Synthesis, thermophysical properties, Hammett acidity and COSMO-RS study of camphorsulfonate-based Brönsted acidic ionic liquids

Accepted Manuscript Synthesis, thermophysical properties, Hammett acidity and COSMO-RS study of camphorsulfonate-based Brönsted acidic ionic liquids

Sabahat Sardar, Cecilia Devi Wilfred, Asad Mumtaz, Zeeshan Rashid, Jean-Marc Leveque PII: DOI: Reference:

S0167-7322(18)30607-X doi:10.1016/j.molliq.2018.09.024 MOLLIQ 9630

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

4 February 2018 19 August 2018 5 September 2018

Please cite this article as: Sabahat Sardar, Cecilia Devi Wilfred, Asad Mumtaz, Zeeshan Rashid, Jean-Marc Leveque , Synthesis, thermophysical properties, Hammett acidity and COSMO-RS study of camphorsulfonate-based Brönsted acidic ionic liquids. Molliq (2018), doi:10.1016/j.molliq.2018.09.024

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ACCEPTED MANUSCRIPT Synthesis, thermophysical properties, Hammett acidity and COSMO-RS study of camphorsulfunate-based Brönsted acidic ionic liquids Sabahat Sardar a,b,*, Cecilia Devi Wilfred a,b, Asad Mumtaza, Zeeshan Rashidb,c, Jean-Marc

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Levequed Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS,

Center of Research in Ionic Liquids (CORIL), Universiti Teknologi PETRONAS, 32610

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32610 Seri Iskandar, Perak, Malaysia

Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610 Seri

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Seri Iskandar, Perak, Malaysia

Iskandar, Perak, Malaysia LCME / SCeM, Université de Savoie Mont-Blanc, 73000 Chambéry, France

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Corresponding Author Email Address: [email protected], [email protected] Contact Number: +60103782709 Abstract

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In this work the synthesis of five new Brönsted acidic ionic liquids (BAILs) bearing same anion (camphorsulfunate) but different cations; 3-(3-sulfopropyl)-imidazolium, 1-methyl-3(3-sulfopropyl)-imidazolium, 1-methyl-3-(4-sulfobutyl)-imidazolium, 1-ethyl-3-(3sulfopropyl)-imidazolium, 1-butyl-3-(3-sulfopropyl)-imidazolium) were synthesized and characterized. The characterization of BAILs was carried out using NMR, FTIR and elemental analysis (CHNS). The thermophysical properties of these ILs such as density, refractive index, viscosity and thermal stability were analyzed in wide temperature window. Furthermore, the effect of alkyl chain length of cations on thermophysical properties was well studied. The experimental values of density were further used to calculate other significant properties such as molar volume, standard molar entropy, lattice energy and thermal expansion coefficients. The Brönsted acidities of investigated ILs were determined using Hammett method. Furthermore, COSMO-RS study was performed to determine δ-surface, and δ-profile of the studied ILs.

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ACCEPTED MANUSCRIPT Keywords: Acidic ionic liquids; Camphorsulfonate; Imidazolium; Hammett acidity; COSMO-RS. 1. Introduction

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Brönsted acidic ionic liquids (BAILs) as environmentally benign media, have attracted extensive research interest in past years and have been widely applied in organic synthesis as catalyst or dual catalyst-solvent due to their remarkable properties like reusability, low vapour pressure, non-inflammability, acidic nature with high thermal and chemical stability [1-4]. The use of ILs as reaction media hence can offer a convenient solution to both solvent emission and catalyst recycling problems [5, 6]. Organic transformations such as esterification reactions [7], cleavage of ethers [8], heterocyclic syntheses [9] and polymerization reactions [10] have been performed using BAILs. Name reactions; Heck reaction [11], Knoevenagel reaction [12], Aldol condensation [13], Friedel-Crafts reactions [14], Diels Alder reaction [11], and Mannich reaction [15] have also been carried out using ILs either solvent or/and catalyst [16, 17]. The reactions have been reported to be conducted in acidic ILs with remarkable selectivity and yield [18]. The tunable physicochemical properties of ILs make them task specific for numerous applications as their physical and chemical properties can be easily adjusted by careful choice of cation and anion [19]. Besides being used in organic and industrial chemistry, ILs have also been extensively used in different areas of science such as CO2 capture [20, 21], biopolymer dissolution [22, 23], liquid-liquid interactions [24, 25], membrane separations [26, 27] and in the conversion of sugars to degraded products (5-HMF, furfural, etc.) without using any catalyst [28]

