Water under extreme confinement in graphene: Oscillatory dynamics, structure, and hydration pressure explained as a function of the confinement width

Abstract

Graphene nanochannels are relevant for their possible applications, as in water purification, and for the challenge of understanding how they change the properties of confined liquids. Here, we use all-atom molecular dynamics simulations to investigate water confined in an open graphene slit-pore as a function of its width w, down to sub-nm scale. We find that the water translational and rotational dynamics exhibits an oscillatory dependence on w, due to water layering.The oscillations in dynamics correlate with those in hydration pressure, which can be negative (hydrophobic attraction), or as high as ∼ 1 GPa, as seen in the experiments. At pore widths commensurable with full layers (around 7.0Å and 9.5Å for one and two layers, respectively), the free energy of the system has minima, and the hydration pressure vanishes. These are the separations at which the dynamics of confined water slows down. Nevertheless, the hydration pressure vanishes also where the free energy has maxima, i.e., for those porewidths which are incommensurable with the formation of well-separated layers, as w 8.0Å. Around these values of w, the dynamics is faster than in bulk, with water squeezed out from the pore. This behavior has not been observed for simple liquids under confinement, either for water in closed nano-pores. The decomposition of the free energy clarifies the origins of the dynamics speedups

Nanoconfined Fluids: Uniqueness of Water Compared to Other Liquids

“…We find that the slit-pore acceptance capacity, defined as the number of confined particles N s / A s normalized by the subvolume area A s , for both fluids has a step-like behavior as a function of δ ( Figure 2 a,c). These steps resemble what has been found for water under similar confinement, 96 , 97 and it is a result of the layering. Indeed, the comparison with Figures S1 and S2 shows that a step starts at values of δ where a new layer appears ( e .…”

“… Longitudinal diffusion coefficient D ∥ , normalized to its large δ value, for the three fluids in a slit-pore, as a function of the plate separation δ. Comparison of the TIP4P/2005-water (blue triangles) 96 with (a) the LJ (black circles) and (b) the CSW (red squares). In both panels, vertical lines mark, approximately, maxima (dotted lines) and minima (dot-dashed lines) for the isotropic fluid (see text).…”

“…The TIP4P/2005 water in a graphene slit-pore has free-energy extrema determining diffusion oscillations, with free-energy/diffusion minima for wall–wall distances fitting complete layers, down to one, and maxima at intermediate distances. 96 In particular, the free-energy minimum for a monolayer originates from an increase of water disorder, despite the corresponding water internal energy increases. For the bilayer, instead, the free-energy minimum is dominated by a minimum in internal energy per water molecule with a larger order.…”

“…For the bilayer, instead, the free-energy minimum is dominated by a minimum in internal energy per water molecule with a larger order. 96 The latter, with a full HB network, is the minimum with the largest mechanical stability, 96 raising the question if it would be so also in a fluid without HBs.…”

Protein Unfolding and Aggregation near a Hydrophobic Interface

“…For the sake of simplicity, we will consider here the case of the projection into two dimensions (2D) of a water monolayer with height nm. Although a confined monolayer of water can have properties quite different from bulk water [ 44 , 45 ], here the dimensionality only affects the number of neighbors of each water molecule but does not change its coordination number (the number of hydrogen bonds formed by each water molecule). Indeed, regardless of whether the model is in 2D or 3D, each water molecule can form up to four hydrogen bonds.…”