In aqueous solutions of long-chain ionic liquids, hydrophobic cations are attracted to the air side, which then facillitates the adsorption of anions to the surface by Coluomb force. This would dictate that the surface properties like surface excess and critical micelle concentration (cmc) depend on the cation type (e.g. length of the alkyl chain in the cation) but not on the change of anion. It was reported, however, that surface tension and cmc of ionic liquid aqueous solutions appear differently with the change of inorganic anions. Detailed surface structure of the IL+water system will be crucial to find out the microscopic origin of this phenomenon.We investigated the composition at the surface and surface structure of the solutions of alkylimidazolium ionic liquids with different halide anions ([C12mim][X], X = Cl, Br, and I) by using X-ray reflectivity, X-ray fluorescence, and surface nonlinear-optical spectroscopy. From the surface tension measurement, it was found that [C12mim]I has the largest surface excess (the smallest surface area per molecules) indicating that larger and more polarizable halide anion promotes the surface activity of [C12mim]+. In [C12mim]Cl or [C12mim]I solutions that formed Gibbs monolayer, inorganic salts having different halide anions were added in the subphase to allow competitive adsporption. For [C12mim]I solution, adsorbed I- ions were not completely replaced by Cl- even in bulk NaCl concentration hundred times higher than that of [C12mim]I. On the other hand, a fair number of Cl- ions were replaced by I- ions when its bulk concentrations is the same as that of [C12mim]Cl. From grazing incidence X-ray diffraction and sum-frequency generation spectroscopy, it was found that [C12mim]Cl solution surface exhibits loosely-packed and disordered monolayer structure whereas [C12mim]I has fully-packed and condensed domains, strongly suggesting the position of adsorbed halide anions are different between the two ILs.

Surface excess and structure of Gibbs monolayer of long-chain ionic liquids

248th National Meeting (2014)

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