [95]. RTILs
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with specified anions were inherited with strong H-bonding acceptance features, such as chlorides and carboxylates, which could dissolve cellulose along with lignin by disturbing their linkages of intramolecular H-bonding. The softened cellulose could be systematically recovered from the solution by the addition of anti-solvents such as water [96]. Furthermore, enzymes easily attacked the disordered assembly of cellulose to convert them into fermentable sugars. In addition to cellulose, there were other natural biopolymers such as wool, wood keratin and silk to
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ACCEPTED MANUSCRIPT process hybrid biopolymer composites with engineering materials. Another efficient fuel known as biodiesel, are nowadays a focal point on behalf of easy processing and good compatibility with the existing setup. Applications of RTILs have also been reported in conversion of triglycerides to biodiesel and glycerol (fatty acid methyl esters) through standardized catalysis
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[97].
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3.6 Paint Additives
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RTILs present a new class of additives in paint technology for improved and systematic drying, finishing and transparency [98]. The colour durability, stability and resistance to rub
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produced by using RTILs as secondary dispersing agents for durable painting, coating and dye
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technology seem to mean that RTILs are likely to replace the existing volatile organic substances in paints and glazes in near future of paint technology [99]. Figure 6 (b), present a comparison
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of painted surfaces with and without using ionic liquids.
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Fig. 6. (a) Solar cooling system, (b) Painted surfaces with and without added ionic liquids.
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3.7 Catalysis
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RTILs are known for homogenous catalysis due to the combined features of their anions and cations. Parshall, for the first time in 1972, described the Pt based hydroformylation of ethane in
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tetraethyl ammonium trichlorostannate with a melting point of 78 °C [18]. Later on, a number of
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groups established RTILs as a novel media for catalytic hydroformylation, olefin dimerization,
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shown in Table 6.
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oligomrisation, hydrogenation and polymerisation. A detailed summary of such reactions is
Table 6 Survey of RTILs based catalytic hydrogenation, C-C, C=O and coupling reactions Reaction Description
Ionic Liquid Scale Hydrogenation Stereo selective hydrogenation [bmim]Cl/ 30 g of aromatic compounds AlCl3 Hydrogenation of 1-pentene [bmim]BrF, PF6, SbF6 with isomerisation to 2-pentene using rhodium catalyst 32
4ml
Advantages Excellent yields and selectivities, efficient under mild conditions Liquid–liquid biphasic protocol realised
Ref. [100]
[101]
ACCEPTED MANUSCRIPT Catalytic cracking of polyethylene Catalytic cracking of high/low [emim]Cl/ AlCl3, 10 g Easy separation and density polyethylene to give [Nbutylpyridine]Cl/ reusable ionic liquid light alkanes AlCl3, Dissolution of kerogen and heavy oil. Heterocyclic [bmim]Cl/AlCl3, PF6 1ml No hazardous workup stages like distillation of azide, catalyst redesign, easy catalyst recycling Asymmetric ring opening reaction Ring opening reactions of [bmim]PF6/SbF6/ 1ml No hazardous workup epoxides catalysed by Cr BF4/OTf stages like distillation of azide, catalyst re-design, easy catalyst recycling Friedal craft alkylation and [bmin] PF6 0.5 g acylation Diels-alder reaction Cycloaddition-reaction [emim]Cl/AlCl3 2.0 g Very good reaction rate catalysed by acid catalyst with high exo and endo selectivity and solubility Dimerization, Oligomerization Polymerization Ethylene polymerisation by [emim]Cl/AlCl3 6.8 g Best product with easy Ziegler-Natta catalyst catalyst recycling Alkylation Alkylation of iso-butane with 2- [bmim]Cl/AlCl3 7 ml Good alkylate with no butene formation of red oil Heck reaction of aryl halides Tetraalkyl ammonium 1.5 g Easy catalyst recycling with butyl acrylate and Phosphonium halides Suzuki cross coupling of aryl [bmim]BF4 0.3 g halide with aryl boronic acid Carbonylation of aryl halides [bmim]BF4/ PF6 2 ml Easy product separation Witting reaction C=C formation [bmim]BF4 2 ml Selective extraction of alkenes
[103]
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Cleavage of compounds
[102]
[104]
[104]
[105]
[106]
[107] [108]
[109] [110] [111]
The table presents a series of RTILs as solvents for homogenous catalysis with unique features like dissolution, stability, reusability for various reactants, precursors and metal complexation. RTILs were addressed to maintain the active sites of catalyst in terms of chemo and regioselectivity [112]. Normally, for heterogeneous catalysis, the catalyst is bound to a specific region with a reduced number of active sites for reaction. Additionally, the separation of catalyst, 33
ACCEPTED MANUSCRIPT unreacted monomers and other by-products is considered a big problem in heterogeneous catalysis. In order to avoid such problems, RTILs gave a platform for both homogenous and heterogeneous catalyses [3]. In light of the above discussion, RTILs present a new class of solvent for catalysis, but the fundamentals of RTILs in terms of kinetics, electrokinetic,
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thermophysical and stability need to be addressed . The author summarises RTILs catalysis in
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three steps.
