Simulations of structural and dynamic anisotropy in nano-confined water between parallel graphite plates

Abstract

We use molecular dynamics simulations to study the structure, dynamics, and transport properties of nano-confined water between parallel graphite plates with separation distances (H) from 7 to 20 Å at different water densities with an emphasis on anisotropies generated by confinement. The behavior of the confined water phase is compared to non-confined bulk water under similar pressure and temperature conditions. Our simulations show anisotropic structure and dynamics of the confined water phase in directions parallel and perpendicular to the graphite plate. The magnitude of these anisotropies depends on the slit width H. Confined water shows "solid-like" structure and slow dynamics for the water layers near the plates. The mean square displacements (MSDs) and velocity autocorrelation functions (VACFs) for directions parallel and perpendicular to the graphite plates are calculated. By increasing the confinement distance from H = 7 Å to H = 20 Å, the MSD increases and the behavior of the VACF indicates that the confined water changes from solid-like to liquid-like dynamics. If the initial density of the water phase is set up using geometric criteria (i.e., distance between the graphite plates), large pressures (in the order of ~10 katm), and large pressure anisotropies are established within the water. By decreasing the density of the water between the confined plates to about 0.9 g cm(-3), bubble formation and restructuring of the water layers are observed.

Continuous melting through a hexatic phase in confined bilayer water

“…Computational studies agree that under two-dimensional nanoconfinement water is structured into layers perpendicularly oriented with respect to the confining direction (monolayer [10][11][12][13][14][15][16][17][18], bilayer [19][20][21][22][23][24][25][26][27][28][29], trilayer and so on [30][31][32][33][34]). Most of the computational studies agree with the existence of a stable monolayer square ice phase [10][11][12][13].…”

How square ice helps lubrication

“…Contrary to the TEM experiments, those simulations indicate an FCC-like lattice. Given the setup of the simulations, we believe that this FCC structure may have been related either to ice VI, or to the rhombic structure found at extremely low pressure in other simulations 12 . While the authors of Ref.…”

“…These studies have found that when water is confined between graphene sheets or graphite, there is clear structure in the direction normal to the surfaces but the in-plane order is typically either liquid-like or hexagonal. In the case of the hexagonal structure there are, however, different opinions on whether or not it is related to the graphite structure 12,14 . Confinement is, however, not required for the hexagonal structure to develop under certain conditions.…”

“…Water confined between hydrophobic surfaces exhibits a complex behavior. A number of highprofile experimental 5-10 and numerical [9][10][11][12][13][14][15]15,16 studies have investigated water confined using graphene or carbon nanotubes under various conditions. These studies have found that when water is confined between graphene sheets or graphite, there is clear structure in the direction normal to the surfaces but the in-plane order is typically either liquid-like or hexagonal.…”

Molecular Dynamics Simulations of Hydroxyapatite Nanopores in Contact with Electrolyte Solutions: The Effect of Nanoconfinement and Solvated Ions on the Surface Reactivity and the Structural, Dynamical, and Vibrational Properties of Water

“…This implies that the hydration shell of the sodium ion is more labile than that of calcium ions. Previous computational studies showed that in nanoconfined environments the diffusion of water is considerably slower than in the bulk [63,64]. However, compared with simulations of graphene oxide layers [63], where the water diffusion in a layer spacing of 15 Å converged to that of bulk water, in the HAP (H = 110 Å)-water system the value of D W (19.3 × 10 −10 m 2 ·s −1 ) is still 15% lower than in the bulk water (see Table 3).…”