Theory of lower critical solution points in aqueous mixtures

Two decorated lattice models of hydrogen bonded mixtures are presented that exhibit lower critical solution temperatures and closed-loop coexistence curves and that account for the strongly asymmetric closed-loop coexistence curves found in many aqueous mixtures. The models are extensions of an earlier decorated lattice model that produces only symmetric closed-loop curves. They incorporate asymmetries in both directional and nondirectional energies as well as the possibility of multiple hydrogen bonding in water. The models are exactly soluble in terms of the spin-1/2 Ising model, and exhibit nonclassical critical behavior at both upper and lower critical solution temperatures. The hydrogen bond energies in the models are in good agreement with those in real systems, and one of the models gives coexistence curves that are in reasonable quantitative agreement with most of the mixtures considered.