Probing Surfactant Self-Assembly and Calculating Critical Micelle Concentrations Using Molecular Dynamics Simulations

Abstract

The effect of concentration and surfactant architecture on aggregation behavior properties such as the free oligomer concentration, mean aggregation number, and inaccessible volume are investigated using parallelized molecular dynamics simulations of a coarse-grained, explicit-solvent model for non-ionic surfactants run on graphics processing units. The computational cost of explicit-solvent simulations at concentrations dilute enough to be near the critical micelle concentration (cmc) is often prohibitive due to the necessity of simulating a large number of solvent beads. Thus, simulations at higher concentrations are used to estimate the cmc. Prior studies have estimated the cmc to be the free oligomer concentration at high total concentrations, despite evidence of a decrease in the free oligomer concentration with total concentration. In the present study, we apply a method previously developed for a lattice model for surfactants to correct the free oligomer concentration and produce a more accurate estimate of the cmc at high total concentrations. We also identify differences in the efficacy of this correction with surfactant architecture and find comparable results to those found for the lattice model. Prior studies have also attempted to equilibrate the more hydrophobic surfactant systems. In the present study, we apply parallel tempering and a more recently developed temperature extrapolation method to one hydrophobic system. The findings in this study can be applied to calculate the cmc's of more complex and realistic models for self-assembling systems more efficiently and accurately.