Cage provides key to water droplet.

Just how tiny is the smallest drop of water? Computer simulations show that a three-dimensional cluster of six water molecules, connected by eight hydrogen bonds, begins to assume the properties of the bulk liquid. In this way, a water hexamer is the smallest drop of water possible, says David C. Clary of University College London.

Starting with just two molecules, Clary and his colleagues systematically modeled water clusters of increasing size to determine their geometries. Groups of up to five molecules join together in a ring, but arrays of six can also take on cagelike or prism structures, he says. In the cage hexamers, the distances between molecules and the distribution of charges are similar to those of liquid water. Clary presented the results of his group's calculations this week at a meeting of the American Physical Society in Los Angeles.

The findings complement other research that examines the elusive behavior of water and ice, for example how molecules interact during melting or freezing. "The basic theory that we've been developing will ultimately be needed to explain those very subtle properties" says Clary.

Clary has also modeled how organic molecules such as benzene associate with water. Although benzene is not soluble, "understanding how benzene interacts with one water molecule -- then two and three and four" can illustrate the process of dissolution.

The simulations match experimental results Richard J. Saykally of the University of California, Berkeley and his colleagues study water clusters with infrared spectroscopy, which reveals the strength of the hydrogen bonds between water molecules.

First, they spray water into very fine droplets, a process that "produces water clusters cooled to temperatures very near absolute zero," Saykally says. Then, they excite the droplets with an infrared laser and record the spectrum of absorbed light frequencies. Analysis of this Spectrum reveals the size and shape of the clusters. Saykally and his coworkers see evidence of the six-molecule water clusters in the ring, cage, and prism configurations.

One of the goals of these studies is to "quantify the force field of water -- the forces that hold liquid water together," Saykally says. The Berkeley group has completed the force field analysis for a two-molecule water cluster, and extending the model to larger ones should be "very easy," he remarks.