Chemical reaction driven phase transitions and critical points

Abstract

Two simple examples of model (mean-field) equations of state for phase equilibrium in chemically reactive systems are examined for ‘‘unexpected’’ phase equilibria. They are, essentially, exactly soluble and give classical critical behavior. One of these leads to a lower critical solution temperature that results in closed-loop coexistence curves similar to those seen in hydrogen-bonding mixtures. The second leads to less familiar, but interesting phase diagrams that exhibit a phenomenon analogous to critical azeotropy. The same phenomena occur in two examples of lattice gas models the partition functions of which can be mapped exactly to that of the Ising model thus resulting in nonclassical critical behavior. These models demonstrate how a chemical reaction can provide a mechanism leading to interesting phase equilibria and critical phenomena.