International Journal of Thermophysics, Vol. 25, No. 5, September 2004 (© 2004) Interfacial Behavior in Type IV Systems 1 A. Mej´ ıa 2, 3 and H. Segura 2 Type IV mixtures exhibit two heteroazeotropic lines, one at low tempera- ture and the other meeting the supercritical range, characterized by the prox- imity of their critical end points (CEPs). Between these CEPs, the liquid phase is homogeneous inside a narrow range of temperature. The aim of this work is to analyze interface properties and wetting transitions for Type IV mixtures. Interfacial tensions have been calculated by means of the gradient theory, applied to binary van der Waals fluids. This approach is able to pre- dict interfacial tension and phase equilibrium using a common equation of state (EOS). Results show that interfacial properties and wetting conditions are governed by the densities and the number of phases involved in equilib- rium, a scenario that changes as temperature evolves from the low- to the high-temperature heteroazeotropic line. KEY WORDS: interface properties; square gradient theory; Type IV behav- ior; wetting transitions. 1. INTRODUCTION Coexisting bulk fluid phases in thermodynamic equilibrium are connected by an interfacial fluid, whose concentration ρ , varies spatially between its bulk fluid phases. Fig. 1. shows, schematically, the typical pattern of ρ as a function of a spatial coordinate z, for a liquid in equilibrium with its vapor. For mixtures, the interfacial fluid may be enclosed by gas/liquid, liquid/liquid, or gas/liquid/liquid bulk fluids, and its ρ(z) behavior is a function of pure fluids and their bulk densities. In fact, ρ(z) may be or not be a monotonic function, as illustrated in Fig. 2. 1 Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A. 2 Department of Chemical Engineering, Group of Thermodynamics, Universidad de Concepci ´ on, POB 160-C, Concepci´ on, Chile. 3 To whom correspondence should be addressed. E-mail: amejia@diq.udec.cl. 1395 0195-928X/04/0900-1395/0 © 2004 Plenum Publishing Corporation