We are all familiar with water, and we see it every day in many forms: in the bulk as a glass of water, in the crystal phase as ice, and the vapor phase as steam. While the behavior of these phases seems predictable, water is an unusual substance that behaves unlike any other small molecule we know of. This fact is particularly notable when water is viewed at small-length scales or confined to small compartments.

An international team of scientists recently discovered some intriguing structural characteristics of water confined in graphene nanocapillaries. In these studies, the researchers deposited a graphene monolayer on a small grid, added a small amount of water, and then covered it with another monolayer of graphene. This sample was left overnight to allow excess water to evaporate, eventually bringing the graphene layers together so that only a small amount of adsorbed water remained between them. The water left behind showed some unusual structural properties.

Structural characteristics of water are influenced by hydrogen bonding among adjacent water molecules. In the liquid state, water exhibits a partially ordered structure. In the crystal state, water molecules begin to conform to more rigid lattice structures, forming ice. As ice, the water molecules typically take on a geometry that is a three-dimensional “tetrahedral” structure, which basically looks like a square pyramid.

In these recent studies, which were conducted at room temperature, the graphene layers forced the water to form a low-dimensional ice with a unique crystal structure.

The water molecules formed layers with square lattices where each molecule interacted with the four molecules surrounding it, forming hydrogen bonds at 90° angles. This square lattice symmetry, which they also saw assembled into bilayers and trilayers, is strikingly different from the normal three-dimensional arrangement in ice, where hydrogen bonds exhibit a bond angle of approximately 109°. The researchers found that this lattice structure could be produced even after certain variables had been changed, including the capillary width, applied pressure, and rigidity of the graphene sheets.

The scientists believe this unique square lattice structure is caused by weaker water-surface interactions than the interactions that occur between water molecules—that can be attributed to both the hydrophobicity of the graphene as well as the confined space. The researchers think this type of phenomenon can explain certain unusual behaviors of water, such as the fast speed at which water is able to permeate hydrophobic nanocapillaries in some contexts.