Local solvent properties of imidazolium-based ionic liquids

Abstract

Solubility parameters such as Hildebrand's solubility parameter or its square, the cohesive energy density ced are often not able to predict solubilities of organic compounds in ionic liquids accurately. It has been suggested that this is an effect of the structural heterogeneity that many ionic liquids exhibit due to the non-polar alkyl chains forming separate domains from the charged parts of the cation and the anion. We here calculated the partial volumes of the polar and non-polar domains in Molecular Dynamics simulations of several 1-alkyl-3-methylimidazolium nitrates and 1-octyl-3-methylimidazolium hexafluorophosphate. We used these partial volumes to calculate the ced of the polar and non-polar domains. For 1-octyl-3-methylimidazolium nitrate the internal pressures Pi of the domains were also calculated. Our results show that ced and Pi are indeed very different in the domains, and that the domains containing the hydrophobic alkyl chains have solvent parameters similar to alkanes. For the domains that contain the charged part of the ions, ced and Pi are much higher, and Pi is comparable to what is found for high temperature molten salts. The results provide a possible explanation why solubility parameters seem incorrect for many ionic liquids, and may be useful in estimating more useful values using computer simulations.