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There are two subclasses of BAILs, one subclass having no functionality on cation with some protic anion (such as HSO4-) while in others functionalities like –SO3H, -CO2H or –H moieties are incorporated into the cation with variable anions [4]. Substituents to conventional homogeneous and heterogeneous acid catalysts, BAILs with –SO3H moiety have revealed potential significance as they are non-corrosive, non-volatile and flexible. Additionally, BAILs with –SO3H functionality are readily soluble in polar solvents like ethanol and acetone but remain notably immiscible with non-polar solvents like alkanes and aromatic hydrocarbons [29-31]. In engineering applications, from product designing to industrial processes, the thermophysical properties of ILs play vital role. In production process such as extraction, adsorption and catalysis, some crucial steps are affected by density and viscosity of concerned IL [32, 33]. With the aim of facilitating the sustainability of ILs, several studies on their physicochemical properties (density, diffusion coefficient, viscosity, melting point, ionic conductivity etc.) have been carried out [34-36]. The aim of the present study is to synthesize BAILs bearing aliphatic cyclic anion with disubstituted imidazolium cation and the influence of temperature on their different thermophysical properties. The five new BAILs were synthesized and the synthetic route was divided into two steps. In first step zwitterions were formed by reacting 1,3-propanesultone and substituted imidazoles. In second step, zwitterions were protonated with camphor-102

ACCEPTED MANUSCRIPT sulfonic acid (CSA) to yield the respective ILs. The spectroscopic techniques (NMR and FTIR) along with elemental analysis (CHNS) were used for the structural confirmation and purity of concerned ILs. The different thermophysical properties such as density, refractive index, viscosity and thermal stability were investigated at specified temperatures. Furthermore Brönsted acidities of the synthesized ILs were also determined using Hammett method. Finally, density functional theory (DFT) calculations were used to determine the

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effect of structural variations of imidazolium cation on the properties of investigated ILs. Experiments on Mannich reaction using these BAILs as catalysts and solvents are currently in progress in our laboratory and will be published elsewhere.

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2. Experimental 2.1. Materials The chemicals utilized for synthesis of ILs were purchased from Sigma-Aldrich and used as received without any purification process. The details of starting precursors are provided in Table S1 of supporting information. 2.2. Synthesis of BAILs

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The synthesis of BAILs has been conducted by using reported methods with some modifications [37-40]. In a typical experiment, 1,3-propane sultone/1,4-butane sultone (0.1 mol) was drawn into two-necked round bottom flask equipped with drying tube and respective imidazole (0.1 mol) was slowly added at ambient temperature. Within 10-30 minutes, the liquid reaction mixture turned to viscous material and further to white solid. The synthesized zwitterions were washed repeatedly with toluene, ethyl acetate and diethylether and dried under vacuum for 24 hours. After drying, the zwitterions (0.1 mol) were further protonated using equimolar concentration of 10-camphorsulfonic acid (0.1 mol) at 85-90 °C for 4-6 hours [41-43]. The synthesized ILs were then washed with ethyl acetate and diethylether and dried under vacuum for 24 h at 70 °C. The synthetic route for 1methylimidazole is shown in Figure 1. n

N

N

O

r.t. N

N

n

S

O

SO3-

85-90 oC

N

CSA

O

N

n

SO3H

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n= 1 and 2 Figure 1: Schematic route for BAIL synthesis 2.3. Characterization 1

H and

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C NMR spectra (Figure S1-10) were recorded in a deuterated solvent (D2O)

using Bruker Avance 500 MHz spectrometer. 10-15 mg of sample was dissolved in 0.8 mL of deuterated solvent to get the NMR spectrum. The chemical shifts were recorded downfield in 3

ACCEPTED MANUSCRIPT parts per million (ppm, δ) from tetramethylsilane (TMS) reference. The multiplicities have been abbreviated as s, singlet; d, doublet; t, triplet; double doublet, dd; and m, multiplet. Carbon, hydrogen, nitrogen and sulphur contents were analyzed using an elemental analyzer (CE Instruments EA-1110). The instrument was initially calibrated using standard calibration sample of known chemical composition before each measurement. The measurement of each IL was made in triplicate and the average values were reported. The synthesized ILs were spectra (Figure S11-15) were recorded from 500-4000 cm-1.

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also characterized using Fourier Transform Infrared Spectrometer (FTIR Schimadzu) and the

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2.4. Measurement of thermophysical properties The water contents of the studied ILs were measured by a coulometric Karl Fischer

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titrator (Model DL 39) using Hydranal Coulomat AG reagent (Riedla-de Haen). All the measurements were performed in triplicate and an average value was reported for each IL.

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Density and viscosity measurements were taken using an Anton Paar viscometer (SVM 3000). The instrument was calibrated using ultrapure Millipore-grade water, for which data

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was established. The required sample volume for the measuring cells was 2.5 mL. During the measurements, the IL was transferred to a syringe and injected into the instrument. The first measurement was taken after the temperature set point was reached. Another measurement

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was taken after the liquid of the vibrating tube was replaced with the one that remained in the

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syringe. The agreement between both values is a measure of the effectiveness of the method. The uncertainties of measurements are u(T) = ± 0.01 K, u(ρ) = ± 5 x 10-6 g cm-3 and u(η) = ± 0.32%.