2. Reactions proceeding with C–O and C–C cleavage.
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1. Reactions proceeding with hydrogenation followed by rearrangement.
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3. Coupling reactions of heteroatom with C and/or C–C. These reactions form the basis for
Air Products
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3.8
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RTILs catalysis and further multistep catalysis are considered future challenges.
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RTILs introduce a new class of air products with exponential performance as compared to competing procedures running for the physical adsorption of different gases on some solid
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surfaces in order to get rid of these hazardous gases [113]. The main problem with these gases is
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their storage and transportation to a safe place at a higher pressure instead of at room atmospheric pressure [114, 115]. It is important to mention here that Complexed Gas
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Technology (CGT) termed RTILs as Guard- I sub atmospheric systems due to their easy handling, adsorption and transportation. RTILs present a green chemistry approach for the storage and transportation of such hazards under the Lewis acid-based concept for a variety of gases. Basic-nature RTILs such as [Cnmim][BF4], are used to store the Lewis acidic gases (such as fluoride and boron) and acidic-nature RTILs (such as [Cnmim][Cu2Cl3] or [Cnmim][Cu2Br3]) are used to store the Lewis basic gases (phosphine, PH3, or arsine, AsH3). In a simple vacuum transfer of highly purified gases, RTILs open a window for electronic industry in terms of 34
ACCEPTED MANUSCRIPT offering a pure and fresh supply of desired gases in special containers as shown in Figure 7 (a) [116]. 3.8.1 Green Cleaners Technologies RTILs as antistatic cleaning agents are used to clean very sensitive and high value surfaces.
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Generally the brushes are moistened by aqueous solutions or sodium chloride solutions as
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wetting agents but the effect of replacing these solutions with RTILs was increased exponentially
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as shown in Figures 7 (b). Beside RTILs find extraordinary applications in biomedicine, coating, implantation, food, biosensors, thermal pumps and the most recently developed green
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applications like DSSCs, phase-changing tools to accumulate universal energy, adsorption/
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desorption and solar cooling [117].
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3.9
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Fig.7 (a) Air Products based on AsH3 and PH3 (CGT) Cylinder Commercial Aids [115]. (b) Cleaning Technology Based on Nozzle Spray Containing NaCl(aq) and IL each after 10 h of operation (c) Purification by Brush bristles coated with IL Film [98]. Liquid Chromatography (LC)
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3.9.1 TLC
Normally silica gel is used for analysis based on HPLC, but silica surface is considered to
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contain silanol groups, so they raise a problem in separation analysis in terms of the broadening
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of the peaks. To avoid this problem, amine based additives are commonly used to block this
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acidified surface peaks broadening [20, 118]. It was reported by Kaliszan et al., that ILs are capable of cutting the effects of silanol on the retention of the drugs during TLC analysis [119].
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Amine additives (ammonia, dimethyloctylamine, triethylamine) failed in suppressing the effects
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of free silanols, but the application of ILs solved the problem of suppressing such effects in a more precise way. ILs applied to such analysis were BF4 containing 1-ethyl-3-methyl, 1-methyl3-hexyl, 1-ethyl-3-methyl, 1-hexyl-3-hepty, 1-ethyl-3-hexylloxymethylimidazolium compounds. Consequently, retention of the analyte and separation of the peptides were highly optimised. It was observed that retention and efficiency was also affected by the presence of different alkyl groups present on the imidazolium cations. ILs were also addressed as additives to distinct nucleotides and other bioactive species. [120].
