Synthesis and physico-chemical properties of two protic ionic liquids based on stearate anion
Accepted Manuscript Title: Synthesis and physico-chemical properties of two protic ionic liquids based on stearate anion Author: Dheiver Santos F´abio...
Please cite this article as: D. Santos, F. Costa, E. Franceschi, A. Santos, C. Dariva, S. Mattedi, Synthesis and physico-chemical properties of two protic ionic liquids based on stearate anion, Fluid Phase Equilibria (2014), http://dx.doi.org/10.1016/j.fluid.2014.05.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and physico-chemical properties of two protic ionic liquids based on stearate anion
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Dheiver Santosa, Fábio Costaa, Elton Franceschib, Alexandre Santosb, Cláudio Darivab and Silvana Mattedia* a
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Programa de Pós-graduação em Engenharia Química, Escola Politécnica, Universidade Federal da Bahia (UFBA),R. Aristides Novis 2 Federação, 40210-630, Salvador-BA, Brazil
b
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Núcleo de Estudos em Sistemas Coloidais—ITP/UNIT, Campus Farolândia, Av. Murilo Dantas, 300, Aracajú, SE, 49032-490, Brazil
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* To whom correspondence should be addressed E-mail: [email protected] Tel & Fax: +55071 3283-9809
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Abstract Over the years, ionic liquids have received increased attention in several fields of application. More specifically a new ionic liquid family was developed, called protic ionic liquids, that have been proven to be biodegradable and they have potentially low toxicity, besides their
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low cost of preparation, simple synthesis and purification are interesting features. Continuing with our systematic work on synthesis and characterization of protic ionic liquids, this work two
new
ionic
liquids,
2-hydroxyethylammonium
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deals with the synthesis, FT-infrared identification and thermodynamic characterization of stearate,
(2-HEAS)
and
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bis(2hydroxyethylammonium) stearate (B2HEAS), based in the neutralization reaction of stearic acid with 2-hydroxyethylamine and bis(2-hydroxyethylamine) (generally known as mono and diethanolamine, respectively). Thermal properties of ionic liquids in function of
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temperature and concentration, along with the volumetric properties were determined. Results showed that both ionic liquids (2-HEAS and B2HEAS) form micelles, suggesting interfacial activity. Also, low negative deviations from ideality for the two binary systems were
Ionic liquids (ILs) are compounds consisting of an organic cation with an organic or inorganic anion, with melting temperature tipically below the boiling point of water at ambient pressure [1]. ILs have interesting properties including non-volatility, non-flammability, high ionic conductivity, chemical and electrochemical stability, among others. Their use as alternative to conventional molecular liquids are remarkably increase in the last years [2-8]. In addition, their chemical and physical properties can be readily adjusted by an adequate selection of cation and anion species. In this scenery, ionic liquids have currently been applied as promising solvents in organic synthesis [9-11], catalysis [8-12], electrochemistry [13] and chemical separation [14]. ILs have intrinsic ionic conductivity at room temperature, and a wide electrochemical window exhibiting good electrochemical stability in the range of 4.0– 5.7 V [15-16]. More recently, the use of proton-conducting ionic liquids (PCILs) was proposed in order to ensure high anhydrous proton conductivities while maintaining sufficient thermal stability. These properties permits the development of Proton Exchange Membrane
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Fuel Cells (PEMFCs), and in this sense PCIL have received increase interest for this application [17-20]. Alvarez et al. (2010) [21-22] synthesized six PILs with N-methyl-2hydroxyethyl amine and organic acids. The spectra obtained by NMR for these ionic liquids suggested the existence of
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an ordered lamellar liquid crystal phase. Some ionic liquids, depending on the size of the alkyl chain, exhibit an aggregation behavior in solutions, improving its application as surfactants in
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wettability modification, detergency, and the displacement of liquid phases through porous media. On the other hand, the stabilization/destabilization of dispersions including foams,
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froths and emulsions are also feasible, suggesting a vast array of practical application areas which are illustrated in terms of mineral and petroleum processing, biological systems, health
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and personal care products, foods, and crop protection [23].
Recently, Maximo et al [24] studied the physico-chemical and rheological characterization of
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reactants of ionic liquids synthesis: oleic acid + monoethanolamine, diethanolamine + oleic acid, stearic acid + monoethanolamine and diethanolamine. The study showed different
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dependence of conductivity by changing the concentration of protic ionic liquids.
hydroxyethylammonium
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In this study we have synthesized two ionic liquids based on stearate anion: 2stearate,
2-HEAS,
bis(2-hydroxyethylammonium)
stearate,
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B2HEAS. Thermal analysis (DSC and TGA), conductivity measurements and volumetric properties of aqueous mixtures (apparent molar volume and molar volume change on mixing) at temperatures of 298.15, 303.15 and 308.15 K were performed to characterize the ILs.
