Revisiting OPLS-AA Force Field for the Simulation of Anionic Surfactants in Concentrated Electrolyte Solutions.

Abstract

Hereby, we developed a set of nonbonded parameters within all-atom optimized potentials for liquid simulations (OPLS-AA) force field for the simulation of concentrated electrolyte solutions of anionic surfactants. More specifically, the aim of this paper is to assess the performance of five sets of atomic charges calculated using different population analyses (DDEC6, CHelpG, CHelpG-SMD, RESP, and CM5), as well as the original set of charges used in the literature for sodium dodecyl sulfate (SDS) simulation. Recently, Farafonov et al. have revised the SDS OPLS-AA force field; however, we were unable to obtain the experimental rodlike micelles using this parameter set on long time scale. In fact, the initial SDS bilayer micelle adopted a rodlike shape transiently and then broke down into spherical micelles. Updating OPLS-AA force field with DDEC6, CHelpG, and CHelpG-SMD charges resulted in stable rod micelles for a long simulation time (1 μs). The atomic charges of Farafonov (taken from Shelley et al.), RESP, and CM5 could not correctly describe SDS in concentrated electrolyte solutions. Analysis of the interaction of SDS with the counterions and solvent highlights the role of a balance of the intermolecular forces that must be met to describe adequately the anionic surfactant electrolyte solutions. Further, the optimization of the SDS Lennard-Jones parameters enabled the Farafonov set to properly reproduce the experimental rod micelle. In addition, we have examined the performance of different parameters of sodium ions: the first developed based on the Kirkwood-Buff integrals (KBI) and the second developed by Joung et al. The excessive ion pairing caused by KBI parameters screens significantly SDS-water interactions, which stabilize the rod micelle. Further, a tight interaction of the Na+-SDS head group resulted in stabilization of the bilayer micelle as observed in the case of Na+ parameters developed by Joung et al.