Volume, surface and UNIQUAC interaction parameters for imidazolium based ionic liquids via Polarizable Continuum Model

Ionic liquids (ILs) have shown great potential as solvent/media for reactions and separations. However, the physico-chemical characteristics of ILs are scarce and the limitless different combinations of cations and anions further complicate the matter. The ternary tie line data along with the binodal curve gives a fair indication regarding the feasibility of the ILs as solvents. Most of the ternary (with ILs) liquid-liquid equilibrium data available in literature has been correlated through the NRTL model as r and q are not available. The absence of the volume and surface parameters poses a hindrance in calculation of the binary interaction parameters for UNIQUAC and UNIFAC models. A novel method has been developed for deriving these volume and surface parameters from the Polarizable Continuum Model (PCM). PCM is widely used for studying solvation effects. Here, the solute is represented by a charge distribution in a molecular shaped cavity embedded in an infinite polarizable dielectric medium. GEnerating POLyhedra (GEPOL), which is based on the concept of solvent excluding surface, is used for calculating this cavity. This novel approach for volume and surface parameters has been verified initially for 71 compounds belonging to several homologous series (paraffins, isoparaffins, olefins, naphthenes, aromatics, alkynes, alcohols, ketones, aldehydes, acids, esters and amines) and 24 solvents. The predicted values of r and q for alcohols, ethers and oxygen containing systems showed significant deviations (2.5-20% for r and 5-25% for q) from the literature values. The values of r and q obtained by PCM method have been applied to 17 ternary systems and 1 quaternary system belonging to these deviant components. The PCM method gives a significantly better fit (average of 60% improvement in rmsd) for all the systems studied. This approach has been used to estimate the structural parameters for 25 dialkylimidazolium based ILs. Subsequently, these values have been used to estimate the UNIQUAC interaction parameters for seven IL based ternary systems, giving a 40% improvement in rmsd over NRTL.