2. 2.1
Experimental Section
Synthesis of ionic liquids
Stearic acid (98%) was provided by Sigma-Aldrich and amines (2-hydroxyethylamine and bis(2-hydroxyethylamine) (99%) were purchased from Vetec. Reactants were used without previous purification this is shown in Table 1 . The synthesis consists of a Brönsted acid-base reaction conducted in stoichiometric ratio. Stearic acid was melted at 353.15 K, and the amine was then added slowly to the melted acid at 353.15 K with continuous stirring. The temperature of the mixture was then increased up to 363.15 K to obtain an homogeneous solution. The solution was then cooled and solidified at room temperature (~298 K). The resulting solid was ground down to form a powder. The powder was stored in a closed
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container for at least 48 hours before any use. This preparation method has been previously used to prepare alkali acid soap crystals, and is similar to previous works [21,22], being adapted to a solid acid. Figure 1 presents the structure of the synthesized ionic liquids.
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The purities of each ionic liquid were checked by 1H NMR. The obtained 1H NMR spectra
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are shown in Figure 2 and 3. The spectra shows that the present impurities were 0.04 and 0.02 in mass fraction for 2-HEAS and B2HEAS respectively. The impurities are due to
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unreacted products as the reaction may not be completed, and as the stearic acid are not volatile it was not removed or impurities present on starting material. The water used was
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double distilled, passed by a reverse osmosis system and further treated with a Milli-Q plus water purification apparatus. The water content of all ionic liquids 2-HEAS and B2HEAS respectively 3541.42 ppm - 0.35% and 7226.21 ppm - 0.76% in weight basis as detected by
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Karl Fischer titration, using a coulometer Karl Fischer (Mettler Toledo, Tritino 870). These results are shown in table 1
2.2
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Chemical characterization of the ionic liquids
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FTIR (Fourier Transform InfraRed) spectra were collected with a Fourier Transform IR spectrophotometer (Shimadzu, IR Prestige 21) using KBr pellets with sweep angle of 4000 to 400 cm-1 accumulating 45 reads with 4 cm-1 of resolution. The backgrounds were collected using KBr spectroscopic grade.
2.3
Physical-chemical characterization
All mixture samples were made up by weight and with an analytical balance (Shimadzu, AY220) with a precision better than 0.0001g. Each mixture was prepared with known mass of ionic liquid and solvent injected into glass vials by means of a syringe. Mixtures were sealed in the vials with an aluminum cap and a rubber plug. Empty spaces in the vials were minimized in order to avoid losses by evaporation and prevent air humidity absorption. Doubly distilled water was used to prepare solutions. The molality of the samples in water were calculated through a mass balance considering the water content of each IL determined through Karl Fischer titration. Electrical conductance measurements were carried out on a
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digital conductivity meter (mCA-150P Tecnopon) with a error of 2.5% and a dipping type conductivity cell with platinized electrodes with a cell constant of 1.1 cm−1. The conductivity meter was calibrated using a standard solution (standard solution of tecnopon MS instrumentation, solution of 146.9 mS/cm). The temperature was maintained constant at 298 ± contribution of the pure solvent present in the prepared ionic liquid.
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1.0 K. The molar concentration of ionic liquid solutions were always corrected to consider the
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Density measurements of synthesized ionic liquid aqueous solutions was carried out with an Anton Paar (DMA-4500 M) digital densitometer thermostatted at each temperature and
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maintained at atmospheric pressure (≈101 kPa). The density measurements protocol includes an automatic correction for the viscosity of the sample. The equipment provides measurements with 5 decimals and the uncertainty of the density measurements was estimated
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to be better than 0.0001 g/cm3.
