Micellization and phase separation in binary amphiphile mixtures

Grand canonical Monte Carlo simulations with histogram reweighting are used to examine the micellization and phase separation of a lattice model for binary amphiphile mixtures in an implicit solvent. The model amphiphiles consist of a neutral, rigid head group that occupies seven lattice sites and solvophobic tail chains of varying length and number. It is found that the critical micelle concentration of the mixtures closely matches theoretical values obtained by assuming that the components form an ideal mixture. The solubility of amphiphiles which undergo a first-order transition to form a disordered liquid phase can be increased by adding an amphiphile that forms micelles, and the phase transitions in these mixtures are between a dense liquid state and a low concentration phase that contains aggregates. In both micelles and dense liquids, the amphiphiles mix indiscriminately and the micelle size and shape depends on the concentration ratio of the two species.