Universal adsorption at the vapor-liquid interface near the consolute point

The ellipticity of the vapor-liquid interface above mixtures of methylcyclohexane (C₇{H}₁₄$) plus perfluoromethylcyclohexane (${C}₇F₁₄) has been measured near the consolute point Tc=318.6 K. The data are consistent with a model of the interface that combines a short-ranged density-versus-height profile in the vapor phase with a much longer-ranged composition-versus-height profile in the liquid. The one-parameter model profile fits the data well for reduced temperatures t=(T-Tc)/Tc>7×{}10^-4. The model predicts a maximum ellipticity at t=7×{}10^-4, which occurs at t=2×{}10^-4 in the experiment. The value of the free parameter produced by fitting the model to the data is consistent with results from two other simple mixtures and a mixture of a polymer and solvent. This experiment combines precision ellipsometry of the vapor-liquid interface with in situ measurements of refractive indices of the liquid phases, and it precisely locates the consolute point.