Dispersion of non-covalently modified graphene in aqueous medium: a molecular dynamics simulation approach

Molecular dynamics were used to simulate the dispersion of graphene in aqueous medium in the presence of a novel organic modifier, sodium salt of 6-amino hexanoic acid (Na-AHA), which non-covalently modifies the graphene surfaces. The modifier molecule contains an ionizable carboxylate head group and an aliphatic tail. The extent of dispersion was estimated by calculating the potential of mean force (PMF) as a function of increasing concentration of the modifier using the thermodynamic perturbation method in conjunction with molecular dynamics simulations. With increasing concentration of the modifier, the PMF changed from a short-range strong attraction to a long-range repulsion at higher modifier concentrations. The simulation results clearly show the adsorption of modifier molecules at the graphene-water interface, which in turn causes the graphene surfaces to acquire a negative charge. Further, the development of a negative electric potential at the graphene surfaces induces a long-range electrostatic repulsion between the graphene sheets, clearly pointing to an electrostatic stabilization of Na-AHA modified-graphene in aqueous medium.