(424b) 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.