Activity coefficients at infinite dilution of organic solutes in the ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate

doi:10.1016/j.fluid.2010.10.008

Fluid Phase Equilibria

Volume 299, Issue 2, 25 December 2010, Pages 198-206

https://doi.org/10.1016/j.fluid.2010.10.008 Get rights and content

Abstract

Infinite dilution activity coefficients $γ_{1}^{\infty}$ and gas–liquid partition coefficients K_L of 30 selected hydrocarbons, alcohols, ketones, ethers, esters, haloalkanes, nitrogen- and sulphur-containing compounds in the ionic liquid (IL) 1-ethyl-3-methylimidazolium methanesulfonate [EMIM][MeSO₃] were determined by gas–liquid chromatography at five temperatures in the range from 318.15 to 353.15 K. Relative contribution of adsorption at gas–liquid interphase to the overall solute retention, as examined by varying sample size and IL loading in the column, was found negligible. Partial molar excess enthalpies and entropies at infinite dilution were derived from the temperature dependence of the $γ_{1}^{\infty}$ values. The linear free energy relationship (LFER) solvation model was used to correlate successfully the K_L values. The LFER correlation parameters and excess thermodynamic functions were analyzed to disclose molecular interactions operating between the IL and the individual solutes. Comparison to [EMIM][NO₃] studied previously revealed [MeSO₃]⁻ anion to have still greater hydrogen bond basicity than that found for [NO₃]⁻. In addition, similarly to [EMIM][NO₃], promising potential of [EMIM][MeSO₃] for applications in solvent-aided separation processes was identified, the selectivities of [EMIM][MeSO₃] for separation of aromatic hydrocarbons and thiophene from saturated hydrocarbons ranking among the highest ever observed with ILs or molecular solvents.

Introduction

In recent years, ionic liquids (ILs) have been increasingly gaining massive interest of researchers because of technologically attractive properties and prospects for a variety of innovative applications [1], [2], [3], [4], [5], [6]. In particular, ILs can offer economically and environmentally favorable alternatives to common organic solvents e.g. in transition metal and enzymatic catalytic reactions and separations of organic substances by means of extraction or extractive distillation. For exploration and design of such applications, knowledge of interactions of process components with ILs is needed. A suitable and widely accepted approach for the description of interactions of volatile organic compounds (VOCs) as solutes with ILs as solvents is the experimental determination of limiting activity coefficients at infinite dilution $γ_{1}^{\infty}$ by gas–liquid chromatography (GLC) [7], [8], [9], [10], [11], [12], [13].

In this work, we examine interactions of selected VOCs with the ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate [EMIM][MeSO₃]:

This IL of the third generation was selected for the study because, on considering the small size and chemical structure of ionic moieties of this IL, its capability for various types of specific interactions and hence its high separation selectivity can be anticipated. Moreover, no experimental data on $γ_{1}^{\infty}$ in this IL have been reported so far.

The methodology of the present investigation is closely parallel to that we used in our recent study devoted to 1-ethyl-3-methylimidazolium nitrate [14]. In this paper, we report thus infinite dilution activity coefficients $γ_{1}^{\infty}$ and gas–liquid partition coefficients K_L for the same set of 30 selected hydrocarbons, alcohols, ketones, ethers, esters, haloalkanes, and nitrogen- or sulphur-containing compounds in [EMIM][MeSO₃] as a function of temperature. The obtained thermodynamic properties are analyzed to disclose the underlying intermolecular interactions governing the observed behavior and to identify the potential of [EMIM][MeSO₃] to be utilized as an entrainer in solvent-aided separations. Thanks to the uniform procedure adopted in both our studies, solvent properties of [EMIM][MeSO₃] and [EMIM][NO₃] can be now well compared.

Section snippets

Theory

In gas–liquid chromatography (GLC), the infinite dilution activity coefficient $γ_{1}^{\infty}$ and the gas–liquid partition coefficient $K_{L} = (c_{1}^{L} / c_{1}^{G})$ for a solute (1) partitioning between a carrier gas (2) and a non-volatile liquid solvent (3) are calculated from the solute retention according to the following equations [15]: $\ln γ_{1}^{\infty} = \ln (\frac{R T m_{3}}{V_{N} p_{1}^{s} M_{3}}) - [\frac{(B_{11} - v_{1}^{L}) p_{1}^{s}}{R T}] + [\frac{(2 B_{12} - {\bar{v}}_{1}^{\infty}) J_{3}^{4} p_{0}}{R T}]$ $\ln K_{L} = \ln (\frac{V_{N} ρ_{3}}{m_{3}}) - [\frac{(2 B_{12} - {\bar{v}}_{1}^{\infty}) J_{3}^{4} p_{0}}{R T}]$ where T is temperature of the column, m₃, M₃ and ρ₃ mass, molar mass and density of the solvent,

Materials

The ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][MeSO₃], M = 206.27 g/mol) was purchased from Solvent Innovations. Its purity according to the producer's specification was 99%, certified water content being <1000 ppm (mass basis). Because of its hygroscopicity, the IL was handled with special precautions to avoid any contact with moisture (see Section 3.3). The IL samples contained in the GLC columns used for retention measurements underwent efficient drying in situ during the

Results

Infinite dilution activity coefficients and gas–liquid partition coefficients in [EMIM][MeSO₃] were determined for a set of 30 selected solutes at 318.15, 323.15, 333.15, 343.15, and 353.15 K. Retention measurements were carried out on three columns of lengths 0.65 m, 1.2 m, and 1.7 m loaded with different amounts of the IL, namely 1.487 g (λ = 0.27), 2.661 g (λ = 0.27) and 5.418 g (λ = 0.34), respectively. The values of $γ_{1}^{\infty}$ and K_L were calculated using Eqs. (1), (2) from measured solute retention and other

Thermodynamic properties of solution and molecular interactions

Fig. 3, Fig. 4, Fig. 5 illustrate for selected solutes various $γ_{1}^{\infty}$ temperature dependences that correspond to widely different thermal effects for the dissolution of these solutes in [EMIM][MeSO₃]. Table 3 lists the limiting partial molar excess Gibbs energies ${\bar{G}}_{1}^{E, \infty} = R T \ln γ_{1}^{\infty}$ of all the studied solutes in [EMIM][MeSO₃] at a reference temperature 323.15 K together with their enthalpy $({\bar{H}}_{1}^{E, \infty})$ and entropy $(T_{ref} {\bar{S}}_{1}^{E, \infty})$ contributions, as calculated from the temperature dependence of limiting

Conclusions

In this work, we have examined interactions of various types of organic solutes with the ionic liquid [EMIM][MeSO₃] through methodical GLC retention measurements. Infinite dilution activity coefficients and gas–liquid partition coefficients of 30 hydrocarbons, alcohols, ketones, ethers, esters, haloalkanes, and nitrogen- or sulphur-containing VOCs in [EMIM][MeSO₃] have been determined over a range of temperatures, which allowed derivation of respective enthalpic and entropic contributions.

Acknowledgment

This work was funded from Ministry of Education of the Czech Republic (grant no. MSM 604 613 7307 and support for specific university research MSMT 21/2010).

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