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Differential scanning calorimetry (DSC) was carried out with a TA Instrument DSC Q20 under nitrogen atmosphere. The dry samples were tightly sealed in aluminum pans. The samples were first equilibrated at 263.15 K and then heated to 373.15 K at a heating rate of 10
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K min-1 followed by quenching to 263.15 K and then reheating to 373.15 K at a heating rate
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of 283.15 K min-1. Melting points (Tm) were determined from the DSC thermograms during the programmed heating scans. Thermo-gravimetric Analysis (TGA) was performed by a
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Shimadzu TGA (50/50H) at a heating rate of 10 K.min-1
under nitrogen flow of
23 mL.min-1. In order to dry the ionic liquids, the ILs were heated up to 373.15 K in the equipment under the nitrogen flow, the temperature were maintained for 20 minutes and then it was cooled to 363.15 K to begin the analysis,
3. 3.1
Results and Discussion
FTIR
FTIR analysis were used to identify the formation of ionic liquids or possible incomplete reactions. FTIR spectra for 2-HEAS and B2HEAS and their reactants are shown in Figures 4 and 5. Reactants are characterized mainly by carboxylic acid and amine groups. For characterization, are expected in general absorption bands features in medium infrared region (4000-300 cm-1). The ILs synthesized in this work presented main bands exactly at expected regions, as described in the literature [21] for its characterization. The spectra does not show
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unexpected bands neither in the reactants nor in the sinthezised ILs. The band attribution for ILs is also shown in the spectra in supplementary material. Figure 4 shows a peak in the wavelength region near to 1600 cm-1 from the carbonyl group, C=O group which is characteristic to absorb radiation at (1550-1610 cm-1). However, the
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effect of ressonance between the C=O and C–O, the carbonyl functional group, together with the C–N, shows (or causes) a decrease in the absorption rate of 25–45 cm-1. Two sharp
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absorption peaks from the C–O group of functional carbonyl appears between 1380 ~ 1500 cm-1. An average absorption peak for the double bond of C–N carbon sp3 attached to the
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nitrogen of the amine at 1020 ~ 1220 cm-1 can be observed. There are absorption peaks in the range of 3100 ~ 3300 cm-1 related to the functional group of primary amines H–N. Close to this band was noticed (2400 ~ 3300 cm-1) a long strip of O–H stretch of the OH group from
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the hydroxyl amine, C–H which overlaps the long carbon chain sp3 hybridized to the structure of the ion stearate (2850 ~ 3000 cm-1). It should also be noted that the OH stretching band
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(3200 ~3600 cm-1) can be influenced by water vapor present in the environment of the FTIR analysis, as well as the group C=O derived from CO2 (1550 ~ 1700 cm-1).
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In Figure 5 the absorption bands present in the range (3100 ~ 3600 cm-1) show superposition
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of the O–H stretching vibrational, with N–H, as both connections have vibrational bands nearby O–H (3200 ~ 3600 cm-1) and N–H (3100 ~ 3500 cm-1) occurs in an overlapping
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absorption bands. In the stretch between 2700 ~ 3000 cm-1 is reported the C–H (CH3) hybridized sp3. In the region from 1550 to 1700 cm-1 the stretching of C=O arising from carbonyl group, the C–N stretch of the amino group in the range between (1100 ~ 1350 cm-1), and C–O in the range of stearate stretch (1000 ~ 1100 cm-1). The stretch band (3200 ~ 3600 cm-1) of O–H suggests traces of water vapor present in the environment of the FTIR analysis as well as the C=O (1550 ~ 1700 cm-1) could be explained by CO2 present in the atmosphere in the equipment enviroment.
3.2
Conductivities of (Water+ IL) Binary System
The measured molar conductivities (Λ) defined by Eq. (1) of the aqueous solutions of ionic liquids as a function of molality concentration of ionic liquid are presented in Table 2 and depicted in Figures 6 and 7.
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Λ=
[ Ke ] = S × cm-1 , [c] mol × cm-3
where : K e = K solution - K water
(1)
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It is observed that the molar conductivity (Λ) decreases with the increasing amount of ionic liquid. At high concentrations of ionic liquid an association relaxation and electrophoretic effects occur between the anion and cation of the ionic liquid, resulting in more aggregates of
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the ionic liquid dispersed in water [23-28]. Accordingly It should be stressed that the salt-rich region promotes a way for displacement of ions by the presence of a network of
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interconnected micellar aggregates [27]. The plot of molar conductivities (Λ) versus concentration m is not linear for aqueous solutions of ionic liquids, indicating interactions of
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ions with each other. Λ0 can be determined by the extrapolation of molar conductivities (Λ) versus of ionic liquid concentration to m = 0.0 mol.Kg-1.
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There are some equations for the analysis of the conductance data The low concentration chemical model of conductivity equation is widely recently applied for the correlation of conductance data in aqueous and noaqueous electrolyte solution. The conductivity data in this
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work was analised using the low concentration chemical model (lcCM) given by Barthel et al.