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ACCEPTED MANUSCRIPT 3.9.2 LC Stationary Phases Joshi et al., examined the imidazolium-based ILs as stationary phases and observed a strong exchange of anions, nucleotides, amines and phase interaction [121]. According to Liu et al., the stationary phases based on silane can be utilized at the maximum extent, thereby achieving quantitative
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and reproducible bonding. Along with sucessive recycling [122]. Another type of stationary phase
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consist of ctylbenzimidazolium-modified silica was prepared by covalent attachment of 1-
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octylbenzimidazole to γ-chloropropyl silica. The proposed material was suggested as potential mixed-mode stationary phase for separation of polycyclic aromatic hydrocarbons, mono-
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substituted derivatives of benzene, anilines, and phenols [123]. Another class of ionic liquid
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based Novel octadecyl stationary phases differ in distribution of imidazolium group have been prepared and comparatively evaluated via linear solvation energy relationships (LSER) model.
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The results were based on the comparative studies of both the stationary phases in term of the
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retention time and difference in selectivity by eluting alkylbenzenes, alkylnaphthalenes, condensed-ring and phenylene polynuclear aromatic hydrocarbons by using LSER [124]. Wang
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et al., merged ionic liquid in the porous polymer monoliths to afford stationary phases with
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enhanced chromatographic performance for small molecules in reversed‐phase HPLC. The formulations were based on the separations of various small molecules including aromatic
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hydrocarbons, isomers where the results suggested that poor resolution and low efficiency of monoliths were significantly enhanced by the addition of ionic liquids into the polymer monoliths [125]. Another strategy based on butylimidazolium bromide surface-confined IL stationary phase was synthesized and investigated for separation of five peptides (Gly-Tyr, ValTyr-Val, leucine-enkephalin, methionine-enkephalin, and angiotensin-II) [126]. An interesting aspect of RTILs for future applications lies in counter-current chromatography (CCC), where
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ACCEPTED MANUSCRIPT they are responsible for both liquid containing stationary and mobile phases. Berthod et al., reported the use of [bmim][PF6] in a ratio of 40:20:40 w/w water–acetonitrile, in order to investigate the distribution constants in different phases [127]. 3.9.3 Micellar Capillary Electrophoresis (MCE)
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Micellar capillary electrophoresis (MCE) is known for its ability to separate both charged
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and neutral analytes. As the name indicates, a suitable surfactant is needed to onset micellization
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for effectiveness and selectivity. For the first time, ILs in micellar electrophoresis were introduced in order to separate both chiral and achiral complexes [128]. Sodium containing
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various surfactants was tested to differentiate various derivatives of aryl, alkyl, phenols and
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ketones. Among various ionic liquids, [bmim][BF4] seemed to work very efficiently in a variety of ways. It was also noticed that the concentration of ILs affected peak determination and
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resolution. It was established that, along with modifiers, RTILs can also be applied as surfactants
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as the micellization performance alkylimidazolium based ionic liquid was examined. Further [bmim]-dodecanesulfonate (BAS) was applied in microprocessor chips and established high-
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electro osmotic flow, also expanding the separation of proteins and nucleotides as shown in
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Figure 8 [129].
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[20, 130]. 3.10
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Fig. 8 Polyphenol Separation Based on (A) [emim]BF4, (B) [bmim] BF4 and (C) [emim] PF6
Biocatalysis
One of the fundamental aspects of RTILs is to maintain the stability and catalytic activity of enzymes suspended in ionic which remain in any other organic polar solvent [131]. RTILs are considered as the favourite solvent for enzymatic catalysis in non-aqueous media. Many biological intermediates including nucleotides, peptides and sugars were investigated for applied 39
ACCEPTED MANUSCRIPT research in biomaterials, pharmaceuticals, chemicals and materials such as hydrogel technology. Biocatalysis, isolation of specific molecules, reuse and separation of RTILs are considered as the main facts for such applications [132]. The lower melting point, lessened vapour pressure, comparable polarity to organics, greater enzymatic stability and reactivity, higher reaction
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kinetics and their reusability reflects the greener approach and controlling pH, dissociation
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shown in Table 7, presents RTILs’ efficiency for biocatalysis .
