Molecular Dynamics Simulation of Mixed Surfactants Adsorption on Graphene Nano-Sheets: Effects of Temperature, Electrolyte, and Alcohol

An effective method for the preparation of stable dispersions of graphene is the direct exfoliation of graphene from graphite flakes in the aqueous solution of surfactants. Physical adsorption of surfactants on graphene surfaces is an important step in dispersing and stabilizing exfoliated graphene sheets in the aqueous medium. Dispersion of graphene with a mixture of surfactants is an effective way to obtain stable graphene sheets. The effect of synergism on a mixture of surfactants can reduce the total concentration of surfactants required for a particular application. The present study employed molecular dynamics simulation to investigate the adsorption of mixed cationic-rich and anionic-rich surfactants onto armchair graphene. We investigated the effects of temperature, electrolyte, and alcohol in the aqueous solution of surfactants on the adsorption process to understand the adsorption and self-assembly mechanism of surfactant mixtures on the graphene surface and also to better optimize the graphene dispersion process. The simulation results suggested the improved stability of these systems by adding an electrolyte to the aqueous solution of the surfactants. The screening effect of electrolyte ions on the electrostatic repulsion between groups of charged heads of surfactants has led to a denser accumulation of surfactants on graphene and more favorable interactions between them. Increasing temperature, however, reduced the system's stability by desorbing the mixed surfactants from the graphene surface. Comparing the surfactant molecules and graphene in terms of energy levels of Lennard-Jones interactions with and without alcohol also showed increased interactions in the absence of alcohol, which helps improve the system stability.