Monte Carlo simulations of a lattice model for amphiphile/solvent systems are used to obtain data for the configurational energy of mixing. Based on the simulation results, the energy of mixing is correlated as a function of the interaction energy and volume fraction of the amphiphile using a Redklich-Kister correlation. The Helmholtz free energy of mixing is obtained by using the Gibbs-Helmholtz relation with the athermal entropy of mixing prediction from the quasichemical approximation as boundary condition. Using this information, an analytic expression for the chemical potential of the amphiphile and solvent is obtained, and the phase behavior for amphiphiles with different structure is obtained. The expressions generated in this work produce coexistence curves that are more accurate than those obtained from the quasichemical approximation.