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The refractive indices of the studied ILs were measured using an ATAGO digital refractometer (RX-5000α). The instrument was calibrated with pure organic solvents

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(methanol and acetonitrile) of known refractive index values and also validated with ILs of known properties. The refractive indices were measured within temperature range of (288.15333.15) K with an accuracy of ± 0.05 K and uncertainties of 3.5 x 10-5. Vacuum dried samples kept in desiccators were directly placed into the measuring cell. Samples were directly introduced in the cell (prism assembly) using a spatula. Triplicate measurements were performed and average values were reported in each case. The thermal decomposition temperatures were measured using a thermogravimetric analyzer (PerkinElmer, pyris V-3.81). IL samples were heated from (323.15 to 823.15) K in a crucible under N2 atmosphere at a heating rate of 10 K/ min with ± 1 K temperature accuracy. Prior to the analysis, the instrument was calibrated to ensure accurate weight and temperature 4

ACCEPTED MANUSCRIPT measurement and the sample was dried in vacuum oven for 24 h. TGA furnace was purged with N2 for at least 10 minutes to eliminate moisture from the system. About 5-10 mg of the IL was placed into an aluminum crucible inside a programmable furnace, hold for one minute at 50 °C with continuous purging of N2 gas. 2.5. Brönsted acidity evaluation of BAILs by Hammett equation The synthesized ILs were dissolved in water and p-nitroaniline was added as an indicator.

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The ILs and indicator were taken at concentration of 3 × 10-2 mol/L and 7.5 × 10-5 mol/L, respectively. The resulting solutions were stirred for overnight. The UV-visible absorption

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study was performed for the determination of Brönsted acidities in terms of Hammett acidity function in the range of 300-800 nm using Cary Series spectrophotometer of Agilent

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Technologies.

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2.6. Quantum Calculation Method

The Density functional theory (DFT) calculations were performed to investigate the effect of

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functional group in imidazolium cation on the molecular properties of the BAILs. All the structures of cations, anion and IL pairs (ion pair) were drawn in Turbomole Version 7.1 software. The geometry optimization was carried out using density functional theory with

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resolution of identity (DFT-RI) level. The triple zeta valence potential (TZVP) was used in

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combination with PB-3LYP basis set to handle the cation-anion complex system [44, 45]. The ideal screening charges on the molecular species were calculated using conductor like screening model for real solvents COSMO-RS (COSMOthermX programme (version C30-

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1601, COSMOlogic GmbH & Co. KG)) [46].

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3. Result and Discussion

IUPAC name of synthesized BAILs

The physical state , chemical structure

3.1. Density measurement for camphorsulfonate-based BAILs The densities (ρ) of pure BAILs were measured in the temperature range of (293.15 to 363.15 K) under atmospheric pressure (0.1 MPa) with 10 K intervals. The temperature dependence of experimental densities is graphically presented in Figure 2 and the respective 5

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density values of studied ILs are illustrated in Table S4. In general, the density values of materials depend on temperature, size and shape of ions, upon how ions are closely packed with each other and cation-anion interaction [47]. For a common anion, density decreases linearly with an increase in temperature and also decreases with increase in alkyl chain length of cation because of large volume of cationic counter ion of IL. The trends of variation of present density values with changing temperature and carbon chain are in good agreement with the literature [48-50] . It was observed that the density increased in order of [imPS][CS] > [1MeimPS][CS] > [1EtimPS][CS] > [1MeimBS][CS] > [1BuimPS][CS]. At 298.15, the values of determined density were 1.303 g.cm-3 for [imPS][CS], 1.264 g.cm-3 for [1MeimPS][CS], 1.256 g.cm-3 for [1EtimPS][CS], 1.2384 g.cm-3 for [1MeimBS][CS], and 1.2297 g.cm-3 for [1BuimPS][CS]. At ambient temperature, the density of [imPS][CS] was found to be 3.08 %, 3.74 %, 5.22 % and 5.96 % higher than [1MeimPS][CS], [1EtimPS][CS], [1MeimBS][CS] and [1BuimPS][CS], respectively. Addition of methyl, ethyl, methylene and butyl group at N-1 position of [1MeimPS][CS], [1EtimPS][CS], [1MeimBS][CS] and [1BuimPS][CS] led to large volume of cationic counter ion which ultimately resulted in lower density of respective ILs in comparison to [imPS][CS]. The densities of ILs were fitted by least-square methods to the following linear equation; 𝜌 (g. c𝑚−3 ) = 𝐴0 + 𝐴1 𝑇 (1)

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where ρ denotes the density of studied ILs (g.cm -3), A0 and A1 are the fitting parameters and T is temperature in kelvin. The fitting parameters along with standard deviation and correlation coefficient for equation 1 are provided in Table S5.

