Peculiarity of the liquid/vapour interface of an ionic liquid: study of surface tension and viscoelasticity of liquid BMImPF6 at various temperatures

We have measured the surface tension and the capillary wave spectra at the liquid/vapour interface of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate at various temperatures up to 400 K. From the weak temperature dependence of the surface tension a low value of the surface excess entropy of approximately 3.5 x 10(5) J K(-1) m(-2) results which is consistent with a strongly aligned surface layer of imidazolium cations previously predicted by MD-calculations. The capillary wave spectra recorded at different wave numbers in the range 170 cm(-1) < or = q < or =500 cm(-1) exhibit strong deviations from the behaviour expected for the free surface of simple liquids. With an extended dispersion relation including the contributions of surface dipole moment density gamma and shear surface excess viscosity mu the spectra have been analyzed. It is found that mu is negligibly small, whereas gamma substantially influences the capillary wave spectra. The electrostatic potential across the interface, which corresponds to the measured dipole moment densities, qualitatively agrees with simulation calculations. The distinct temperature dependence of gamma suggests that with increasing temperature an order-disorder transformation occurs in the surface layer.