CHEMICAL FREEZING OF PHASE-SEPARATION IN IMMISCIBLE BINARY-MIXTURES

We discuss the thermodynamic and kinetic conditions under which chemic al reactions may prevent the coarsening terminating spinodal decomposi tion and freeze the unmixing of binary mixtures at some early, pattern forming, stage of evolution. Under very general conditions, we establ ish that (i) this pattern freezing phenomenon can only occur in nonequ ilibrium systems the level of dissipation of which exceeds a finite, n onzero threshold value; (ii) at least two independent chemical process es must take place; (iii) chemistry must be destabilizing, which requi res that at least one of these processes must be autocatalytic; (iv) p attern formation is possible even outside of the spinodal region, i.e. , without involving a phase separation phenomenon, in unsymmetrical mi xtures where the potential energies between pairs of identical particl es are sufficiently different. This latter condition replaces for noni deal chemically reacting binary mixtures the unequal diffusion coeffic ients condition which governs the appearance of Turing patterns in the classical reaction-diffusion theory.