Figure 2: Density (ρ) as a function of the temperature (T) for the studied BAILs; [imPS][CS] (a), [1MeimPS][CS] (b), [1EtimPS][CS] (c), [1MeimBS][CS] (d), and [1BuimPS][CS] (e). Estimation of volumetric properties The experimental densities were further used to calculate other density dependent thermodynamically derived properties such as standard molar volume (Vm), molecular 6

ACCEPTED MANUSCRIPT volume (V), standard entropy (S°), crystal energy (UPOT), and isobaric thermal expansion coefficients (αp) of the studied ILs (Table 1). Standard molar volume Standard molar volume (Vm) is the volume occupied by one mole of a substance at standard temperature and pressure. The Vm values of the studied ILs were calculated at ambient temperature and atmospheric pressure using the below mentioned equation; 𝑉m (cm3 . mol−1 ) = 𝑀 / 𝜌

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(3)

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Where Vm is molar volume in cm3.mol-1, M is the molecular weight in g.mol-1, and ρ is the density in g.cm-3 at 298.15 K. The calculated values of Vm are listed in the Table 1. The Vm values of the studied ILs follow the order of [imPS][CS] < [1MeimPS][CS] < [1EtimPS][CS] < [1MeimBS][CS] < [1BuimPS][CS]. The molecular volumes (V) of the present BAILs were also calculated from molar volume and Avogadro’s constant (NA = 6.02245 x 1023 molecules per mol.) using the following equation; (4)

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V = 𝑉m / 𝑁A

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The calculated molecular volumes of the ILs are also displayed in the Table 1. The molecular volume (V) of ILs increase in the same order as that of Vm and is observed to be [imPS][CS] < [1MeimPS][CS] < [1EtimPS][CS] < [1MeimBS][CS] < [1BuimPS][CS]. Standard entropy

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The values of standard entropy (S°) were calculated from molecular volume using the relation suggested by Glasser [51] as given in the equation 5; 𝑆 ° (J. K −1 mol−1 ) = 1246.5 𝑉(nm3 ) + 29.5

(5)

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where S° is standard entropy in J.K-1mol-1 and V is molecular volume of ILs in nm3. As the equation does not need any structural information so can be used to calculate the standard entropy of both amorphous solids as well as ILs. The values of standard entropy of ILs are presented in Table 1. It can be observed that the standard entropy (S°) follows the same trend as standard molar volume (Vm). Crystal energy

The lattice energy of a salt is associated with potency of interactions between its ions and it predicts their relative stabilities. In accordance with Glasser’s theory, the crystal energy (UPOT) can be estimated using the equation 6; 𝑈POT (kJ. mol−1 ) = 1981.2 (𝜌 / M)1/3 + 103.8

(6)

where ρ and M are the density in g.cm-3 and molecular mass in g.mol-1, respectively. The calculated UPOT values are shown in Table 1. As expected, the investigated ILs have significantly lower crystal energies than that of inorganic fused salts (alkyl halides) [52]. For example, the crystal energy for CsI, referring lowest crystal energy among alkyl halides, is 7

ACCEPTED MANUSCRIPT 613 kJ.mol-1 [53]. The lower lattice energy values for ILs renders them as liquid at ambient temperature [54, 55]. The low UPOT values of ILs in comparison with alkali chloride (e.g., NaCl) are result of asymmetric and bulky nature of the cation, which facilitates steric hindrance causing decreased interactions among cation and anion. The UPOT values of studied ILs decrease in order of [imPS][CS] > [1MeimPS][CS] > [1EtimPS][CS] > [1MeimBS][CS] > [1BuimPS][CS], which was opposite to trend of molar volume. The finding rationalizes that more compact ILs had higher lattice energy value.

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Table 1. Molar volume (Vm), molecular volume (V), standard entropy (S0) and crystal energy (UPOT) of present BAILs at 298.15 K and at atmospheric pressure [imPS] [1MeimPS] [1EtimPS] [1MeimBS] [1BuimPS] [CS] [CS] [CS] [CS] [CS] 3 -1 Vm (cm mol ) 324.17 345.28 358.85 363.83 389.22 3 V (nm ) 0.5383 0.5733 0.5958 0.6042 0.6463 0 -1 -1 S (J.K .mol ) 700.49 744.12 772.16 782.63 835.11 -1 UPOT (kJ.mol ) 392.2 386.2 382.59 381.32 375.15 Isobaric thermal expansion coefficient

1

𝜌

𝜕𝜌

. (𝜕𝑇)

𝑝

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𝛼𝑝 = −

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The relationship between density and temperature can be used to calculate coefficient of thermal expansion (αp). The isobaric thermal expansion coefficients (αp) of the ILs were estimated from the experimental densities using equation (7). = −

𝐴3 𝐴2 +𝐴3 𝑇

(7)

