Phase separation and liquid crystal self-assembly in surfactant-inorganic-solvent systems

The behavior of surfactant-inorganic oxide-solvent systems is studied using lattice Monte Carlo simulations. Under no inorganic condensation conditions, these systems phase separate into a liquid crystal phase that contains mainly surfactant and inorganic oxide, in equilibrium with a solvent-rich phase. In the systems studied, the solvent and the inorganic oxide have favorable interactions with the surfactant head, but the inorganic oxide-surfactant interactions are stronger than the solvent-surfactant interactions, which leads to a phase separation, regardless of the oxide-solvent miscibility. The formation of ordered liquid crystal phases is observed in the phase containing a high surfactant concentration, and the structure of this phase depends on the system composition and strength of the interactions. The formation of hexagonal and lamellar structures at different conditions is in qualitative agreement with experimental evidence on the formation of surfactant-silica liquid crystals and the synthesis of MCM-41 type materials. The effects of temperature and surfactant architecture are also investigated. We show that the increase in surfactant solubility in the solvent-rich phase with temperature can result in a lamellar to hexagonal transformation and that surfactants with small head/tail ratios favor the formation of lamellar phases.