(692c) AQUA IONS-GRAPHENE INTERFACIAL and CONFINEMENT BEHAVIOR: Insights From Molecular Simulation and Their Practical Implications

The behavior of water at aqueous-solid interfaces has been extensively analyzed by theoretical, simulation and experimental approaches confirming our expectation that water's structural and dynamical properties are modified at the interfacial region. In particular, the behavior of water at carbon surfaces in the presence of ions has recently attracted considerable attention due to the practical interest associated with the electrochemistry underlying energy-storage devices and biosensor technology. Two different, yet interrelated, physical scenarios are of crucial interest here: (a) the formation of the inhomogeneous solid-fluid interfacial (SFI) region when a fluid becomes in contact with a solid surface, and (b) the overlapping of two approaching SFI regions to become confined-fluid environments. Differential ion hydration in confined environments plays a central role in defining the inhomogeneous distribution of species, and consequently, all properties associated with the potential overlapping of electric double layers. In this context molecular simulation becomes a versatile tool to link every microscopic details of the intermolecular forces describing the system and the resulting structural and dynamical behavior of the SFI and confined fluid. For that purpose, and as part of an ongoing investigation, here we discuss the behavior of metal-chloride aqueous solutions in contact with and under confinement between (both charged and uncharged) graphene plates as predicted by isothermal-isobaric molecular dynamics simulation. We place emphasis on the distinct hydration behavior of lithium(+1), barium(+2), and yttrium(+3) for plates separation between 9Å-15Å, spacing range where de-solvation and ion expulsion appear to be a key mechanism underlying the structural behavior of the overlapping SFI. The simulation systems are set in such a way that we simultaneously characterize the behavioral differences between the species under interfacial and confinement effects, while keeping the same global (PTX) state conditions. This characterization involves the axial profiles of species concentrations, charge density, electric field as well as potential, as well as interpreted in terms of the hydrogen bonding structure, ion-coordination, and surface charge. Finally, we discuss the electrochemical implications of the predicted behavior and some modeling strategies to address these issues properly.