Simulations of water nano-confined between corrugated planes

Abstract

Water confined to nanoscale widths in two dimensions between ideal planar walls has been the subject of ample study, aiming at understanding the intrinsic response of water to confinement, avoiding the consideration of the chemistry of actual confining materials. In this work, we study the response of such nanoconfined water to the imposition of a periodicity in the confinement by means of computer simulations, both using empirical potentials and from first-principles. For that we propose a periodic confining potential emulating the atomistic oscillation of the confining walls, which allows varying the lattice parameter and amplitude of the oscillation. We do it for a triangular lattice, with several values of the lattice parameter: one which is ideal for commensuration with layers of Ih ice and other values that would correspond to more realistic substrates. For the former, the phase diagram shows an overall rise of the melting temperature. The liquid maintains a bi-layer triangular structure, however, despite the fact that it is not favoured by the external periodicity. The first-principles liquid is significantly affected by the modulation in its layering and stacking even at relatively small amplitudes of the confinement modulation. Beyond some critical modulation amplitude, the hexatic phase present in flat confinement is replaced by a trilayer crystalline phase unlike any of the phases encountered for flat confinement. For more realistic lattice parameters, the liquid does not display higher tendency to freeze, but it clearly shows inhomogeneous behaviour as the strength of the rugosity increases. In spite of this expected inhomogeneity, the structural and dynamical response of the liquid is surprisingly insensitive to the external modulation. Although the first-principles calculations give a more triangular liquid than the one observed with empirical potentials (TIP4P/2005), both agree remarkably well for the main conclusions of the study.

Nanoconfined Fluids: Uniqueness of Water Compared to Other Liquids

“…Hence, these properties are specific of confined water, possibly related to its HBs. Indeed, the HB network and its specific geometry are held responsible for the crystallization of subnm confined water into bilayer ices at ambient conditions in experiments ,, and simulations ,− and its re-entrant melting by changing the slit-pore size. ,, To understand better how it relates to the subnm speed-up and the bilayer strong-stability, we analyze the water HB network in the detail in the next section.…”

Confinement-Controlled Aqueous Chemistry within Nanometric Slit Pores

“…To begin with fundamental physical properties, the phase behavior of water changes when cylindrically confined within CNTs, − which has been found to be extremely diameter-dependent, as well as when confined to planar lamellar environments within slit pores such as parallel graphene sheets; ,,, see also ref for fundamental discussion including the issue of confinement-induced phase transitions. Additional aspects like the corrugation of the confining media at the atomic scale and as well as flexibility have been found to furthermore affect the phase equilibria of confined water.…”

Nanoconfined Water within Graphene Slit Pores Adopts Distinct Confinement-Dependent Regimes

“…Too many studies to be comprehensively addressed here have disclosed a plethora of remarkable and surprising properties of nanoconfined water. Let us highlight just a few out of many examples: Nanoconfined water shows almost frictionless flow within graphene sheets and carbon nanotubes − but not within boron nitride nanotubes; its phase behavior depends on both the spatial dimensionality of the confinement and its strength as shown by using carbon nanotubes of different diameter as well as slit pores with different spacings − and also depends on corrugation; last but not least, nanoconfinement has been found to even change chemical equilibria in opposite ways with respect to homogeneous bulk water depending on the dimensionality of the confinement as discovered for the self-dissociation reaction of water in two-dimensional versus one-dimensional confinement . Such examples make it very clear that water responds extremely sensitively to the specific confinement conditions .…”