Influence of high pressure on solubility of ionic liquids: experimental data and correlation

Abstract

The solid–liquid phase equilibrium (SLE) in binary mixtures that contain a room-temperature ionic liquid, IL, and an organic solvent: {1-ethyl-3-methylimidazolium tosylate, [EMIM][TOS] (1) + cyclohexane, or benzene (2)} and {1,3-dimethylimidazolium methylsulfate, [MMIM][CH₃SO₄] (1) + hexan-1-ol (2)} have been measured under very high pressures up to about 900 MPa at the temperature range from T = 328 to 363 K. The termostatted apparatus for the measurements of transition pressures from the liquid to the solid state was used. The pressure–temperature–composition relation of the high pressure solid–liquid phase equilibria polynomial based on the Yang model was satisfactorily used. The freezing and melting temperatures at a constant composition increase monotonously with pressure. The high pressure experimental results obtained at isothermal conditions (p–x) were interpolated to well known T–x diagrams. Additionally, the SLE of binary systems {[EMIM][TOS] (1) + cyclohexane, or benzene (2)} at normal pressure have been measured by a dynamic method from 270 K to the melting point of the IL. Experimental solubility results are compared with values calculated by means of the non-random, two-liquid equation (NRTL) utilizing parameters derived from SLE and liquid–liquid equilibria (LLE) results. The experimental results at high pressures were compared for every system to these at normal pressure. The paper includes thermophysical basic characterization of pure ionic liquids obtained via differential scanning calorimetry (DSC), temperatures and enthalpies of melting. The influence of the interaction between the IL and solvent on the solubility at 0.1 MPa and high pressure up to 900 MPa was discussed.