Study on association and thermal stability of binary imidazolium ionic liquid mixtures

Ionic liquid (IL) mixtures with specific properties can be designed by the combination of different ionic liquids (ILs), but there are few studies on the physicochemical properties of IL mixtures with different ratios. Thus, this work explores the association and thermal stability of binary imidazolium IL mixtures. In particular, the physicochemical properties of binary IL mixtures at different ratios were investigated. Four different ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), 1-butyl-3-methylimidazolium nitrate ([BMIM][NO₃]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][TFO]), and 1-butyl-3-methyl-imidazolium trifluoroacetate ([BMIM][CF₃COO]), were selected as the parent salts to form the different binary imidazolium IL mixtures. These IL mixtures were tested at different temperatures (25, 30, 35, 40, 45, and 50 °C) and ratios (1:9, 3:7, 5:5, 7:3, and 9:1) by viscometer, conductivity meter, and then by isothermal thermogravimetric analysis in the temperature rise range of 30–500 °C. It was found that the increase in temperature decreased the association of IL mixtures, thus destroying the association behavior between the parent salts. Accordingly, different ratios of the parent salts had no significant effect on the thermal stability of some IL mixtures. Moreover, the results showed that smaller anions were more likely to combine with cations to form ion clusters or ion pairs, enhancing the association of binary IL mixtures. Thus, it increased the thermal stability of IL mixtures.