Dispersion of graphene using surfactant mixtures: Experimental and molecular dynamics simulation studies

The capability of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), cationic-rich and anionic-rich mixtures for dispersion of graphene nanosheets in an aqueous medium was assessed through both experimental and molecular dynamics (MD) simulation methods. The experimental results, especially measurements of ζ-potential showed that the dispersing power of pure SDS and the anionic-rich mixture is more than that of pure CTAB and cationic-rich mixture due to the smaller size of the SDS and more adsorption of it on the surface of graphene compared with CTAB. Nonetheless, stable dispersed graphene nanosheets were obtained at a lower total surfactant mixture concentration compared to when used alone, suggest the synergistic effect of catanionic mixtures. Regarding environmental impacts and costs, using surfactant mixtures for dispersing graphene nanosheets is promising. Moreover, MD simulations were used to examine surfactant mixture assemblies' structure on graphene and explain the experimental results, which showed that the random adsorption model first changes to the monolayer model and then the hemispherical model with an increased surfactant concentration. The ζ-potential about surfactant mixture-graphene assemblies was estimated using the results of MD simulation and Poisson's theory, and the results conformed to the experimental results favorably. Eventually, interactions between two surfactant mixture-graphene assemblies were evaluated through calculating the potential of mean force (PMF), and it was found that increasing surfactant surface coverage would lead to an enhanced repulsive barrier of PMF. Understanding the mechanism of interactions will help the design and selection of appropriate surfactants, the optimization of the process and the improvement of graphene dispersion in aqueous solutions of surfactants.