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where αp, ρ, T and subscript p are thermal expansion coefficient, density, absolute temperature and constant pressure, respectively. Furthermore, A2 and A3 are the fitting parameters. The values of αp as a function of temperature are displayed in Table S6. The variations of αp values with the temperature were not significant for the present ILs. Thermal expansion coefficients of studied ILs increase slightly with increasing temperature and show an inverse relation with corresponding densities. The calculated αp values are in the range of (5.15-6.41) × 10-4 K-1 which shows the characteristic ILs behavior [56]. 3.2. Refractive Index measurement for camphorsulfonate-based BAILs The optical properties of prepared ILs were studied by determining their refractive indices (nD) within the temperature range of (288.15 to 333.15) K under atmospheric pressure as summarized in Figure 3. As shown in Table S7, among the studied BAILs, [1EtimPS][CS] has the highest refractive index value while [1MeimPS][CS] has lowest refractive index value. The increasing order of refractive indices of the studied ILs is; [1MeimPS][CS] < [1MeimBS][CS] < [1BuimPS][CS] < [imPS][CS] < [1EtimPS][CS]. The refractive index was found to be linearly decreased with increasing temperature. The investigated BAILs have comparative refractive indices with high-refractive material such as liquid immersion oil (nD = 1.51) and quartz crystal (nD=1.54) [57], which shows that these ILs may be 8

ACCEPTED MANUSCRIPT used as optical materials. No direct relation between the refractive indexes and molar volume of present ILs was observed. Almeida et al. [58] reported that the refractive index slightly depends on the volume of anion. Deetlefs et al. [59] reported that refractive index would be higher when the materials are more tightly packed to each other. Furthermore, refractive index of ILs also depends on the nature of the functional groups attached with imidazolium alkyl chain [60]. The reported data of refractive index values are in good agreement with above statements. The temperature dependence of the nD were further correlated using linear equation (8),

𝑛D = 𝐴4 + 𝐴5 𝑇

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(8)

The standard deviations (SDs) and fitting parameters of refractive indices for the studied

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ILs are provided in Table S8.

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Figure 3: Temperature dependence of the refractive indices (nD) of selected BAILs; [imPS][CS] (a), [1MeimPS][CS] (b), [1EtimPS][CS] (c), [1MeimBS][CS] (d), and [1BuimPS][CS] (e). 3.3.Viscosity measurement for camphorsulfonate-based BAILs The pattern of viscosity (η) fluctuation in ILs can be evaluated from interactions present between the ions, such as van der Waals, hydrogen bonding and columbic forces [61]. An increase in such interactions results in increased viscosity and vice versa. Viscosity also depends on other factors such as charge density, shape and size of ions, interactive forces and conformational changes in alkyl chain [62]. The weak hydrogen bonding interactions between ions of ILs may lead to enhanced ion mobility and consequently viscosity would be low, but only limited ILs are known to have low viscosity [63]. The viscosities of studied BAILs are determined in the temperature range of (293.15 to 363.15) K under atmospheric 9

ACCEPTED MANUSCRIPT pressure as shown in Table S9. The viscosity of the studied ILs markedly decreased with increase in temperature (Figure 4) and this might be due to increased movement of ions at elevated temperature which weakens the interionic interactions among the ions resulting in low viscosity [64]. The viscosities of ILs increased in order [imPS][CS] < [1MeimPS][CS] < [1EtimPS][CS] < [1MeimBS][CS] < [1BuimPS][CS]. It was found that the structures of different cations have a pronounced effect on the viscosities of ILs. The viscosities of BAILs

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increased with increase of cationic chain length that might be due to overlapping of decreased electrostatic interactions over increased dispersive forces along the alkyl chain. Generally, with increase in alkyl chain length the overall strong interaction contribution between counter

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ions decreases and contribution of weaker dispersion forces increases [65].. When alkyl chain

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length increases, the van dar Waals interactions increase among the ions which in turn contribute in resistance to flow resulting in increased viscosity. Experimental viscosities (η)

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were fitted as a function of temperature using the equation (9). ln 𝜂(mPa. s) = 𝐶0 +

𝐶1 𝑇

+

𝑐2 𝑇2

(9)

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where η is the dynamic viscosity, T is the temperature and C0, C1, and C2 are the

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adjustable parameters. The fitting parameters are listed in Table S10.

Figure 4: Viscosity as a function of temperature for the present BAILs; [imPS][CS] (a), [1MeimPS][CS] (b), [1EtimPS][CS] (c), [1MeimBS][CS] (d), and [1BuimPS][CS] (e). Seddon et al. [66] proposed that ILs show non-Arrhenius behavior hence Vogel-FulcherTammann (VFT) equation can be applied to investigate the variation of transport properties 10

ACCEPTED MANUSCRIPT with respect to temperature. The experimental viscosity values of present ILs were correlated using VFT model using following equation: 𝐵𝜂

𝜂 (𝑇) = 𝜂𝑜 . exp[ 𝑇−𝑇 ]

(10)

𝑜

Where ηo is the high temperature limit of viscosity, Bη is fitting coefficient controlling the curvature and To is Vogel temperature that typically lies a few tens of degrees below T g. At

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298.15 K, the derived coefficients of VFT equation and viscosity are presented in Table 2. It was observed that the correlated viscosities were in good agreement with the experimental

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data.

free movement of ions in the liquid. 𝐸𝜂

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𝑙𝑛 𝜂 = 𝑙𝑛𝜂∞ +

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The Arrhenius equation in logarithmic form can be used to calculate the energy required for

(11)

𝑅𝑇

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Where η is dynamic viscosity, η∞ is viscosity at infinity temperature, Eη is activation energy, R is universal gas constant (8.314 J.K-1.mol-1) and T is temperature in Kelvin, respectively.