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[29] as described by eq. 2 to 4. For associated electrolytes, as in the present case, data analysis
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is carried out by a nonlinear least-squares iterations with coefficients J1 and J2 of eq. 2 preset
are the molar conductivities at c is concentration, and Λ0 are the molar conductivities
at infinite dilution,
is the fraction of oppositely charged ions acting as ion pairs. The
coefficients of Eq. (2) reflect the relaxation and electrophoretic effects. In Eq. (2), S is the coefficient of the Debye-Hückel-Onsager and E depends only on the properties of the solvent
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and the charge on the ions, both were taken from Barthel et al. [29] and ( K 0a ) is the equilibrium constant of ion association, that this study provides values for both ionic liquids
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in aqueous solution and root mean square deviation between experimental and calculated
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values with lcCM shown in the Table 3.
3.3
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Thermal analysis of ionic liquids
The Fig 8 shows the endotherm of melting of 2-HEAS and B2HEAS obtained at the heating
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rate of 10 K min-1. There is observed that the samples presents an onset temperature for melting of 348.7 K and 331.68 K with ΔHm = 166.8 and J/g and ΔHm = 138.9 J/g enthalpy of
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melting, for 2-HEAS and B2HEAS respectively, confirming that they are ionic liquids (data are collected and shown in Table 4, as well as the uncertainities of measured data). From Fig
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9 it can be suggested that the degradation temperature of the IL are around 605 K to 2-HEAS.
3.4
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B2HEAS however is lower stable as it is degradated in two steps 450 and 650 K.
Properties of Mixtures
Physico-chemical properties, especially volumetric, for binary mixtures involving salts compounds are interesting for process design and knowledge of molecular interactions. Molar volumes are among the most often determined properties of binary mixtures. To know the solute-solute and solute-solvent interaction behavior, the partial molar properties of dilute solutions are significant. Some works can be found in the literature regarding the volumetric properties of aqueous solutions involving ionic liquids [30-31]. Such results can contribute to clarify intermolecular interactions between different species in solution. The obtained density of the mixtures are shown in Table 5.
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The molar volume change on mixing can be calculated by the following equation
Vmm = Vm − x1Vm1 − x2Vm2
(6)
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Where Vm is the molar volume of solution containing one mole of (water + IL), x1 and x2 are the mole fractions of components 1 (water) and 2 (IL), respectively, and Vm1 and Vm2 are
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the molar volumes of pure components in their stable aggregation state [32]. The apparent
be expressed as:
(Vm − xiVmi ) xi
(7)
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V ϕi =
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molar volume of water in IL ( V ϕ1 ), and the apparent molar volume of IL in water ( V ϕ2 ) can
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Apparent molar volumes, V ϕ , were determined using the Eq.(8) from measured solution densities ρ and pure water densities ρ 0 at atmospheric pressure and measured temperature,
ρ
−
1 ⎛ ρ0 − ρ ⎞ ⎜ ⎟ mi ⎝ ρρ 0 m ⎠
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MM i
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V ϕi =
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where MM i is the molecular mass of the salt, and m is the solution molality (mol.kg−1).
(8)
Combining Equations (6 and 7): ⎛ Vm m ⎞ V ϕ 2 = Vm2 + ⎜ ⎟ ⎝ x2 ⎠
(9)
rearranging Eq. (9):
Vmm (V ϕ2 − Vm2 ) = x1 x2 x1
(10)
The apparent molar volumes for the 2-hydroxyethylammonium stearate, 2-HEAS, bis(2hydroxyethylammonium) stearate (B2HEAS) in water from (298.15 to 308.15) K are depicted in Figs. 10 and 11 and Table 6 that show the variation of apparent molar volumes of
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the investigated solutions as a function of the ionic liquids concentration. From these figures, it is evident that the apparent molar volume present a great variation with increasing concentration of the ionic liquid within the concentration range investigated here. In fact, the variation of Vφ with the ionic liquid concentration was always found to be linear [33].
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The molar volumes change on mixing is shown in Table 6 and Figures 12 and 13 were negative over the entire composition range for the two binary systems, and the absolute values
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of molar volumes change on mixing hardly increases with increasing temperature from (298.15 to 308.15) K. The negative molar volumes change on mixing indicated that a more
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efficient packing and/or attractive interaction occurred when the protic ionic liquids and the water were mixed. Generally, low negative deviations from ideality in these two binary systems were due to the interstitial accommodation and intermolecular interactions such as
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electrostatic, dipole, and hydrogen bonding of these ionic liquids with water. On the other hand, Holbrey et al. [33-34] found that ionic liquids exhibited liquid clathrate formation in
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solvents with various ratios, and ratio was corresponded to the minimum value.