Predicting the hygroscopicity of imidazolium-based ILs varying in anion by hydrogen-bonding basicity and acidity

Abstract

Most of ILs are hygroscopic. Hygroscopicity of ILs has a significant influence on the structure and properties of ILs. There should be correlation between hygroscopicity and the other properties of ILs. In this study, eight imidazolium-based ILs varying in anion were used to investigate the relationship between their hygroscopicity parameters (experimental water sorption capacity within 3 h W_3h, saturated water sorption capacity W_∞ derived from the modified two-step model, degree of difficulty to reach sorption equilibrium 1/k, and water sorption rate kW_∞ derived from the modified two step model) and their property parameters (electric field ΔE, polarity E^N_T, hydrogen-bonding acidity α, hydrogen-bonding basicity β, dipolarity-polarizability π*, and the difference between hydrogen-bonding basicity and acidity β − α). We found that β − α had a very strong positive correlation coefficient ρ ≈ 0.99 with the hygroscopicity parameters. However, the respective species, i.e., β and α are less correlated with hygroscopicity. The E^N_T and ΔE are much less correlated with the hygroscopicity of ILs than those of β − α, β and α, while π* has almost no correlation with hygroscopicity of ILs.