Interfacial phase transitions in imidazolium-based ionic liquids

We report measurements of the surface tension and capillary wave spectra at the liquid/vapour interface of the ionic liquids [C₄mim]⁺X^- (X^- = Cl^-,AlCl₄^-) and compare these with recent results of [C₄mim]⁺PF₆^-. Special attention is given to the temperature dependence of the surface dipole moment density γ(T), which has been derived from the capillary wave spectra with the aid of the properly extended dispersion relation. For [C₄mim]⁺PF₆^- an order-disorder interfacial phase transition with a critical temperature of T_c = 385 K is found, which has not been reported before. At higher temperatures a first order ferroelectric surface phase transition is indicated in γ(T) of both [C₄mim]⁺PF₆^- and [C₄mim]⁺Cl^- melts. Increasing the anion size in [C₄mim]⁺AlCl₄^-, these interfacial phase transitions are suppressed.