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

Abstract

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 ∼3.5 × 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 ≤ q ≤ 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 γ and shear surface excess viscosity μ the spectra have been analyzed. It is found that μ is negligibly small, whereas γ 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 γ suggests that with increasing temperature an order–disorder transformation occurs in the surface layer.