Activity coefficients at infinite dilution and physicochemical properties for organic solutes and water in the ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium trifluorotris(perfluoroethyl)phosphate

Highlights

• ${\gamma }^{\infty }$ and K_L for 65 solutes in the IL [C₂OHmim][FAP] were determined by IGC.

• Partial molar thermodynamics functions $\mathrm{\Delta }{G}_1^{\text{E,}\infty }$, $\mathrm{\Delta }{H}_1^{\text{E,}\infty }$ and $\mathrm{\Delta }{S}_1^{\text{E,}\infty }$ were calculated.

• Selectivities and capacities for alkanes/thiophene separation problems were calculated.

• LFER system constants as a function of T for [C₂OHmim][FAP] were calculated.

• Results were compared to other ILs based on the same cation and anion.

Abstract

This work presents new data of activity coefficients at infinite dilution, γ^∞ of different organic solutes and water in the 1-(2-hydroxyethyl)-3-methylimidazolium trifluorotris(perfluoroethyl)phosphate, [C₂OHmim][FAP] ionic liquid. Values of γ^∞ were determined for 65 organic solutes, including alkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons, alcohols, thiophene, ethers, ketones, esters, 1-nitropropane, aldehydes, acetonitrile and water by inverse gas chromatography within the temperature range from (318.15 to 368.15) K. The basic thermodynamic functions, such as partial molar excess Gibbs energies, $\mathrm{\Delta }{G}_1^{\text{E,}\infty }$, enthalpies, $\mathrm{\Delta }{H}_1^{\text{E,}\infty }$ and entropies, $\mathrm{\Delta }{S}_1^{\text{E,}\infty }$ at infinite dilution were calculated from the experimental γ^∞ values obtained over the temperature range. Additionally the gas–liquid partition coefficients, K_L were determined. Experimental values of gas–liquid partition coefficients were used to determine the coefficients in the Abraham solvation parameter model (LFER). Results are compared to previously investigated ionic liquids with the same [C₂OHmim]⁺ cation and [FAP]⁻ anion. The selectivity and capacity at infinite dilution for alkanes/thiophene extraction problems were calculated from experimental γ^∞ values to verify the possibility of investigated ionic liquid as an entrainer in liquid–liquid extraction.

Introduction

Over 20 years, ionic liquids (ILs) have been the subject of intensive studies. Generally speaking, ionic liquids are salts, which are liquid at room temperature or their melting point is lower than 100 °C. An important feature of ILs is that their physicochemical properties can be simply adjusted by modification of the structure of both the cation as well as the anion. One of the most important features of ionic liquids is their negligible vapor pressure. This is very important property especially for solvents in liquid extraction. Actually used solvents in separation processes are often toxic and volatile what makes them environmentally unfriendly. Therefore there is a need for looking for better environmentally friendly solvents for new and existing industrial processes. To use an IL in a particular industrial process, knowledge is needed about the interaction of the IL with different solvents. Activity coefficients at infinite dilution, γ^∞ provide such a basic knowledge. From these measurements the selectivity ($S_{\mathit{ij}}^{\infty }={\gamma }_i^{\infty }/{\gamma }_j^{\infty }$) and capacity ($k_j^{\infty }=1/{\gamma }_j^{\infty }$) can be directly calculated for different separation problems.

In this work, which is a continuation of our systematic study on interactions between ionic liquid and different solutes, the activity coefficients at infinite dilution for organic solutes and water in the ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium trifluorotris(perfluoroethyl)phosphate, [C₂OHmim][FAP] were determined within the temperature range from (318.15 to 368.15) K using inverse gas chromatography (IGC). Based on these results the gas–liquid partition coefficients, K_L, and basic thermodynamic functions such as partial molar excess Gibbs energy $\mathrm{\Delta }{G}_1^{\text{E,}\infty }$, enthalpies $\mathrm{\Delta }{H}_1^{\text{E,}\infty }$ and entropies $\mathrm{\Delta }{S}_1^{\text{E,}\infty }$ at infinite dilution were determined.

Ionic liquids are considered as extractants in separation of sulfur compounds from fuels [1], [2], [3], therefore the selectivities and capacities at infinite dilution for alkanes/thiophene extraction problems were calculated and compared to other ILs based on the same [FAP]⁻ anion [4], [5], [6], [7], [8], [9]. Additionally results were compared to ionic liquid based on the same cation with bis(trifluoromethylsulfonyl)-amide, [NTf₂]⁻ anion [10].