Self-assembly of the surfactant mixtures on graphene in the presence of electrolyte: a molecular simulation study

The insolubility of graphene nanosheets in aqueous media has been a limitation for the practical applications of this unique material. The non-covalent functionalization of graphene with mixed surfactants is an effective procedure for preparation of stable graphene dispersions. Physical adsorption of surfactants onto graphene surfaces is a significant stage in the dispersion of graphene nanosheets in the aqueous environment. Studies have shown that the presence of electrolyte greatly affects the adsorption process of pure surfactants on solid surfaces. Examination of mixed surfactants adsorption on graphene in the presence of electrolyte will help to better understand the mechanism of molecular interactions between the graphene nanosheets and mixed surfactants. Therefore, in this study, we used molecular dynamics simulations to investigate the adsorption of mixed surfactants on graphene in the presence of electrolyte and to study the morphology of assemblies formed from mixed surfactants on graphene. We investigate the effects of electrolyte concentration and surface coverage of surfactants' mixture on the adsorption process and structural changes in the assemblies formed on graphene nanosheets. We have found that increasing the ionic strength of electrolyte reinforces stretching of surfactants adsorbed on graphene toward the aqueous phase, and this leads to a clear volume expansion in the structure of graphene-surfactant mixture hemi-spherical micelles. In fact, screening effect of electrolyte ions on electrostatic repulsion between charged head-groups made molecules approach each other, leading to a more compact assembly of surfactants on graphene surface. As hemi-spherical micelles of mixed surfactants-graphene expanded, the steric repulsion between these micelles also increased, which in turn, inhibited the re-aggregation of graphene sheets covered with surfactants.