IntroductionIn 1975, Lang and Widom published their well-known arti-Ž . cle on liquid᎐liquid᎐liquid equilibria LLLE in systems of waterq benzene q ethanolq ammonium sulfate at atmo-Ž . spheric pressure Lang and Widom, 1975 . In that publication, equilibrium data are presented at different temperatures up to very close to the tricritical point, the temperature at which the three phases become identical. During experiments to determine the liquid᎐liquid phase behavior of water qbenzeneqcaprolactamqammonium sulfate systems, phase behavior similar to that for water qbenzene qethanolq ammonium sulfate systems was encountered.The practical importance of LLLE is, for example, for the waterqoilqsurfactant systems in enhanced oil-recovery processes, where capillary forces in reservoir pores are reduced by surfactant flooding. Also, for improving the performance of an extraction process, understanding of the phase behavior of systems exhibiting three liquid phases can be essential Ž . Garcia-Sanchez et al., 1996 . Equilibria of three liquid phases can occur when two-liquid phase regions overlap. LLLE received considerable interest, mainly inspired by the possibility of a tricritical point, as a test for theories of critical phenomena. Despite this degree of interest, the amount of Ž . three-liquid phase equilibrium data is limited Bocko, 1980 . For electrolyte solutions, most LLLE data were determined Ž . for systems with surfactants Kahlweit et al., 1988 et al., 2002 . For these systems, the patterns of the three-liquid phase equilibria with increasing salt concentration are similar Ž . to those investigated by Knickerbocker et al. 1979Knickerbocker et al. , 1982 for a variety of waterqhydrocarbonqalcoholqsalt systems.The phase-behavior pattern observed for LLLE in quaternary electrolyte solutions is a progression with increasing salt concentration from two liquid phases at low salt concentrations, a three-liquid phase region at higher salt concentrations, and again a two-liquid phase region at still higher salt concentrations. The three-phase region is limited by two critical tie-lines, where two of the three phases become identical. Ž In this pattern, temperature or pressure Sassen et al., 1989; . Andersen et al., 1999 can play a role similar to that of the salt concentration. With increasing temperature, the size of the LLLE-region decreases. At a given pressure in a quaternary system, a point with a certain temperature and composition exists, where the three coexisting phases become identi-Ž . cal. This is the tricritical point. Lang and Widom 1975 attempted to locate this tricritical point for the system waterq benzeneqethanolqammonium sulfate.The amount of literature on the modeling of LLLE using an excess Gibbs energy model is quite limited. Negahban et Ž . al. 1986Negahban et Ž . al. , 1988 In this article, LLLE are reported and modeled for the system waterqbenzeneqcaprolactamqammonium sulfate in a temperature range of 293 K to 330 K. The system waterq benzeneqcaprolactamqammonium sulfate is of imp...
