Halide Ions Effects on Surface Excess of Long Chain Ionic Liquids Water Solutions

The interfacial structure and composition of water solutions with alkylimidazolium ionic liquids varying in their halide anions ([C(12)mim][X], X = Cl and I) were investigated by X-ray near-total-reflection fluorescence spectroscopy and X-ray reflectivity measurements. We demonstrate that X-ray fluorescence and reflectivity techniques provide a more direct measurement of surface adsorption. Furthermore, we show that for [C(12)mim][Cl] and [C(12)mim][I] solutions with mixed inorganic salts (NaI, NaCl), I- ions replace Cl- above the critical micelle concentration (CMC) of [C(12)mim][Cl] at much lower concentrations of NaI whereas NaCl concentrations a hundred times higher than the CMC of [C(12)mim][I] only partially replace the I- at the interface. Our surface-sensitive X-ray diffraction and spectroscopy provide two independent tools to directly determine the surface adsorption of ionic surfactants and the interfacial composition of the surface films.