Aqua Ions-Graphene Interfacial and Confinement Behavior: Insights from Isobaric-Isothermal Molecular Dynamics

We carry out a systematic microstructural characterization of the solid-fluid interface (SFI) of water and simple metal chloride aqueous solutions in contact with a freestanding plate or with two such plates separated by an interplate distance 0 <= h (angstrom) <= 30 at ambient conditions via isothermal-isobaric molecular dynamics. With this characterization, we target the interrogation of the system in search for answers to fundamental questions regarding the structure of the "external" and "internal" (confined) SFIs, the effect of the differential hydration behavior among species, and its link to species expulsion from confinement. For water at ambient conditions, we found that the structure of the "external" SFIs is independent of the interplate distance h in the range 0 <= h (angstrom) <= 30, that is, the absence of wall-mediated correlation effects between "external" and "internal" SFIs, and that for h < 9 angstrom the slit-pores dewet. Moreover, we observed a selective expulsion of ions caused by the differential hydration between the anion and the cations with a consequent charging of the slit-pore. All these observations were interpreted in terms of the axial profiles for precisely defined order parameters, including tetrahedral configuration, hydrogen bonding, and species coordination numbers.