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

Interfacial tensions σ 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 σ for the noncritical interface shows the predicted behavior: σ and dσdT appear continuous while d²σdT² changes sign. Results for the αγ interface suggest high adsorption. The tricritical exponent for σ varies across the three-phase region and appropriate ratios of σ do not approach unity, in disagreement with a van der Waals-type theory of interfaces.