Hydrogen Bond Network in Liquid Water

In water, the hydrogen-bond network shows distinct structural properties: hydrogen bonds in water can form polygons with different number of sides. See illustration of two-dimensional non-short-circuited hydrogen-bond polygons in Fig. 3.

Figure 3: The dash line and solid line represent hydrogen bonds. If water molecules form polygons connected by hydrogen bonds, we use solid lines to highlight them. The polygons, abce and cde, are counted, but the pentagon abcde is not counted. Only the non-short-circuited polygon is included in calculating the distribution.

Liquid water structure differs from any known ice or hydrate crystal orientation. Enumeration of non-short-circuited polygons of hydrogen bonds in water has been conducted to demonstrate the relation between hydrogen bond strength and size distribution of polygons [21]. We may count the distribution of polygons in water or solvation shell to study structural properties and the strength of hydrogen bonds.

When an isolated apolar solute was placed in water, the surrounding water molecules typically form a hydration shell, in which hydrogen bond network should differ from that of bulk water. This is shown in a recent study [11], which computes distribution of hydrogen-bond network in the hydration shell and the shell-bulk interface. The distribution of hydrogen-bonded polygons also gives us a tool to characterize water structure at the vicinity of the solute. For example, a large number of pentagons relative to hexagons and larger polygons lends support for clathrate-like structure on the hydrophobic surface.