COSMO-RS-based theoretical investigation of equilibria in protic ionic liquids

Ionic liquids have become an increasingly important field of research. Protic ionic liquids, a subgroup of ionic liquids, have been shown to be excellent solvents even for poorly soluble materials such as cellulose or polyamides. Still, the composition of such liquids, e.g., their ionicity and the nature of formed aggregates, is mostly unknown and experimental attempts to shed light on this have been mostly unsuccessful. Herein, we report the development of the Python-based COSMO-PIL program (COnductor-like Screening MOdel for Protic Ionic Liquids) to analyze the composition of protic ionic liquids using theoretical methods based on density functional theory (DFT) and the COSMO-RS (COnductor-like Screening MOdel for Real Solvents) implicit solvation model. Experimental inputs such as dielectric constants are not necessary. The COSMO-PIL program demonstrates the influence of acid/base ratios and residual water on ionic liquid composition and aids in understanding the reported poor ionicity of these liquids. Most notably, we explain this "ionicity gap" in protic ionic liquids, whereby even superbases fail to achieve full acid deprotonation, with a polarity mismatch between the anion and the cation, requiring residual acid as a mediator.