THE STRUCTURE OF THE LIQUID-VAPOR INTERFACE OF N-HEXANE PLUS PERFLUORO-N-HEXANE ALONG THE PATH OF CRITICAL COMPOSITION THROUGH THE CRITICALEND-POINT BY NEUTRON REFLECTIVITY

The surface structure of the mixture n-hexane + perfluoro-n-hexane has been determined using specular neutron reflection. Above the upper cr itical end point (UCEP) at 22.65 degrees C the preferential adsorption of the perfluoro-n-hexane is present as a macroscopic layer, becoming richer in perfluoro-n-hexane as the UCEP is approached. This adsorbed layer is capped by a monolayer of perfluoro-n-hexane. The monolayer o f perfluoro-n-hexane exists in the approximate concentration range x(p erfluoro-n-hexane) = 0.2-0.7 as reported in a previous paper. Just bel ow the UCEP the surface is composed of a monolayer of perfluoro-n-hexa ne capping a macroscopically thick layer of the perfluoro-n-hexane-ric h phase. Further below the UCEP the surface consists of the perfluoro- n-hexane-rich phase, the disappearance of the monolayer being attribut ed to its composition-dependent existence.