Order–Disorder Transition at the Liquid/Vapor Interface of Imidazolium Based Ionic Liquids: A Surface Light Scattering Study

Abstract

The liquid/vapor interface of imidazolium based ionic liquids, [C6mim][PF6] and [C6mim][BTI], has been probed by capillary wave spectroscopy at various temperatures up to 400K. Separately, the corresponding surface tensions have been measured by the maximum bubble pressure method. Interpretation of the capillary wave spectra with a modified dispersion relation yields the surface dipole moment density γ(T) as a function of temperature. In both systems γ(T) exhibits a characteristic decrease with temperature which indicates an interfacial order–disorder transition. Within the mean field approximation, fits of γ(T) yield the following critical temperatures for this transition: T c = 364K for [C6mim][BTI], and T c = 383K for [C6mim][PF6].