Monte Carlo simulations of model nonionic surfactants

The focus of this work is on aggregation and micellization in a lattice model for nonionic surfactants. Formation of micellar aggregates as a function of temperature and surfactant chemical potential was studied by histogram reweighting grand canonical Monte Carlo simulations. Two different sets of site-site interactions were utilized. The critical micellar concentration (CMC) was determined from the equation of state obtained from simulations in small systems. Results were obtained for the micellar size distributions. The introduction of effective repulsive site-site interactions between nearest-neighbour head-head and head-tail groups, in addition to the attractive tail-tail segmental interactions considered in earlier investigations, was shown to cause a significant decrease in the CMC. It was also found to attenuate the dependence of micellar size on surfactant chemical potential. The observed dependence of the CMC on temperature for the present model disagrees with experimental results for the n-alkyl m-ethoxyl series of surfactants in water. This is attributable to the failure of the model to take into account the (entropic) hydrophobic effect important for amphiphilic aggregation in aqueous solutions.