Water Under Extreme Graphene Confinement: Surface Corrugation Effects On the Wet-Dry Transition

The wetting of solid surfaces by water (or any other fluid for that matter) is a phenomenon of relevance involving natural biochemical and geochemical processes as well as energy-related industrial applications, and current evidence indicates that the surface wettability becomes strongly affected by the surface topography (corrugation).   In fact, it has been known for some time that surface roughness can significantly modify the contact angle of a fluid, a phenomenon frequently described in terms of the meso-scale models of Wenzel [1] and Cassie[2].   However, theory and simulation of solid-water interfacial phenomena involving nanoscopically-flat surfaces indicate that the interfacial region extends only a few molecular diameters; consequently, the effect of surface roughness on interfacial phenomena might become more significant when the surface roughness is within the same characteristic length-scale, i.e., of a few angstroms [3].  

More importantly, it is not just the effect of surface roughness on the solid-water interfacial behavior ¾ on the inhomogeneous solid-fluid interfacial (SFI) region when a fluid becomes in contact with a solid surface ¾ but also, the compounding effect resulting from the overlapping of approaching SFI regions with the formation of confined-fluid environments.  Because experimental nano-scale studies of SFI's are significantly challenging and their outcome must be interpreted in terms of pre-defined models, the molecular simulation of precisely defined realistic model systems can provide direct molecular-based microscopic information to aid the understanding of microscopic processes and the relationships between wetting and surface structure/roughness.   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 water at and under confinement of SFI's.   

Therefore, the overarching goal of this effort is the molecular-based investigation of the surface-corrugation effect of finite-size graphene plates on the interfacial and confinement behavior of water at ambient conditions, to address key issues regarding the microscopic links between the strength of the graphene-water interactions, the evolution of their hydro-philic/phobic nature with plate corrugation, and the impact on the local microstructural and thermodynamics properties of interest.  For that purpose, we performed isothermal-isobaric molecular dynamics simulations to study the behavior of water under confinement between uncharged graphene plates, to simultaneously characterize the behavioral differences between water at interfacial and under confinement, while in equilibrium with its own bulk [4]. Using the slit-pore (graphene flat-plates) configuration as a reference system, we introduce four precisely defined corrugation patterns that preserve the original pore volume, a feature that facilitate 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, as well as interpreted in terms of the corresponding profiles for hydrogen bonding structure and water coordination. 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 quasi-elastic neutron scattering (QENS) experiments by calculating the simulated scattering spectra and interpreting them in terms of microscopic information from the corresponding molecular dynamics simulation [5].

            [1]       Wenzel, R. N. Industrial & Engineering Chemistry 1936, 28, 988-994.

            [2]       Cassie, A. B. D.; Baxter, S. Transactions of the Faraday Society 1944, 40, 546-551.

            [3]       Striolo, A. Adsorption Science & Technology 2011, 29, 211-258.

            [4]       Chialvo, A. A.; Cummings, P. T. The Journal of Physical Chemistry A 2011, 115, 5918-5927.

[5]       Mamontov, E.; Wesolowski, D. J.; Vlcek, L.; Cummings, P. T.; Rosenqvist, J.; Wang, W.; Cole, D. R. Journal of Physical Chemistry C 2008, 112, 12334-12341.