Water molecules clump more loosely than previously thought

Favorite liquid revisited.

The physicist of antiquity called it one of nature's fundamental elements; third-graders know its chemical formula; and all known forms of life need it to exist. Yet what water really is - at least in its liquid form - is still, to a large extent, a mystery. A team led by scientists at Stanford Synchrotron Radiation Laboratory (SSRL) and Stockholm University now has achieved a breakthrough in understanding the structure of liquid water. They found that water molecules clump much more loosely than previously thought.

The findings appeared April 1 in Science magazine's advance publication website. ''The results overturn 20 years of research in the physical chemistry of water,'' says team leader Anders Nilsson, a chemical physicist at the Stanford Linear Accelerator Center (SLAC). ''It's going to be a big shock in the whole field.''

The SSRL is a division of SLAC, a U.S. Department of Energy (DOE) facility operated by Stanford University. The project was a collaboration between researchers at SSRL, Stockholm University, Linköping University (Sweden) and the University of Utrecht (Holland).

In ice, each water molecule is surrounding by 4 other molecules in a tetrahedral arrangement (left). The new result on liquid water shows that the molecules are connected only with 2 others. This implies that most molecules are arranged in strongly hydrogen bonded rings (middle) or chains (right) embedded in a disordered cluster network connected mainly by weak hydrogen bonds. The oxygen atoms are red and the hydrogen atoms grey in the water (H2O) molecules.

Structure of the First Coordination Shell in Liquid Water
[ Illustration by H. Ogasawara ]

As its H2O formula suggests, each water molecule is made of two atoms of hydrogen and one of oxygen. In ice, water molecules are arranged in a crystal structure, with each molecule typically linked to four others through what chemists call hydrogen bonds. In a hydrogen bond, electrostatic forces stick together a hydrogen atom from one molecule with the oxygen atom from a different molecule. The oxygen can form two hydrogen bonds, so a molecule can link to as many as four others - with two links through its oxygen and one through each of its hydrogens.

Although they are 10 times weaker than the covalent bonds within the molecule itself, hydrogen bonds between molecules still take a lot of energy to break up - which is why ice melts so slowly. Even in liquid water, molecules spend most of their time clumped together by hydrogen bonds, though not in a static pattern as in ice. ''Hydrogen bonds in liquid water form and break very fast, on the order of every picosecond (one trillionth of a second),'' says SLAC physicist Uwe Bergmann, a co-author of the research paper. The ephemeral patterns formed by bonding in the liquid are still far from being understood, but are thought to be responsible for the peculiar properties of water, including its relatively high boiling point, its high viscosity and - last but not least - its ability to sustain the chemical reactions inside a living cell.

For the past 20 years, the consensus among researchers has been that, at any given time, a molecule of water typically forms three or four hydrogen bonds - 3.5 on average. ''What we find,'' Bergmann says, ''is that there are not 3.5 hydrogen bonds, but only 2.'' Each molecule could still form up to four bonds, the research suggests, but two would be of different, much looser kinds.

The authors point out that the earlier estimate of 3.5 was based on theoretical assumptions that became commonly accepted because, when applied in computer simulations, they gave results consistent with known properties of water, such as the unusually high amount of energy that is required to heat it up. ''Nobody had anything to object to the prevailing model, so it became the truth,'' Nilsson says.

But the difficulty of ''seeing'' the actual molecules in action meant a dearth of real data. ''There has not really been new experimental information about water in the last 20 years, except for data from neutron studies,'' Nilsson says. ''The amazing thing is that hardly anything is known about the unique properties of liquid water.''

The new result now reopens the hunt for the structure of liquid water. ''It resurrects models that were considered inappropriate,'' Bergmann says. One possibility, he says, is that water molecules could arrange in chains or even in closed rings. Eventually, the outcome could be a better understanding of the chemistry of the cell, which is notoriously hard to imitate using different liquids. ''Nobody has a clear answer to why water is essential for life,'' Nilsson says.

The research was the first to apply a technique called X-ray absorption spectroscopy to the local structure of water. The technique, developed by SSRL, among other laboratories, bombards a material with X-rays that are finely tuned to excite particular electrons in a molecule's structure. Careful measurement of the scattered radiation reveals the motions of the excited electrons, which, in turn, reveal what bonds molecules are forming. The experiments used intense X-ray sources at the Argonne National Laboratory and the Lawrence Berkeley National Laboratory, both of which are DOE facilities.

The team is now working on several projects to extend their results. ''We want to study water in a whole range of pressures and temperatures,'' Bergmann says. SPEAR3, SSRL's newly upgraded, state-of-the-art X-ray source that formally opened Jan. 29, would be the ideal place for that. ''We propose to build a new facility at SPEAR3 where the structure of water would be a large part of the scientific drive,'' he says.

''Water covers the majority of the Earth's surface, is present in all forms of life and is perhaps the most important natural resource for humanity. Despite its familiarity and years of rigorous study, water can still yield remarkable surprises,'' says Patricia Dehmer, director of the DOE Office of Basic Energy Sciences. ''This collaboration ... has given a new understanding of the molecular bonding in liquid water.''

In addition to Nilsson and Bergmann, the other SLAC scientists included in the five-year collaboration are Philippe Wernet (first author of the paper, now at the BESSY laboratory in Berlin), Hirohito Ogasawara and Lars Naslund.

By Davide Castelvecchi, 02-April-2004