Continuous melting through a hexatic phase in confined bilayer water

Abstract

Liquid water is not only of obvious importance but also extremely intriguing, displaying many anomalies that still challenge our understanding of such an a priori simple system. The same is true when looking at nanoconfined water: The liquid between constituents in a cell is confined to such dimensions, and there is already evidence that such water can behave very differently from its bulk counterpart. A striking finding has been reported from computer simulations for twodimensionally confined water: The liquid displays continuous or discontinuous melting depending on its density. In order to understand this behavior, we have analyzed the melting exhibited by a bilayer of nanoconfined water by means of molecular dynamics simulations. At high density we observe the continuous melting to be related to the phase change of the oxygens only, with the hydrogens remaining liquid-like throughout. Moreover, we find an intermediate hexatic phase for the oxygens between the liquid and a triangular solid ice phase, following the Kosterlitz-ThoulessHalperin-Nelson-Young theory for two-dimensional melting. The liquid itself tends to maintain the local structure of the triangular ice, with its two layers being strongly correlated, yet with very slow exchange of matter. The decoupling in the behavior of the oxygens and hydrogens gives rise to a regime in which the complexity of water seems to disappear, resulting in what resembles a simple monoatomic liquid. This intrinsic tendency of our simulated water may be useful for understanding novel behaviors in other confined and interfacial water systems.

Simulations of water nano-confined between corrugated planes

“…The structural and dynamical properties of the honeycomb ice, square tubes ice, and the triangular phases appearing in Fig. 1 have been already well described in previous works [6,10] and α does not affect them significantly (see Appendices B and E). However, when α = 0.8 and 1.0 a different form of ice stabilizes at ρ=1.47 g cm −3 , which we call intercalated honeycomb ice.…”

“…From C OO , we calculate the characteristic time τ of the C OO curve, which is defined by C OO (τ ) = 0.5. For α ≤ 0.4, the oxygens arrange themselves into a triangular lattice showing similar structural and dynamical features as the ones observed in the hexatic phase at α = 0 [10]: similar RDFs (see Appendix D), diffusivities (D = 6 10 −7 cm 2 s −1 at T = 300 and α = 0.4 versus D = 9.5 10 −7 cm 2 s −1 for α = 0.0 [10]) and oxygen-oxygen first-neighbor correlation functions [ Fig. 2 (a)].…”

“…When α ≤ 0.6, at ρ = 1.37 g cm −3 water remains being liquid. At ρ = 1.47 g cm −3 and low temperatures, the square-tubes ice observed for flat confinement [10] stays stable for α ≤ 0.6. In this solid, the molecules arrange into tubes and the hydrogen bonds tend to point towards the oxygens from the same tube (see Appendix C).…”

“…Although the triangular structuring is still present in the xy density histogram of the oxygens, water seems to behave as a dense triangular liquid. Therefore, the indicators employed in this section suggest that the continuous melting reported in [10] washes out for α ≥ 0.6.…”

“…In addition, one expects that a modulation inserting a lattice of energy maxima and minima along the plane should hinder continuous shear, and thus constrain the appearance and diffusion of the dislocations, responsible for the lack of long-range translational order of the hexatic phase [23][24][25][26]48]. These dislocation appear when the density of the ideal triangular ice (expected to be at ρ = 1.76 g cm −3 [10]) is decreased.…”