Near-critical liquid/liquid interfaces of simple and complex mixtures probed by ellipsometry

Measurements of the coefficient of ellipticity ϱ of light reflected at liquid/liquid interfaces of five binary mixtures are presented and analyzed in terms of theories which account for the interfacial thickness on the basis of the intrinsic profile and capillary waves. For simple mixtures of organic compounds exhibiting a lower miscibility gap and aqueous systems of weak amphiphiles like oxyethylene butylether (n-C4E1 and i-C4E1) with an upper miscibility gap, the experimental results for ϱ are consistent with current composite theories of the interfacial thickness. However, for an aqueous system of a strong amphiphile (C10E4, a nonionic surfactant) the results for ϱ deviate significantly from the calculated values at temperature |T − Tc| > 2 K. This is attributed tentatively to the formation of a layer of surfactant in the interface of this water-continuous system.