Revised Implicit Solvent Model for the Simulation of Surfactants in Aqueous Solutions. 2. Modeling of Charged Headgroups at Oil-Water Interface.

The revised implicit solvent model (ISM-2) for the simulation of cationic surfactants in water was proposed in the previous study (J. Phys. Chem. B 2005, 109, 11762):  no water molecules of the solvent are explicitly treated, and their effects are incorporated using the solvent-averaged interactions between the surfactant segments in water, where the interactions between the hydrocarbon sites of the surfactants are allowed to vary depending on their surroundings. In the present study, the representation of a charged headgroup at the liquid-liquid interface between the hydrocarbon oil and the implicit water has been improved, where the free energy change due to the transfer of the charged headgroup across the interface is taken into account. The present model (ISM-3) has been applied to the molecular dynamics simulations of (i) the single preformed micelle of 30 n-decyltrimethylammonium chloride (C10TAC) cationic surfactants in water and (ii) 343 C10TAC surfactants uniformly dispersed in water, where the corresponding systems are also simulated using the ISM-2 for comparison. The first simulations showed that the ISM-3 as well as the ISM-2 is applicable to the simulation of the preformed micelle of the average aggregate size for C10TAC. The second simulations demonstrated that the ISM-3 can represent the surfactant self-assembling plausibly, while the ISM-2 fails to do so because of the rude treatment of the charged headgroups at the interface. The results will be compared with those from experiments and atomistic model simulations.