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Table 7 RTILs Catalysed Enzymatic Reactions Enzymes/ Microorganism
Reactions
Thermolysin α-chymotrypsin
Peptide synthesis Transesterification
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Reduction of ketones 10:1 Biphasic (Ionic Liquid: H2O) Oxidation of guaiacol 25% Ionic Liquid in water Oxidation of 25% Ionic Liquid in water syninggaldazine Lipases Transesterification Biphasic (Ionic Liquid: H2O) Perhydrolysis 25% Ionic Liquid in water Transesterification Biphasic (Ionic Liquid: H2O) Polyester synthesis 25% Ionic Liquid in water
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Formate dehydrogenase Baker’s yeast Peroxidases Laccases C
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B-Galactosidase
PCL
Comments
Proteases 25 % ionic liquid in H2O Biphasic (Ionic Liquid: H2O) Esterease Transesterification Biphasic (Ionic Liquid: H2O) Glycosidase N-Acetyllactosamine Biphasic (Ionic Liquid: H2O) synthesis Oxidoreductases Regeneration of NADH Biphasic (Ionic Liquid: H2O)
BSE
Cal-B
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constant, low are some of the bases for the application of RTILs in biocatalysis. A brief report, as
Ref
[131] [131] [133] [134]
[135] [136] [137]
[138] [131] [139] [140]
3.11 Electrochemical Industry High ionic conductivity, lower volatility and vapour pressure represent the aspects of RTILs that are applicable in electrochemical devices, fuel cells, batteries, solar cells and applied 40
ACCEPTED MANUSCRIPT cells. Electrochemistry deals with RTILs’ interfacial features, extraction, voltammetric analysis and transportation of metallic ammonium ions across membrane [141]. Many efforts have been made to design such an electrolyte system with significant stability and precision to efficiently replace the present liquid organic based electrolytic solutions particularly based on lithium salts
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in RTILs. However their practical availability and advanced chemistry is under discussion as the
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structural interface of RTILs/electrodes is still unknown. It has also been reported that RTILs are
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unstable at reduced or low voltages, e.g., when in contact with highly negative electrodes, such as lithium metal and/or graphite anodes [142]. Serious attention is required to investigate the
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vital reactions happening at the RTIL-solid electrode site. Both experimental (for instance
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impedance spectroscopy) and computational modelling is necessary to explore basic electrochemistry with new techniques and implementations may be appropriate to shed some
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light on the basic electrochemistry of ionic liquids. The above discussions only suggest
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investigating advanced chemistry, as modern electrochemical approaches frequently deal with the application of RTILs in various fields. The reported properties, including very high
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conductivity (4.5V in comparison with 1.2V) in aqueous electrolytes, electro, hydro and thermal
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stability, and in flammability are the main studied features which form the basis for RTILs as supreme electrolytes in electrochemical strategies for batteries, fuel cells, capacitors, actuators,
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photovoltaic, electrochemical and optical sensors. Takaya et al., [143] investigated ammonium salt with a methoxyethyl group in combination with tetrafluoroborate (BF4-) and bis(trifluoromethyl sulfonyl)imide [TFSI; (CF3SO2)2N−] anions. The oxidation-reduction potential, some thermophysical properties like conductivity investigation practically considered them for electrochemical capacitors. Consequently, they held very big potential (6.0 V); highly ionic conductivity (4.8 mS cm−1 at 25 ◦C) made them an ideal candidate for electrochemical
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ACCEPTED MANUSCRIPT sensors. The standard oxidation and reduction potential was investigated and the limiting current density was defined. Such systems showed an improved c stability compared to phenyl based ILs due to their non-electron conjugated system. Therefore, were considered better electrolytes for a high operating voltage electric double layer capacitor (EDLC). A typical voltammogram for ILs
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is shown in Figure 9, with respective parameters shown in Table 8.
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Fig. 9 Cyclic Voltammogram of the ILs Based on DEME Cation and EMI-BF4 at 25 ◦C [143].
