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 ĝ^H,E were evaluated from thermodynamic analysis of surface tensions for the 1-decyl-3-methylimidazoulium bromide (C ₁₀mimBr)-tetraethylene glycol monooctyl ether (C₈E ₄) and 1-decyl-3-methyl-imidazolium tetrafluorobrorate (C ₁₀mimBF₄)-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 ĝ^H,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 ĝ^H,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-C₈E ₄ system.