In chemistry, a watercluster is a discrete hydrogen bonded assembly or cluster of molecules of water. Many such clusters have been predicted by theoretical models, and some have been detected experimentally, in various contexts such as ice, and bulk liquid water, in the gas phase, in dilute mixtures with non-polar solvents, and as water of hydration in crystal lattices. The simplest example is the water dimer2. Water clusters have been proposed as an explanation for some anomalous properties of water, such as the unusual variation of density with temperature. Water clusters are also implicated in the stabilization of certain supramolecular structures. They are expected to play a role also in the hydration of molecules and ions dissolved in water.
Detailed water models predict the occurrence of water clusters, as configurations of water molecules whose total energy is a local minimum. Of particular interest are the cyclic clusters n; these have been predicted to exist for n = 3 to 60. With increasing cluster size the oxygen to oxygen distance is found to decrease which is attributed to so-called cooperative many-body interactions: due to a change in charge distribution the H-acceptor molecule becomes a better H-donor molecule with each expansion of the water assembly. Many isomeric forms seem to exist for the hexamer 6: from ring, book, bag, cage, to prism shape with nearly identical energy. Two cage-like isomers exist for heptamers 7, and octamers 8 are found either cyclic or in the shape of a cube. Other theoretical studies predict clusters with more complex three-dimensional structures. Examples include the fullerene-like cluster 28, named the water buckyball, and the 280-water-molecule monster icosahedral network. The latter, which is 3 nm in diameter, consists of nested icosahedral shells with 280 and 100 molecules. There is also an augmented version with another shell of 320 molecules. There is increased stability with the addition of each shell. There are theoretical models of water clusters of more than 700 water molecules, but they have not been observed experimentally.
Experimental observations
Experimental study of any supramolecular structures in bulk water is difficult because of their short lifetime: the hydrogen bonds are continually breaking and reforming at the timescales faster than 200 femtoseconds. Nevertheless, water clusters have been observed in the gas phase and in dilute mixtures of water and non-polar solvents like benzene and liquid helium. The detection and characterization of the clusters is achieved with infrared spectroscopy techniques such as far-infrared vibration-rotation-tunneling spectroscopy. The hexamer is found to have planar geometry in liquid helium, a chair conformation in organic solvents, and a cage structure in the gas phase. Experiments combining IR spectroscopy with mass spectrometry reveal cubic configurations for clusters in the range W8-W10. When the water is part of a crystal structure as in a hydrate, x-ray diffraction can be used. Conformation of a water heptamer was determined using this method. Further, multi-layered water clusters with formulae 100 trapped inside cavities of several polyoxometalate clusters were also reported by Mueller et. al...
Bulk water models
According to the so-called in silico method quantum cluster equilibriumtheory of liquids W8 clusters dominate the liquid water bulk phase followed by W5 and W6 clusters. In order to facilitate a water triple point the presence of a W24 cluster is invoked. In another model bulk water is built up from a mixture of hexamer and pentamer rings containing cavities capable of enclosing small solutes. In yet another model an equilibrium exists between a cubic water octamer and two cyclic tetramers. However, in spite of much model-making, no model yet has reproduced the experimentally-observed density maximum.
The structure of water
A water cluster is composite of two phases, an outer liquid phase and an inner gas phase.
