|Water: Dissolving the Controversy|
|Contact: Lynn Yarris, [email protected]|
You would think that if scientists could agree upon anything, it would be water. After all, almost everyone knows that H2O is the symbol for water; two atoms of hydrogen attached to one atom of oxygen form a water molecule. What could be simpler than that? And yet chemists have been disagreeing for the past 40 years over how water molecules arrange themselves in a liquid drop.
In this scientific controversy, one side has argued for what is called the two-state model. The other side has argued for what is called the continuum model. Now, strong new evidence provided by scientists at Berkeley Lab and the University of California at Berkeley may have finally tipped the balance in favor of the continuum model.
Chemically simple though it may be, water is an unusual substance. Most substances with comparable molecular weights become gases under the same conditions as water becomes a liquid. And unlike most other substances, when liquid water freezes it expands in volume rather than contracts.
Scientists have long agreed that liquid water's unique behavior is due to the hydrogen bonds that connect one water molecule to another. A single water molecule is V-shaped, but because the oxygen atom is more electronegative than the hydrogen atoms, all 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 another water molecule, which is called a hydrogen bond.
It is known that every individual water molecule can form hydrogen bonds by pointing its two hydrogen atoms towards the oxygen atom of one of its adjacent neighbors. When water exists as ice, the hydrogen bonds form a complete network, with each molecule donating two bonds and accepting two bonds from its neighbors. When the same amount of water is melted and becomes liquid, it loses about ten percent of those hydrogen bonds. The controversy has revolved around the nature of the missing hydrogen bonds.
The two-state camp believes that water molecules toggle between ice-like and broken hydrogen bond structures. The continuum camp says that ice-like and broken hydrogen bond structures are not two distinctive states, but instead are extreme examples of a smooth distribution of possible distortions.
In a paper recently published in the Proceedings of the National Academy of Sciences (PNAS), the Berkeley Lab and UC Berkeley scientists explain how changes in temperature affect measurements made through Raman spectroscopy, a technique widely used in physical and chemical research to characterize substances. Scientists who advocate the two-state model of liquid water have based their arguments on the temperature dependence of liquid water's Raman spectra.
"We show, using a combined experimental and theoretical approach, that many of the features of the Raman spectrum considered to be hallmarks of the two-state system actually result from a continuous distribution of intact hydrogen bonds," says Richard Saykally, an award-winning chemist who holds joint appointments with Berkeley Lab's Chemical Sciences Division and UC Berkeley's Chemistry Department. "These features include the asymmetric band profile, the isosbestic point" indicating temperature invariance "and van't Hoff behavior."
Saykally, who has spent a good portion of his scientific career nailing down the elusive nature of liquid water, is one of the principal authors of the PNAS paper, along with Phillip Geissler, a theoretical chemist with Berkeley Lab's Materials Sciences Division and the UCB Chemistry Department. Other authors are Jared Smith, Christopher Cappa, Kevin Wilson, and Ronald Cohen, who all hold joint appointments with Berkeley Lab and UC Berkeley. Geissler provided the advance in the theory of liquid spectroscopy that made it possible to interpret the Raman spectral features in a manner consistent with the continuum model.
At any given moment, some of the hydrogen bonds in a quantity of liquid water must be greatly distorted; otherwise the water would be in a solid rather than a liquid state. However, according to first author Jared Smith, a member of Saykally's research group who performed the key experiments, their new and more detailed Raman spectroscopy measurements show that any broken hydrogen bonds in a sample of water reform so quickly within approximately 200 femtoseconds that they have no impact.
"The widely held notion that there exists a stable population of water molecules with broken hydrogen bonds in the liquid appears to be incorrect," said Smith. "Hydrogen bonds in liquid water are continually breaking and reforming and moving around. Our experimental results and calculations are in excellent agreement with the continuum model."
Given that 90 percent of our blood consists of water and that the human body is 60 percent water, it is important that scientists nail down water's microscopic structure. Among other applications, the biological simulations used in various research endeavors, including medical studies, are based on models that mimic the continuum model of liquid water.