Investigation of the mixing behavior of surfactants by lattice Monte Carlo simulation.

We have studied the behavior of binary surfactant mixtures using the Monte Carlo (MC) simulation technique with a three-dimensional lattice model of a binary surfactant mixture, in which the constituent surfactant species are represented by a series of connected grid sites. Head-head interactions, alone and along with tail-tail interactions, among identical surfactant species were varied to imitate non-ideal mixing and to manipulate the net attractions and repulsions between surfactant species. We found that the head-head and tail-tail interactions affect both the mixed critical micelle concentration and distribution of aggregates. The simulation results are analyzed in the light of the phase separation model, which considers micelles as separate bulk pseudo-phase. Our studies reveal that regular solution theories do not present a satisfactory description for such systems. The discrepancies observed between the theoretical and simulation results for the studied systems could be attributed to the nonrandom mixing effect in simulation, which is neglected in regular solution theory.