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The empirical parameters η∞ and Eη are reported in Table 2.

0.0539 ±

1083.94 ±

163.38 ±

0.053

6.1

1.3

0.3129 ±

620.74 ±

200.58 ±

0.485

9.4

3.2

0.0423 ±

1186.72 ±

160.41 ±

0.039

5.3

0.9

778.91 ±

201.63 ±

0.085

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1.6

0.0370 ±

1281.69 ±

161.68 ±

0.102

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2.2

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[imPS][CS]

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Table 2. Fitting parameters of Vogel-Fulcher-Tammann (Eq. (10)) and Arrhenius equations (Eq. (11)) at T=298.15 K ILs ηo / mPa.s Bη / K To / K η∞ / Eη / J.mol-1

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[1MeimPS][CS]

[1EtimPS][CS]

[1MeimBS][CS] 0.1218 ±

[1BuimPS][CS]

mPa.s 36647.61 61.7

± 0.032 35321.5 ±

107.2

0.024 38729.75

36.94

± 0.031 44429.11

5.48

± 0.042 42469.11

15.5

± 0.034

The activation energy (Eη) is the minimum energy required by ions to move across each other in medium. By lowering Eη, the movement of ions becomes more feasible. As seen from 11

ACCEPTED MANUSCRIPT Table 2, the activation energies of studied ILs are close to imidazolium based ILs [67]. From Figure 5 (A) and (B), the measured viscosity (symbols) was fitted with VFT equation (dotted

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lines) and Arrhenius equation (dotted lines), respectively.

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Figure 5. Plot of experimental viscosity (symbols); (A) VFT fitting (dotted lines) and (B) Arrhenius fitting (dotted lines), as a function of temperature for [imPS][CS] (a), [1MeimPS][CS] (b), [1EtimPS][CS] (c), [1MeimBS][CS] (d), and [1BuimPS][CS] (e).

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3.4.Thermal Gravimetric Analysis (TGA) The thermal stability of present ILs was studied by thermogravimetric method (TGA) under nitrogen atmosphere. TGA thermogram is presented in Figure 6. The studied ILs showed high thermal stability and change in slope of TGA curve was found and calculated by instrumental software Pyris V-3.81. As reported earlier, increase in alkyl chain length or introduction of functional groups on the structure of either cation or anion results in decrease in thermal degradation [68, 69], but reverse findings were observed from the present work. Also, thermal stability of ILs depends on the coordinating nature of anionic counterpart [69, 70], i.e. strongly coordinating anions result in decrease of decomposition temperatures of ILs 12

ACCEPTED MANUSCRIPT and poorly coordinating anions increase the thermal stability. So from the present TGA thermogram, it may be depicted that the increase in alkyl chain length of cation might result in poor coordination with camphorsulfonate anion that ultimately increased the thermal stability of respective IL. The thermal degradation temperatures calculated for [imPS][CS], [1MeimPS][CS], [1EtimPS][CS], [1MeimBS][CS] and [1BuimPS][CS], were found to be 510.1, 534.3, 547.8, 561.9, and 557.9 K, respectively. Thus thermal stabilities show the trend

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[imPS][CS] < [1MeimPS][CS] < [1EtimPS][CS] < [1BuimPS][CS] < [1MeimBS][CS].

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Figure 6: Thermogravimetric analysis of the studied ILs between (323.15and 823.15) K at a scan rate of 10 K.min-1; [imPS][CS] (a), [1MeimPS][CS] (b), [1EtimPS][CS] (c), [1BuimPS][CS] (d), [1MeimBS][CS] (e). 3.5. Hammett Acidities of studied BAILs The Hammett acidities of investigated BAILs were determined using Hammett method and

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measured by evaluating the protonation extent of indicator in the ILs. Following the procedure proposed by Gilbert et al. [71], the acidities of camphorsulfonate-based BAILs were characterized by UV/Vis spectroscopy using p-nitroaniline as indicator. The ILs and pnitroaniline were dissolved in water with concentrations of 3.0 x 10-2 molL-1 and 7.5 x 10-5 molL-1, respectively. The resulting solutions were stirred overnight and then their UV spectra were recorded. The maximal absorbance of unprotonated p-nitroaniline was observed at 381 nm in water. The experimental results are displayed in Figure 7 and Table 3. As shown in Table 3, the Hammett acidity function (H0) can be expressed as: H0=pKa(ln)+log([In]/[InH+]) 13

(10)

