Effect of Counterionic Species on the Adsorption of Tetradecyltrimethylammonium Ion at the Polarized Nitrobenzene–Water Interface

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Abstract
The effect of counterions, F−, Cl−, and Br− on the adsorption of the hexadecyltrimethylammonium ion (HTMA+) at the polarized nitrobenzene–water interface has been studied by measuring electrocapillary curves. The interfacial tension in the negative branch of the electrocapillary curve, where the interface is polarized such that the potential in the aqueous phase is negative with respect to that in the nitrobenzene phase, was lowered due to the specific adsorption of HTMA+ ion at the interface. The magnitude of interfacial tension depression in the negative branch became larger in the order F−, Cl−, and Br−, indicating a specific counterion binding at the interface in this order. The adsorption free energy of the HTMA+ ion, ΔGa0, for the adsorption of HTMA+ ion at the interface from the nitrobenzene phase was found to depend markedly on the anionic species present in the aqueous side of the interface. This ΔGa0 values can be taken as a quantitative measure of the counterion specificity at the interface. The meaning of the fact that the change in the shape of the electrocapillary curves depends on the counterion species is discussed in detail in terms of the thermodynamics of the oil–water interface.