Tricitical and Critical-End-Point Behavior of Interfacial Tensions in Fluid Mixtures

Interfacial tensions $\sigma$ in the tricritical region of mixtures of ethane with higher alkanes were measured by surface light scattering. At each of the two critical end points $\sigma$ for the noncritical interface shows the predicted behavior: $\sigma$ and $\frac{d\sigma }{\mathrm{dT}}$ appear continuous while $\frac{d^2\sigma }{d{T}^2}$ changes sign. Results for the $\alpha \gamma$ interface suggest high adsorption. The tricritical exponent for $\sigma$ varies across the three-phase region and appropriate ratios of $\sigma$ do not approach unity, in disagreement with a van der Waals-type theory of interfaces.