Table 8 Physical Properties of the Investigated RTILs [143] Ionic Liquid
Density at 20 Molar. Conc ◦ C / (g cm−3) (mol dm−3)
DEME-BF4 DEME-TFSI EMI-BF4
1.18 1.42 1.24
5.02 3.33 6.26 42
Tm (◦C)
Tf (◦C)
Tg (◦C)
Td (◦C)
9 n.d 15
-35 n.d -51
n.d -91 -87
318 383 391
ACCEPTED MANUSCRIPT 3.11.1 Ion Selective Sensors Reference electrodes based on RTILs are implemented currently due to a variety of electrochemical advantages related to RTILs. In order to produce very high sensitivity for ion detection, RTILs are reported to be used with Ag/AgCl microelectrodes in polyvinyl chloride
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membranes [dmim][Cl], producing a solid electrolyte [144]. Kakiuchi et al., [145] also
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Potentiometry by ion selective electrodes is reported to be a very fast, accurate and comparatively cheap method to sense many species at very low detection limits and high
graphite
powder,
N-octylpyridinium
hexafluorophosphate
[OPy][PF6],
and
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selectivity. Kachoosangi and Faridbod., [146, 147] reported potentiometric ion sensors
[bmim][PF6]. It was concluded that such a combination responded to various ions due to the
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partition of ions between the electrode membrane and the aqueous phase due to the lack of
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ionophores in the assembly of electrodes. Furthermore, Solid-state ion selective electrodes are under investigation to encourage polymerisation for a variety of monomers. One such reported
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polymer, known as PEDOT, a conducting polymer, was electro-polymerised with the help of
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RTILs containing large organic anions. The experiment resulted in an improved potentiometric reaction in 10−5 to 10−1M KCl(aq) [148]. Therefore, RTILs, due to their ionic nature and highly
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plasticising ability, attracted the researchers to formulate different membranes for ion selective electrodes. Coll et al., also reported on the above mentioned membranes for specific ion selection in the electrodes by using a hydrophobic ionic liquid [bmim][PF6]. Furthermore, [bdmim][Tf2N] was used in order to induce higher ionic selectivity by plasticising the PVC and PMMA membranes. This resulted in a stable and highly selective response to hydrophobic anions and cations and surfactant composition analysis was achieved for such electrodes [147].
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ACCEPTED MANUSCRIPT 3.11.2 Voltammetric Sensors Voltammetry is one of the electro analytical methods where RTILs are used for a high degree of detection and selectivity. [bmim][PF6], [bmim][Tf2N] and [bmim][BF4] were reported to detect a very low level of chlorides (ppb) in comparison to typical square wave, linear sweep
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and cathodic stripping voltammetry [149, 150]. Wang et al., reported the imidazolium-based
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polyelectrolyte system to analyse the flow-injection in different electrochemical analysers by
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using the voltammetric approach [151]. All the electrodes (working, reference and auxiliary) were employed side-by-side within the flow passage and shielded with a thin film of RTILs. It
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has been noted that without disturbance from water-air bubbles, the electrochemical species are
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diffused from the channel via a thin film on the surface of the working electrode with negligible time loss. Kanakobu et al., reported the higher electocatalytic activity of a carbon paste electrode
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by using [omim][PF6] and [OPy][PF6] using the redox analysis, where an increase in the
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electocatalytic activity of the electrode was noted due to the addition of RTIL [152]. 3.11.3 Gas Sensors
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RTILs, characterised by inherent conductivity, negligible vapour pressure and broad ionic
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organisation, do not require any supporting electrolyte solution to develop gas sensors for O2, CO2, and NH3 [153, 154]. Platinum (Pt), glassy carbon, and gold (Au) or) electrodes containing
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RTILs are reported to detect the superoxide radical (O2•−) produced by the in situ reduction of O2 at a very low level (ppb) [155]. Accordingly, O2 sensors built on PVC membranes containing [emim][BF4] showed a high sensitivity, widespread detection range, and outstanding reusability and reproducibility. It has been reported that with the high concentration of CO2 in the sample, cyclic voltammetry gave an enlarged cathodic peak current due to the production of (O2•−) and the decreased peak current with reverse scan of oxidation. This indicates that the generated (O2•−)
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2−
. Another kinetic
investigation based on ([N6, 2, 2, 2][Tf2N] and [emim][Tf2N] addressed an amperometric way to detect CO2 [156]. The reaction between the (O2•−) radical and CO2 was seen to progress in a similar mechanism in both cases, which reflects the gas sensor applications related to RTILs.
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Figure 10 shows a conceptual diagram sketching the sensing of CO2, NH3 and other gases as
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reported by A. Inaba et al., [157].
