Mixing properties of two-dimensional lattice solutions of amphiphiles

Lattice Monte Carlo simulations of two-dimensional amphiphile solutions are used to examine the accuracy of the mixing properties predicted by lattice theories such as the Flory-Huggins theory, random-solution approximation, and quasichemical approximation. The internal energy, Helmholtz free energy , and entropy of mixing have been calculated from the configurational energ y data obtained from the simulations, and the effect of nonrandom mixing on these properties has been determined. The quasichemical approximation pred icts the entropy and Helmholtz free energy of mixing accurately for the amp hiphile solution, but fails to predict the energy of mixing, due to the pre sence of microphase (self-aggregation) separation, which is beyond the reac h of the quasichemical approximation, a mean-field theory. Helmholtz free e nergy of mixing is predicted accurately, and the shielding of the solvophob ic segments in the microphase leads to small energies of mixing compared to the entropy of mixing. (C) 2000 Academic Press.