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

The dimensionality of a system largely determines the nature of any long range order that is possible in the solid phase. For this reason considerable effort has been directed towards elucidating the behaviour of materials under confinement and at interfaces. This has ranged from theoretical studies that predict that a truly two-dimensional solid cannot exist to studies on the use of confinement to promote solidification in thin films. In this paper we review the results of recent molecular dynamics simulations of water confined to a slab geometry. The simulations predict that the freezing and melting of a monolayer and a bilayer of liquid water can be induced at ambient conditions by changing the distance between two confining parallel walls. The confined ice phases are stable only for a small range of plate separations. This can be explained by the ability of the structure of the confined ice phases to achieve optimal distance and angle of hydrogen bonds only for a small range of gap values. Above the film thickness of a bilayer, the degree to which the solid phase is enhanced due to confinement is insufficient to freeze liquid water at ambient conditions. It is also found that a bilayer of liquid water can supports two different phases of liquid water that differ in the local ordering at the level of the second shell of nearest neighbours and in the density profile normal to the plane. These high- and low-density phases of confined liquid water suggest the intriguing possibility of a liquid-liquid transition as a function of the film thickness.