Density and molar volume predictions using COSMO-RS for ionic liquids.: An approach to solvent design

The specific density and molar liquid volume of 40 imidazolium-based ionic liquids were predicted using the COSMO-RS method, a thermodynamic model based on quantum chemistry calculations. A molecular model of ion pairs was proposed to simulate the pure ionic liquid compounds. These ion-paired structures were generated at the B3LYP/6-31++G** computational level by combining the cations 1-methyl- (Mmim(+)), 1-ethyl- (Emim(+)), 1-butyl- (Bmim(+)), 1-hexyl- (Hxmim(+)), and 1-octyl-3-methylimidazolium (Omim(+)) with the anions chloride (Cl-), tetrafluoroborate (BF4-), tetrachloroferrate (FeCl4-), hexafluorophosphate (PF6-), bis(trifluoromethanesulfonyl)imide (Tf2N-), methylsulfate (MeSO4-), ethylsulfate (EtSO4-), and trifluoromethanesulfonate (CF3SO3-). Satisfactory agreement with the available experimental measurements was obtained, showing the capability of the current computational approach to describe the effect of the anion nature and cation substituent on the volumetric properties of this family of ionic liquids. Thus, calculated and experimental density values of ionic liquids (and also other common solvents) were fitted by linear regressions with correlation coefficients R > 0.99 and standard deviations SD < 20 kg/m(3). Consequently, molar liquid volumes were also predicted very accurately by COSMO-RS, indicating the suitability of the ion-pair model to describe intermolecular interactions of pure ionic liquids. In this sense, the sigma-profiles of the ion-paired molecules were used to qualitatively analyze the influence of cation and anion natures of ionic liquids on their volumetric properties. As a result of the analysis, we propose the charge distribution area below the sigma-profile (S sigma-profile) as a simple a priori parameter to characterize the contributions of cation and anion to the ionic liquid behavior as tool to design solvents.