Understanding Surfactant Stabilization of MoS2 Nanosheets in Aqueous Dispersions from Zeta Potential Measurements and Molecular Dynamics Simulations

The sonication-assisted exfoliation of MoS2 in aqueous media in the presence of ionic surfactants to give stable dispersions is an attractive procedure for obtaining single or few-layered nanosheets, as it is easily scalable and does not involve toxic or high boiling solvents. Here, we have investigated the origin of the stability of aqueous dispersions of MoS2 nanosheets obtained by sonication in the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB) by zeta potential measurements at different ionic strengths and molecular dynamics (MD) simulations. Our measurements show that the dispersions are stabilized by electrostatic repulsive interactions between the delaminated MoS2 nanosheets, which acquire a positive charge because of the adsorption of the cationic surfactant. MD simulations were performed to understand the interaction between MoS2 nanosheets and the CTAB surfactant chains in the dispersion and the structure and arrangement of the adsorbed surfactant chains. Our simulations are able to reproduce the experimentally measured variation of the zeta potential with ionic strength. In addition, the relative contribution and role of different intermolecular interactions between various components of the dispersion was estimated by simulating the potential of mean force (PMF) between two surfactant-adsorbed MoS2 sheets. On the basis of experiment and simulations, we are able to establish that the stability of aqueous dispersions of MoS2 in the presence of an ionic surfactant can be understood based on classical models of charged interfaces.