(704g) Concentration Effects in Simulations of Non-Ionic and Ionic Surfactant Micellization

Surfactants are well known for self-assembling into micellar aggregates from free surfactants at the critical micelle concentration (cmc). Although some micellar properties above the cmc are assumed to be constant, such as the free surfactant concentration and the degree of co-ion association, experiments and simulations have shown that the free surfactant concentration decreases above the cmc. We have performed histogram reweighting grand-canonical ensemble Monte Carlo simulations supplemented with cluster moves to study the micellization of implicit-solvent, non-ionic lattice¹ and ionic off-lattice² surfactants. Molecular dynamics simulations of the ionic surfactants were also performed for large systems. We calculate the cmc, osmotic pressure, concentration of free surfactants and aggregation number distributions³. For ionic surfactants, we also obtain the degree of co-ion association. Above the cmc, the free surfactant concentration decreases for both non-ionic and ionic surfactants. From simulation configurations, we offer an explanation for the decrease of free surfactants using a methodology developed for non-ionic surfactants to calculate the excluded volume³ and theory developed for ionic surfactants⁴. Our finding suggest that simulation studies that use the free surfactant concentration at high surfactant loadings as a proxy for the cmc provide underestimates, potentially by large factors, of the true cmc. Models are proposed for reliable extrapolations of results from simulations at high loadings back to the cmc.

[1] Larson, R. G. “Monte-Carlo Lattice Simulation of Amphiphilic Systems in 2 and 3 Dimensions”, J. Chem. Phys. 1988, 89, 1642-1650

[2] Jusufi, A., Hynninen, A.P., Panagiotopoulos, A.Z., “Implicit Solvent Models for Micellization of Ionic Surfactants”, J. Phys. Chem. B, 2008, 112, 13783-13792

[3] Santos, A.P., Panagiotopoulos, A.Z., “Determination of the critical micelle concentration in simulations of surfactant systems”, J. Chem. Phys., 2016, 144, 044709

[4] Jusufi, A., LeBard, D.N., Levine, B.G., Klein, M.L., “Surfactant Concentration Effects on Micellar Properties”, J. Phys. Chem. B, 2012, 116, 987-991