Development of a Coarse-Grained Model for the Surfactant Family of Linear Alkylbenzene Sulfonates

Abstract

A coarse-grained (CG) model has been developed for the anionic surfactant class, linear alkylbenzene sulfonates (LAS), which are the most widely used synthetic surfactants. The development work started from a systematic examination of tens of CG water models with different resolutions, interaction potentials (Lennard-Jones and Morse), and cut-off distances. The relationships between the parameters under specific choices of the above options and the thermodynamic properties, such as density, surface tension, and compressibility, were found to fit simple mathematical equations. The limits of applicability of these CG water models were explored by checking the melting temperature. Considering both efficiency and accuracy, a CG water model which includes three water molecules in one CG site was chosen. Correspondingly, the LAS molecules were mapped into CG sites each contains approximately three heavy atoms and connected hydrogens. Structural data obtained from atomistic simulations and thermodynamic data from experiments were used as targets to parameterize standard potential forms for bonded and non-bonded interactions. An extensive evaluation of the CG model for a series of different alkane molecules (aliphatic or aromatic, linear or branched) shows that the present model is not only reliable, but also transferable. This point is crucial to assure that the model is capable of representing different isomers and homologues in the LAS family. The resulting model is easily implemented into standard MD codes. The added computational efficiency permits the simulation of the self-assembly of LAS solutions starting from a random configuration. The model is shown to accurately reproduce the phase behavior of solutions of pure isomers of sodium dodecylbenzene sulfonate, despite the fact that phase behavior was not directly taken into account in the parameterization.