Water makes a splash

11/11/04

Recent experimental results threatened to overturn 100 years of scientific research into the mysterious nature of liquid water, but new experimental results say ... not so fast! A team of scientists with the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California, Berkeley, has shown that the energy required to "measurably distort" the molecular structure of liquid water is the same as the energy required to melt ice. This could explain why a study last spring out of Stanford University seemed to contradict what was has long been believed about the molecular structure of liquid water.

Using the ultrabright x-ray beams at Berkeley Lab's Advanced Light Source and a unique experimental technique of their own, the Berkeley researchers, led by chemist Richard Saykally, found that 1.5 kcal/mol is the average energy required to distort or bend a hydrogen bond in both solid and liquid water. The Stanford measurement of these hydrogen bond distortions was based on theoretical calculations rather than experiments. As a result, it appeared that most of the molecules in liquid water only interact with two other water molecules, as opposed to the traditional picture in which nearly every water molecule interacts with four other water molecules.

"Our results certainly do not disprove the conclusions of the elegant Stanford experiment, but we do present an alternative way to interpret their experiments that is consistent with the standard view of liquid water structure," says Saykally, who holds joint appointments with Berkeley Lab's Chemical Sciences Division and UC Berkeley's Chemistry Department.

The results of the Berkeley experiments are reported in the October 29 issue of the journal Science. Saykally is principal author of the paper. The other authors are Jared Smith, Christopher Cappa, Kevin Wilson, Benjamin Messer and Ronald Cohen, who all hold joint appointments with Berkeley Lab and UC Berkeley.

Water is the most abundant liquid on our planet. It covers 70 percent of the Earth's surface and makes up 60 percent of the human body. Blood may be thicker than water, but not by much, since 90 percent of it is water. Despite water's ubiquitous presence in our lives, it remains a mystery. Whereas most substances contract when they solidify, water expands, making it less dense as a solid than as a liquid. Our lives depend upon liquid water but, given its light molecular weight, water at room temperature should be a gas.

The key to understanding the strange but vital properties of liquid water is to fully understand its structure. Consisting of two hydrogen atoms joined to a single atom of oxygen, water is one the smallest and most simple of all molecules, but it is able to form unique kinds of chemical bonds with other water molecules. A single water molecule is V-shaped, but because the oxygen atom is more electronegative than the hydrogen atoms, the electrons in the molecule tend to gather towards the oxygen end, creating a slightly negative pole there and a slightly positive pole on the hydrogen side. The polarity of each water molecule results in a weak attraction between it and other water molecules, called a hydrogen bond.

In the traditional scientific picture of liquid water, every individual water molecule forms four hydrogen bonds -- two that are electron acceptors and two that are electron donors through which it connects to its four nearest neighbors. The result is a network of tetrahedrons. These are the same bonds that exist when water is in the solid ice state. Under the standard view of water, when ice melts, only about 10 percent of the tetrahedral hydrogen bonds are broken. This retention of intact hydrogen bonds has long been thought to be the source of liquid water's unusual properties.

As Saykally once explained in the film by David Suzuki, The Sacred Balance, "The way I like to think about it, it's like water has two hands and two feet. The hands of water are the hydrogens that are more or less positively charged, and the feet are electron pairs that are the negative part associated with oxygen. And these two hands want to grab the feet of two other water molecules, and the two feet want to interact with the hands of two other water molecules. So in each water molecule, hydrogen bonds to four others, making very extensive networks in the liquid."

However, in April, scientists at Stanford University reported a series of tests, using x-ray absorption spectroscopy and x-ray Raman scattering techniques, that indicated a radically different molecular arrangement for water. In their tests, they found that in room-temperature liquid water, more than 80 percent of the hydrogen bonds between water molecules were broken. On the average, they found each liquid water molecule formed only two hydrogen bonds -- one electron donor and one electron acceptor. From this they concluded that in the liquid state, water molecules form a network of rings or chains, rather than the tetrahedrons formed when water becomes ice.

"Experimental measurements, however, necessarily define hydrogen bonds in terms of the particular technique being used to make them," says Saykally. He and his colleagues used a different technique, called total electron yield near-edge x-ray absorption fine structure (TEY-NEXAFS) of liquid water microjets, in which the spectrum of liquid water is measured over a wide range of temperatures. This gave them the energy that would be required to distort the hydrogen bonds in liquid water enough to yield a picture of water's structure similar to what was found at Stanford. They used this new technique to measure the spectra of normal and supercooled water between minus-27 and 15 degrees Celsius.

"We found that the Stanford results arise from from relatively small distortions of an ice-like hydrogen bond," Saykally says, "and that the same results could be expected even for nearly perfect tetrahedral configurations in the liquid water molecules."

Because liquid water plays such a critical role in life and a great many other physical and chemical processes, scientists will continue to study its structure and how that structure can change when liquid water interacts with something else. This latest round of experiments is not the final word, but another clue towards solving what continues to be a scientific enigma.

Source: Eurekalert & others

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