The structure of the liquid-vapour interface of n-hexane + perfluoro- n-hexane along the path of critical composition through the critical end- point by neutron reflectivity

摘要

The surface structure of the mixture n-hexane + perfluoro-n-hexane has been determined using specular neutron reflection. Above the upper critical end point (UCEP) at 22.65℃ 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 of perfluoro-n-hexane exists in the approximate concentration range x(perfluoro-n-hexane) = 0.2-0.7 as reported in a previous paper. Just below the UCEP the surface is composed of a monolayer of perfluoro-n-hexane capping a macroscopically thick layer of the perfluoro-n-hexane-rich phase. Further below the UCEP the surface consists of the perfluoro-n-hexane-rich phase, the disappearance of the monolayer being attributed to its composition-dependent existence.