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 Tc = 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