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Fig. 10 A proposed gas sensor
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3.11.4 Gelation of RTILs and their Use in Biosensors Development of biosensors containing RTILs is a predominantly worthy of consideration. The maximum strength of enzymes in RTILs in comparison to mostly organic solvents facilitated and improved the stability of different enzymes and proteins even at a higher temperature [158]. Dagade et al., investigated the hemin catalysis by initiating through an electron acceptor in ionic liquid solution [159]. It was concluded that H-bonding and electrostatic interactions are responsible for a high kinetic barrier for the unfolding of enzymes and the 45
ACCEPTED MANUSCRIPT catalytic activity of hemin increased by increasing the volume of the ionic liquid. Further, the thermal stability, higher activity and the structure disruption of enzymes immobilised in IL-based matrices were investigated. It was concluded that the choice of the RTILs for specific application matter as some of them inactivate enzymes. Another effort to produce a biosensor based on the
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in-situ functionalization of SCNTs in [bmim][PF6] in order to fabricate a glucose sensor showed The CNT- ILs based
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the capability to covalently bind the glucose oxidase (GOD) [160].
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biosensors were tested with analytical skill for biological molecules such as Lactic acid (LA), ascorbic acid (AA), dopamine (DA) and (NADH) in order to further increase the sensitivity and
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thus the conductivity of the biosensors [161].
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Nitric oxide (NO) was generated in both normal and abnormal tissues and was able to react very rapidly with O2, haemoglobin enzymes, proteins and other bio-oxidative molecules in
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vitro and also in vivo. It is important from the medical and biochemical points of view.
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Biosensors based on ([hmim][PF6]) showed an efficient sensing of NO by the electrochemical– chemical (EC) oxidation mechanism within the 100nM to 100mM range [162]. Lenzo et al.,
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tested [emim][NTf2]-based ammonia sensors based on the electro-oxidation of hydroquinone
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[163]. Such a sensor was without any absorbed water, thus with no moisture and humidity it may find application in hi-tech analysis where ammonia and related gases can be detected in an
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RTILs are thus the catching material in sensor technology. RTILs-based sensors are considered a hot favorite research field with inherited sensitivity, selectivity, reusability, separation and an extremely low detection limit. RTILs based sensors act as functioned as a nonvolatile electrolyte and do not need a membrane for sample separation. More importantly,
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nucleic acids, peptides and proteins for targeted applications have been described by Benedetto
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et al.,[164-166]. Accordingly, the role of RTILs in pharmaceutical, drug delivery, preservation of
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nucleic acids, molecular- biomedicine and bio-nanotechnology are promising as next generations technologies. The fundamental of RTILs is dependent of their selectivity, sensitivity and bio-
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availability at physiological conditions. Currently, RTILs have been applied for a variety of
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targeted applications, regardless their side effects by producing toxic species, radicals and ions from the decomposition and side reactions of anions like [HSO4]-, [BF4]-, [PF6]- and
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[PF6]- respectively. On the other hand, the variety of complex interactions between the
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biomoleculeas and RTILs are also important but unknown to result in complex behaviors and
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interfaces at physiological conditions.
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3.12.1 Interaction of RTILs with phospholipid bilayer The effect of RTIL on salt concentration [NaCl, MgCl2, KCl] was investigated in hydrated
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environment and the results were established by various experiments and simulations as shown in Figure 11 [167]. The results suggested that the cations of respective RTILs can enter the bilayer system with the ability to interact with the carbonyl and phosphonium anions. Further, tri and divalent cations were observed with higher interactions with the bilayer molecules as compared to monovalent cations, with a steady increase in transition temperature and bending ragidity of the overall system for desired applications. Consequently, the interfacial tension
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phospholipid bilayer by using long chain hydrophobic cation or anions respectively [168, 169].
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Fig. 11 Interaction of RTIL and phospholipid bilayer controlling the effect of salt.
