Selection rules for estimating the solubility of C4-hydrocarbons in imidazolium ionic liquids determined by machine-learning tools

Abstract

Solubilities of C-4-hydrocarbons, 1,3-butadiene (13BD), trans/cis-2-butene (T2B and C2B), 1-butene (1B), isobutene (i-But), isobutane (i-B), and butane (B), in 3267 different imidazolium-type ionic liquids (ILs) in a temperature range from 273.15 to 373.15 K were estimated by means of the COnductor-like Screening MOdel for Realistic Solvents (COSMO-RS) calculations. Simple temperature-dependent mathematical expressions were developed to predict the solubility of 13BD, C2B, T2B, 1B, i-But, i-B, and B at any temperature in a range from 273 to 373 K. The COSMO-RS results for each hydrocarbon considered were then analyzed using machine learning tools, induding association rule mining and decision tree classification, using semi-empirically derived molecular descriptors of ILs. It was found that the polarizabilities of both cation and anion, together with the anion's CPK (space filling model) area, are the most important descriptors for determining the affinity of ILs towards C-4-hydrocarbons. Results also present the selection rules for imidazolium ILs, offering opportunities for the rational design of new ILs by using these simply-determined structural descriptors to meet the desired solubility (or selectivity) requirements for each C-4-hydrocarbon considered.