Thermodynamic study on the interaction of imidazolium salts and POE-type nonionic surfactant in the adsorbed film

The composition of the adsorbed film and the excess Gibbs energy of adsorption \( {\widehat{g}}^{\mathrm{H},\mathrm{E}} \) were evaluated from thermodynamic analysis of surface tensions for the 1-decyl-3-methylimidazoulium bromide (C₁₀mimBr)–tetraethylene glycol monooctyl ether (C₈E₄) and 1-decyl-3-methyl-imidazolium tetrafluorobrorate (C₁₀mimBF₄)–C₈E₄ systems, where the counter anion of imidazolium salts is different from each other. The higher miscibility of two components compared to an ideal mixing and thus negative \( {\widehat{g}}^{\mathrm{H},\mathrm{E}} \) were observed in the former, which comes from the ion–dipole interaction between imidazolium cation and the oxyethylene group of C₈E₄. On the other hand, the lower miscibility and thus positive \( {\widehat{g}}^{\mathrm{H},\mathrm{E}} \) were observed for the latter. Such differences were attributed to that BF₄ ⁻ forms two hydrogen bonds and has stronger affinity with the cationic head group of C₁₀mim⁺ than Br⁻. This results in that the ion–dipole interaction between C₈E₄ and C₁₀mim⁺ cation is diminished in the C₁₀mimBF₄–C₈E₄ system.