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3.12.2 Interaction of RTILs with nucleic acids (RNA/ DNA)
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The negatively charged character of the nucleic acids including RNA/ DNA play a key
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role to interact with a variety of cations associated with diferent RTILs [170]. Thus, a variety of reactions, stability and different ways of tunning the interactions between nucleic acids and
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RTILs can be originated for high tech- targeted applications like bio-nanobiotechnology [165]. Another striking aspect of RTILs aim to solubilize the RNA/ DNA for separation, processing and
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time and temperature-dependent stability applications [171]. Advantages related to RTIL-nucleic acid interaction are necessary to produce hydrated solutions to extend the lifetime of RNA/ DNA at standard conditions, to preserve the nucleic acid for extended time, to save and reduce the expenditure of medical treatment and finally to reserve and revive the fossils [172]. As RNA is less stable than DNA, so the emerging research is focusing on the solubility and processing of DNA for desired applications as described by Arguably et al., [173]. Accordingly, the results
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ACCEPTED MANUSCRIPT suggeste that DNA is soluble in (20% hydrated) choline based ionic liquids. The exceptional stability of DNA was obderved for six months at room temperature confirmwed by retaining of the UV- visible and Fluorescence spectral peaks. Further, the the stabilization of DNA structure and the absence of denaturation peak up to 900 C was confirmed by Fluorescence spectroscopy.
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Finally, the electrophoric measurements showed that the composition and size of the DNA were
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unchanged for six months kept at room temperature. The study contributed a lot to the
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modification, processing, stabilization and functionalize the DNA for bi- nanotechnology applications to arise.
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3.12.3 Interaction of RTILs with proteins and amino acids
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The better solubility enzymatic-action, and thermo-chemical aspects of proteins in RTILs perhaps exceeds expectations. The interaction of protein (amino acid) and RTILs have been
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focused to explore the phase performance, solubility, biphase- and ternary phase separation, bio-
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products, bio-synthesis and bio-catalysis [174]. Similarly, another study report on the stabilization of amino acid in ammonium based RTILs by investigating the change in free energy
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to transform one mole of amino acid from water to RTIL solution [175]. Furhter, the protein-
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RTIL interaction is presented in term of Hofmeister series by measuring the ability of solute to salt-in and salt-out proteins [176]. The Hofmeister series also contribute to calculate the
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electrophoric mobility, biological and enzymatic activity of different proteins andd amino acids in solution. Consequently, the destabilizing effect of cation was observed with increasing hydrophobic chain length, which favorably interact with hydrophobic amino acid chain groups. The effect of RTIL on the structure of protein was investigated by using solvation dynamics measurements fluorescence correlation spectroscopy respectively [177, 178]. Some of proteins have been reported with inverse enzymatic activity while majority of proteins like lypases have
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ACCEPTED MANUSCRIPT been reported with increased enzymatic activity after interacting with RTILs [179]. Regarding interaction of RTILs with proteins, another report describes the formation of amyloid fibres, i.e. fibrous aggregates of proteins and peptides that might indeed represent the thermodynamically stable form of these systems as shown in Figure 12 [180]. Concerning the vast bio-
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nanotechnology and pharmacological interest, still there is no report to address the mechanism
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and effects of such interactions. So, a significant research is still needed to explore through in
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vivo experiments, computer modeling and simulations establish the fundamental of interactions
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between RTILs and various proteins with different chain lengths at physiological conditions.
Fig. 12 Interaction of RTILs and proteins for nano-biotechnology applications. 5.0 Summary RTILs are proved to be the useful reaction media and catalyst in various processing and synthetic routes. The author aimed to visualize the future, advance chemistry and unseen applications of RTILs. Understanding the internal properties and analytical chemistry of RTILs 50
ACCEPTED MANUSCRIPT prior to relating it is considered the key criteria for RTILs application. The built-in functionalities of the desired RTILs have the potential to expand and modernize industrial setup. RTILs can be synthesized by combining a variety of organic cation and asymmetrical anions in a simple reaction. The processing schemes are confident enough to produce novel RTILs which
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must be applied in dissolution processes, ion-selective sensors, energy generation, catalysis,
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bioseparation, additives and electro-analytical methods. Consequently, the development of new
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synthesis and relevant application is quite certain to allow RTILs in challenging tasks. RTILs have challenged both our experimental and our intellectual abilities. Detailed descriptions of how
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ionic liquids interact with solute species to change their reactivities have begun to emerge, yet
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much remains to be discovered.
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Acknowledgment
The author is highly grateful to the School of Materials and Mineral resources engineering,
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USM, for sponsoring this research under the project Grant FRGS-203/ PBAHAN/ 6071337.
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Conflict of interest
2. 3. 4. 5. 6.
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