MONTE-CARLO SIMULATION OF MICROSTRUCTURAL TRANSITIONS IN SURFACTANT SYSTEMS

In a lattice model of mixtures of idealized surfactant, oil, and water molecules, the microseparation of hydrophobic components (oil and surfactant tails) from hydrophilic ones (water and surfactant heads) is simulated by a Monte Carlo technique. In water, symmetric surfactants, i.e., with heads as long as the tails, achieve lamellar or hexagonal-cylindrical order as the temperature is reduced; the lamellae and cylinders form at surfactant concentrations that are similar to the concentrations at which symmetric block copolymers mixed with homopolymers have been found to form these structures. The lamellae containing tails can have many holes; as the temperature is reduced the holes attain hexagonal order within each layer. At low concentrations in water, symmetric surfactants form spherical micelles; the size distribution of these is computed, as well as the critical micelle concentration. When the surfactant tail is larger than the head, the micelles are cigar shaped or cylindrical. Cylindrical micelles can intersect each other to form a bicontinuous phase. Other disordered bicontinuous phases, including a symmetric spongelike phase, are observed, and the validity of film theories for these phases is examined.