ACCEPTED MANUSCRIPT where pKa(ln) is pKa of p-nitroaniline indicator solution, [In] and [InH+] represents molar concentrations of protonated and un-protonated form of p-nitro aniline indicator, respectively. Based on UV spectra of Figure 7, the quantitative data of H0 values was calculated. The corresponding H0 values were 1.6, 1.542, 1.544, 1.667 and 1.553 for [imPS][CS], [1MeimPS][CS], [1EtimPS][CS], [1MeimBS][CS] and [1BuimPS][CS], respectively (Table 3). The results showed that -SO3H functionalized ILs possessed strong Brönsted acidities and

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the acidity of [1MeimPS][CS] was strongest among the examined BAILs and acidity of [1EtimPS][CS] was very close to that of [1MeimPS][CS]. The decreasing order of the absorbance of ILs was observed as follows: [1MeimBS][CS] > [imPS][CS] > [1BuimPS][CS]

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> [1EtimPS][CS] > [1MeimPS][CS].

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Figure 7: The UV/vis spectrum according to Hammett method; [imPS][CS] (a), [1MeimBS][CS] (b), [1EtimPS][CS] (c), [1MeimPS][CS] (d), and [1BuimPS][CS] (e)

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Table 3. Hammett function values of investigated BAILsa IL Absorbance [In]%a [InH+]% b None 1.174 100 0 [imPS][CS] 0.943 80.32 19.68 [1MeimPS][CS] 0.917 78.11 21.89 [1EtimPS][CS] 0.918 78.19 21.81 [1MeimBS][CS] 0.970 82.62 17.38 [1BuimPS][CS] 0.922 78.53 21.47

Entry 1 2 3 4 5 6

H0 c -1.600 1.542 1.544 1.667 1.553

a

[In]= molar concentration of indicator

b

[InH+]= molar concentration of protonated indicator

c

H0=pKa(ln)+log([In]/[InH+]); (pKa=0.99); solvent: water; c(In)= 7.5 x 10 -5 molL-1; c(sample)= 3.0 x 10-2 molL-1; temperature= 25 °C

14

ACCEPTED MANUSCRIPT It has been found that [1MeimPS][CS] and [1EtimPS][CS] showed higher acidity (H0=1.542 and 1.544, respectively) as compared [imPS][CS] (H0=1.600). Similar findings were reported by Zhao et al. [72], where ILs contained alkyl-substitution at N1-position of imidazolium ring. They reported Brönsted acidities of ILs bearing -SO3H functionality attached as side chain on imidazolium cation. The stronger acidities ranging 1.760-1.883 were attributed to the existence of -SO3H group attached on side chain of imidazolium ring.

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Due to the electron withdrawing effect of imidazolium ring, the lone pair electrons of the oxygen atoms of -SO3H group may be attracted toward imidazolium moiety to promote the release of hydrogen ions, hence increasing the acidities. Furthermore, increase in side alkyl

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chain length of the cation can also lead to poor symmetry of whole IL molecule which in turn

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can cause deviation of electron [72]. Hence, the constrain of labile proton in -SO3H group is weakened [73]. The stronger acidities of reported BAILs (H0=1.542-1.667) can be attributed

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to nature of camphosulfonate anion as well. Wang et. al. [74] reported that acidities of –SO3H functionalized ILs depend both on the characteristics of the anion and on the sulfonate group of cation. When the sulfonic acid and anion interaction is stronger, the IL has a stronger

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acidity. In addition to alkyl –SO3H group, the anion can also considered as available acidic site [75]. In present study, camphorsulfonate anion and imidazolium containing sulfonic acid

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group are both alkylsulfonic acids (albeit one contains a distant cationic group). They both may have a similar pKa and thus the proton can be located on both the imidazolium sulfonic

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acid and the camphorsulfonic acid, which therefore can increase the number of possible acidic sites in the respective IL resulting in higher acidities. For establishment of such equilibrium between the two sulfonic acids, orientation of both molecules is significant. To

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attain equilibrium, the imidazolium sulfonic acid molecule is likely to be located in neighbor of the camphorsulfonate molecule either face-to-face direct contact or a few water molecules

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may also aid in bridging of the two sulfonate molecules. The higher acidities of present ILs show that these BAILs can be used as acidic catalyst and/or solvent. 3.6. COSMO-RS computational study

The advance level ab initio density functional theory (DFT) calculations, were performed to investigate the effect of functional groups of the cation on the molecular properties of ILs. Initially, the individual structures of imidazolium based cations and anion were optimized, then cation-anion paired structures, each containing one cation and (7,7-dimethyl-2oxobicyclo [2.2.1] heptan-1-yl) methane-sulfonate, abbreviated as [CS], were optimized to

15

ACCEPTED MANUSCRIPT obtain ILs molecular structure. The database of stable structures of the studied ILs is given in supporting information (Table S11 and S12). The carbon atom between two nitrogen atoms N1-C2-N3 is relatively more acidic than other carbon atoms of the imidazolium ring whereas a weak delocalization in the ring is present (Figure 8) due to π-electrons between C4 and C5 [76]. Moreover, Hydrogen atom attached to oxygen atom at the function group (-SO3H) contains another acidic moiety. This can easily be

