EXACT RESULTS FOR THE LOWER CRITICAL SOLUTION TEMPERATURE IN AN ASYMMETRIC MODEL OF AN INTERACTING BINARY MIXTURE

We have obtained exact results for the lower critical solution temperature (LCST) in a two-dimensional lattice model of an asymmetric binary mixture of large and small molecules (squares A and triangles B, respectively) in which both couplings AA and AB are present. We find that for a LCST to exist in addition to the upper critical solution temperature it is required that the free volume be nonzero, the Flory-Huggins coupling parameter 1T(AB-12AA) be positive, and that -AA be much larger than T. The last two conditions imply that both AA and AB must be negative. © 1994 The American Physical Society.