(592h) Coarse-Grained Molecular Dynamics Simulations of Self-Assembly of Ionic and Nonionic Surfactants

We apply two CG force fields, i.e. the Martini force field developed by Marrink et al. with a 4:1 mapping of heavy atoms to CG beads, and the SDK model developed by Shinoda et al. with a 3:1 mapping, to the self-assembly of ionic and nonionic surfactants in aqueous solutions. Although the SDK model accurately predicts a water/air interfacial tension (at 293K) to be 74 mN/m while it is ~33 mN/m with the Martini force field, both models result in similar micellar structures for both ionic sodium dodecyl sulfate (SDS) surfactants and nonionic alkyl poly (ethylene glycol) (CnEm) surfactants. The respective micelles have comparable sizes and very similar radial distribution functions calculated with these two CG models. However, the kinetic exchange of surfactants among SDS micelles in the Martini force field is slower than that in the SDK model. The quicker surfactant exchange in the SDK model is traced to the slightly lower energy barrier for a surfactant to escape from the micelle and the longer range electrostatic interactions implemented via the particle mesh Ewald summation. We show that even though both models greatly speed up the simulations compared to the atomistic force fields, it remains difficult to achieve equilibrium for the self-assembly of surfactants with hydrophobic tails longer than 8 or 9 carbons without extensive computational resources. Interestingly, when implicit water replaces the explicit waters in the Martini force field, the gains in the simulation speed allow this problem to be overcome and a reasonable equilibrium micelle size distribution is obtained for SDS. This opens the door for many other studies, such as adsorption and competitive adsorption of surfactants onto hydrophobic surfaces.