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observed from the sigma surface of the molecules generated by COSMO shown in Figure 8 (b). These acidic moieties are the key to comprehend the properties of studied BAILs. Figure

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8 (b) shows the surface charge distribution of 1-Methyl-3-(3-sulfopropyl)-imidazolium. The

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red color represents the negatively charged moiety due to the basic oxygen atom present at the functional group of the cation whereas, blue color shows the positively charged moiety of acidic hydrogen present at C2, C4, C5 and SO3 sites of the imidazolium head. Hydrogen atoms

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located at C4 and C5 are considered neutral compared to C2. Hence, [CS] will preferably interact with either SO3-H or C2-H site. Thus, ILs will be stable when [CS] will be in between

other cations is displayed in Figure 9.

N+

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N

3

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2

(a)

(b)

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HO3S

1

4

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5

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both locations as shown in Figure 8 (d). The COSMO-surface charge density distribution of

(c)

(d)

16

ACCEPTED MANUSCRIPT Positive

Neutral

Negative

(b) [1MeimBS]

(d) [1MeimPS]

(e) [imPS]

(c) [1EtimPS]

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(a) [1BuimPS]

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Figure 8 (a): Molecular structure of [1MeimPS], (b): sigma surface of [1MeimPS], (c): sigma surface of [CS], (d): sigma surface of [1MeimPS][CS]

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Positive

Neutral

Negative

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Figure 9: COSMO-Surface charge density distribution of studied cations

Figure 10: σ- profile of functionalized cations with alkyl chain length C0 to C4. 17

ACCEPTED MANUSCRIPT The qualitative and quantitative information regarding the 1-substituted functionalized cations with different chain length can be analyzed from their σ-profile (Figure 10) which shows the screening charge density in term of surface polarity (σ) of the molecule. In general, σ range from -1 to 1 showing non-polar region whereas σ value greater than 1 represents hydrogen acceptor region and smaller than -1 represents hydrogen bond donor region [77]. The sigma profile of studied cations is in the range of (-2.5 to 1.4) e.nm-2 with different peak

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heights. The effect of chain length is shown in the sigma profile heights of the cations, as the chain length increases the peak height of the cation increases, which indicates the strength of non-polarity of the cations. The peaks at 1 e.nm-2 of these cations represent the presence of -

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SO3H functional group which induces the hydrogen bond acceptor property in the

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imidazolium cation. The only anomaly found in the peak height of the [1MeimPS] of the sigma profile. It may be the in accuracy of the COSMO-RS software. The effect of alkyl

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chain length in the functional group can be seen in the sigma profile of [1MeimPS] and [1MeimBS] in Figure 11. The [1MeimBS] which contain 4 carbon atoms in the alkyl chain of its functional group is more non-polar than [1MeimPS] which contains 3 carbon atoms,

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making Brönsted acidity of [1MeimPS][CS] higher than [1MeimBS][CS] (Table 3, entries 3 and 5). The functionalized cations studied in this work contain hydrogen bond donor due to

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imidazolium ring and hydrogen bond acceptor sites due to the functional group whereas the

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alkyl chain length induces the non-polarity in the cation.

Figure 11: σ- profile of cations with different chain length of functional group. 4. Conclusion 18

ACCEPTED MANUSCRIPT In the present study, five camphorsulfonate-based BAILs were synthesized and characterized. The pure ILs were further used for determination of thermophysical properties such as density, refractive index, viscosity and thermal stability. Effect of temperature and cationic chain length on physicochemical properties was well studied. Furthermore, Brönsted acidities and COSMO-RS study were also carried out to understand the effect of substitution on acidic properties of synthesized BAILs. From density values, different volumetric

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properties (molar volume, standard molar entropy, lattice energy and thermal expansion coefficients) were also estimated using well estabilished empirical equations. The studied BAILs had comparative refractive indices with high-refractive material, suggesting their use

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as optical materials. Owing to lower viscosities and strong Brönsted acidities of studied ILs,

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they could be effectively used as catayst and/or solvent at synthetic level. Thermal stability was also determined using thermogravimetric method (TGA) and high thermal stability of

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synthezied BAILs make them applicable for high temperature reactions at industrial level. ACKNOWLEDGMENT

D

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The authors would like to thank Center of Research in Ionic Liquids (CORIL) at Universiti Teknologi PETRONAS for the financial support from the grant number URIF/0153AA-B35.

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ACCEPTED MANUSCRIPT

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ACCEPTED MANUSCRIPT Highlights 

Five camphorsulfonate-based ILs with imidazolium and substituted imidazoliumbased cations were synthesized and characterized Effect of cation and temperature on physicochemical properties were determined

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The Brönsted acidities of investigated ILs were evaluated using Hammett equation

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COSMO-RS model was used to determine δ-surface, δ-potential and δ-profile of the

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