STRUCTURE OF THE VAPOR-LIQUID INTERFACE NEAR THE CRITICAL-POINT

We measured the thicknesses of the vapor-liquid interfaces near the cr itical points of carbon dioxide (CO2), sulfur hexafluoride (SF6), and trifluoromethane (CHF3) using ellipsometry. The data (when scaled by t he refractive index difference DELTAn and the correlation length xi) a re in agreement with other ellipticity data for binary and pseudobinar y mixtures at low pressures. Fully constrained theories of the interfa ce correctly predict the temperature dependence and scaling of the thi ckness but systematically overestimate the thickness itself by 15%-20% . The theory can be brought into agreement with experiment when an int rinsic interfacial stiffness is added to the theory. A novel feature o f the present measurements is that the effects from pressure-induced w indow strain were measured and mitigated by using a cylindrically-symm etric pressure cell with floating seals.