The liquid–vapor interface of a binary liquid mixture near the consolute point

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Abstract
The liquid–vapor interface above mixtures of isopropanol (i-C3H7OH) and perfluoromethylcyclohexane (C7F14) has been studied in the vicinity of the consolute point (Tc=363 K). As three-phase coexistence is approached, the excess fluorocarbon adsorbed at this interface increases; the adsorption is expected to diverge at Tc for a mixture of the critical composition. A simple model of the interface which incorporates the adsorption anomaly is compared with our ellipticity measurements. Both the model and our data yield ellipticities which have a finite maximum at 0.1 K above Tc. (In general, the ellipticity is not a monotonic function of the adsorption.) The calculation of the ellipticity uses an exact numerical integration of Maxwell’s equations for a model dielectric constant vs height profile. The model dielectric constant profile for the critical composition is consistent with a short-ranged density vs height profile between the vapor and the liquid mixture as well as a much longer ranged composition vs height profile expected at a boundary of a mixture near its consolute point. The liquid–vapor correlation length (0.6 nm) was estimated from the ellipticity data far from Tc. The correlation length for the composition profile was found by combining our interfacial tension data taken below Tc with a compilation based on two-scale-factor universality. Ellipticity data for noncritical compositions are presented; however, their analysis will be presented elsewhere.