Water confined to a slab geometry: a review of recent computer simulation studies

Abstract

The dimensionality of a system largely determines the nature of any long range orderthat is possible in the solid phase. For this reason considerable effort has beendirected towards elucidating the behaviour of materials under confinement andat interfaces. This has ranged from theoretical studies that predict that a trulytwo-dimensional solid cannot exist to studies on the use of confinement to promotesolidification in thin films. In this paper we review the results of recent moleculardynamics simulations of water confined to a slab geometry. The simulations predictthat the freezing and melting of a monolayer and a bilayer of liquid water can beinduced at ambient conditions by changing the distance between two confiningparallel walls. The confined ice phases are stable only for a small range of plateseparations. This can be explained by the ability of the structure of the confined icephases to achieve optimal distance and angle of hydrogen bonds only for a smallrange of gap values. Above the film thickness of a bilayer, the degree to which thesolid phase is enhanced due to confinement is insufficient to freeze liquid water atambient conditions. It is also found that a bilayer of liquid water can supports twodifferent phases of liquid water that differ in the local ordering at the level ofthe second shell of nearest neighbours and in the density profile normal to theplane. These high- and low-density phases of confined liquid water suggest theintriguing possibility of a liquid–liquid transition as a function of the film thickness.