Thickness of fluid interfaces near the critical point from optical reflectivity measurements

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Abstract
The optical reflectivity of fluid interfaces near the critical point is analyzed in the light of recently improved estimates of the surface tension and measurements of the bulk correlation length. In contrast to earlier analyses, it becomes possible to discriminate between the two leading phenomenological theories of fluid interfaces, the intrinsic structure theory originated by Maxwell and van der Waals and the capillary wave theory as developed by Buff, Lovett, and Stillinger. For one of the two systems studied, cyclohexane–methanol, capillary wave theory is found to fit the reflectivity data well, while intrinsic structure theory is in strong disagreement with the reflectivity data. A simple combination of the two theories does not significantly improve the performance of capillary wave theory. For the second system studied, sulfur hexafluoride, neither capillary wave theory nor intrinsic structure theory fits the reflectivity data well, while their combination gives partial agreement with experiment.