Molecular Dynamics of Graphene-Electrolyte Interface: Interfacial Solution Structure and Molecular Diffusion

Graphene-based applications often take place in aqueous environments, and they benefit from a molecular-level understanding of aqueous salt solutions in contact with graphene surfaces under different conditions. We study the aqueous solutions of electrolytes (LiCl, NaCl, KCl, MgCl2, and CaCl2) near the interface with a graphene sheet using classical molecular simulations. In order to model the graphene ion interactions accurately, we use the effective polarizable model of Williams et al. (J. Phys. Chem. Lett. 2017, 8, 703). In order to thoroughly characterize the solution structure at the graphene surface, in addition to standard structural properties, we employ our novel intermolecular bond definition based on the spatial distribution functions, which provides numbers of water-water and water-ion intermolecular bonds per water molecule and number of water molecules per ion as functions of the distance from the graphene surface in a completely self-consistent manner. This thus allows summations of the bonds and quantitative comparisons of the bonds between different species in the solution. Our analysis shows that the interfacial structure exhibits a competition between strong water structuring, formation of ion dense adsorption layers, and strong hydrogen and ion-water bonds in the solution; what is particularly interesting are the observed charge compensation and the mutual symmetries of intermolecular bonding. Finally, we evaluate the lateral mobility of water and ions separately in the interfacial and bulk regions, showing significant reduction of the dynamics of both the water and the ions in the interfacial region compared to the bulk phase.