Micellization and phase separation of diblock and triblock model surfactants

The phase behavior and micellization of several model lattice diblock and triblock surfactants have been investigated by histogram-reweighting grand canonical Monte Carlo simulations. By studying the system-size dependence of the calculated phase diagrams, it has been found that for the cases studied (for which interactions are short ranged and temperature independent) each surfactant system either micellizes or phase separates, but never both. These results suggest that the experimentally observed behavior, where the same aqueous surfactant solution shows both phase separation and micellization under different conditions, is a consequence of the unusual solvation properties of water. The tendency to self-assemble is responsible for appreciable deviations from quasichemical theory even in systems that do not form micellar aggregates but are close to the boundary of macroscopic phase separation. For the micelle-forming systems, the surfactant volume fraction at the critical micellar concentration, φ_cmc, has been calculated from the point where a change of slope in the osmotic pressure versus surfactant volume fraction plots is observed. In all cases investigated, φ_cmc was found to increase with increasing temperature. As a consequence, positive values of the heat of micellization were obtained. For surfactant architectures close to macroscopic phase separation, the cluster size distributions are broad and extend to very large aggregation numbers indicating the presence of elongated micellar aggregates. This was also confirmed by an examination of typical configurations. Triblock systems, with symmetric architecture, behave in a similar manner, and architectures where the solvent-insoluble block is on the outside tend to phase separate over a broader range of parameter space than the triblock where the middle block is solvophobic. These results provide a growing understanding of the role of interactions and chain architecture on the self-assembly of surfactant systems and can be employed to benchmark existing theories in this area.