COSMO-RS-Based Theoretical Investigation of Equilibria in Protic Ionic Liquids

Ionic liquids have become an increasingly important field of research. A subgroup, protic 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 COSMOPIL program (COnductor-like Screening MOdel for Protic Ionic Liquids) to analyze the composition of protic ionic liquids via theoretical methods based on density functional theory (DFT) and the COSMO-RS (COnductor-like Screening MOdel for Real Solvents) implicit solvation model. The inputs of experimental data such as dielectric constants is not necessary. The COSMO-PIL program demonstrates the influence of acid/base ratios and residual water on the ionic liquid composition and allows the understanding of the reported poor ionicity of those liquids. Most notably, we explain this “ionicity gap” in protic ionic liquids, as even superbases fail to achieve full acid deprotonation, with a polarity mismatch between anion and cation, requiring residual acid as mediator.