Interfacial roughness in a near-critical binary fluid mixture: X-ray reflectivity and near-specular diffuse scattering

Measurements are presented of the X-ray specular reflectivity and near-spec ular diffuse scattering of the interface in a near-critical mixture of hexa ne and perfluorohexaae. A lineshape analysis of the scattered intensity at each temperature yields Values for the interfacial tension and interfacial width. The temperature variation of the tension and width so-obtained are c onsistent with current understanding of this interface, which holds that th ere is, firstly, an intrinsic width over which the fluid density Varies smo othly from one coexistence composition to the other; and, secondly, that th e interface acquires an additional and larger statistical interfacial width as a result of capillary fluctuations.