(180g) Analysis of Thermodynamic Properties of Coarse Grained Surfactant Models

Surfactants are used for a wide range of industrial and biological applications. Composed of a hydrophilic head group and a hydrophobic tail group, they self-assemble into various structures in aqueous solutions. Atomistic models cannot reach the necessary time and length scales for self-assembly in dilute solutions. Therefore, various coarse grained models have been developed. A recent group of coarse grained models provides semi-quantitative information. However, these models have not been tested on their ability to reproduce experimentally measured critical micelle concentrations (cmc's). In this work, we analyze the MARTINI model (Marrink et al., J. Phys. Chem. B, 2004, 108, 750-760; Marrink et al., J. Phys. Chem. B, 2007, 111, 7812-7824) and the model developed by Shinoda and coworkers. (Shinoda et al., Molecular Simulation, 2007, 33, 27-36), for their ability to reproduce the cmc's of various zwitterionic and non-ionic surfactants. We obtain long (several ns) molecular dynamics trajectories at multiple temperatures allowing the determination of cmc's and aggregate size distributions and their trends. Our results suggest that that the models underpredict the critical micelle concentrations by 1-2 orders of magnitude for zwitterionic surfactants, with smaller but still significant deviations for nonionic ones. While the monomer surfactant concentrations reach equilibrium, the aggregate size distributions do not reach equilibrium. Overall, while the results from these models are in the correct regions, the underprediction indicates the need of further refinement in order to create quantitative, transferrable models for surfactant self-assembly.