(363e) Surface Corrugation Effects On the Behavior of Water Under Extreme Graphene Confinement
Significant theoretical, experimental and simulation efforts have been aimed at understanding the behavior of water in contact with solid surfaces. The emerging picture from these studies confirms the expectation that the structural and dynamical properties of water are significantly perturbed at the fluid-solid interfaces. The structural perturbation is characterized by an inhomogeneous local density distribution whose dynamical counterpart translates into anisotropic behavior of the resulting translational and rotational molecular motions.
An interfacial phenomenon of great relevance in biochemical and geochemical processes as well as energy-related industrial applications, is solid surface wettability. In fact, current evidence points to the strong effect of surface topography (i.e., curvature and corrugation) on the surface wettability, whose most obvious macroscopic evidence is the modification of the fluid’s contact angle, a phenomenon frequently described in terms of the meso-scale models of Wenzel  and Cassie .
Molecular-based simulation of solid-water interfacial phenomena at nanoscopically-flat surfaces show that the interfacial regions extend only a few molecular diameters, therefore, any surface corrugation-driven interfacial phenomenon might be significantly enhanced as the surface corrugation becomes commensurate to the solvent molecular characteristic length-scale, i.e., a few angstroms for water .
Our current work has been motivated by the experimental evidence of the strong influence of surface roughness on the wettability of carbon surfaces in contact with aqueous environments, as well as the macroscopic and controversial Wenzel’s prediction of an enhancement of the solid-fluid hydro-philicity (phobicity) with increasing roughness (corrugation) over that of the corresponding flat surface.
Therefore, the aim of this work is to provide essential molecular-based insights into the hydro-phobic/philic nature of the water-(corrugated graphene) surface interactions, by addressing the following central issues: (a) the link between the corrugation-driven hydration free energy changes in the association process involving graphene plates and the resulting water-graphene interfacial tension, (b) the simultaneous effects of surface corrugation plus confinement on the water’s thermodynamic response functions and the resulting macroscopic modeling implications, (c) the ill-defined nature of Wenzel formula to describe the water-corrugated graphene interfacial behavior at the nanoscale.
For that purpose, we performed isobaric-isothermal (locally grand canonical) molecular dynamics simulations to study the behavior of water under extreme confinement between uncharged finite-sized graphene plates, to simultaneously characterize the behavioral differences between water at interfacial and under confinement, while in equilibrium with its own bulk [4,5]. Using the slit-pore (graphene flat-plates) configuration as a reference system, we introduce precisely defined corrugation patterns that preserve the original pore volume, a feature that facilitates the interpretation of simulation results. The characterization involves the axial profiles of the potential of mean force (PMF), local isothermal compressibility, diffusivity, and thermal expansivity. Moreover, the profiles of the solvent contribution to the PMF in conjunction with that of the local isothermal compressibility are then used to assess the surface-corrugation effects on the hydro-philic/phobic nature of the water-graphene interactions and on the (oscillatory) onset of wet-dry transition. We also make contact with quasielastic neutron scattering (QENS) experiments by calculating the simulated scattering spectra and interpreting them in terms of microscopic information from the corresponding molecular dynamics simulation .
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