We drink it. We bathe in it. It's part of our everyday life, but the driving force behind one of the fundamental properties of water, its pH, has defied explanation for decades. Scientists at the University of California, Berkeley, and the University of Rochester, however, have created the first model of how water becomes acidly neutral-a characteristic on which all life depends. The findings should help researchers understand and control other complex chemical reactions as well, ones that could be used to create medicines and better materials. The research appears in the March 16 issue of Science.
The advent of high-speed computers and the development of new algorithms have given the team of researchers the ability to create a simulation of a kind of molecular split so rare and brief that it's impossible to witness in real life. In 10 hours, a single watched molecule could be expected to split in about 100 femtoseconds-about a thousandth of a trillionth of a second. It would be the equivalent of waiting the entire age of the universe to see a one-second twitch.
Since the team couldn't catch the split by chance, they developed a complex computer simulation that showed how a proton is torn away from a water molecule. The pH is a measure of the number of protons, or hydrogen nuclei, that are pulled from a water molecule and roam freely. The number of these free protons determine how water behaves when it comes in contact with other substances, playing a crucial role in nearly any biological process that includes water. Since the 1950s, scientists such as Nobel Prize winner Manfred Eigen have been trying to catch water in the act of splitting, but so far the mysterious process has avoided both observation and modeling.
"This reaction is very complex," says Christoph Dellago, assistant professor of chemistry at the University of Rochester. "It's as if the water acts as both the 'splitter' and the 'splitee' at the same time."
The research team found that the brief reaction happens when, by pure chance, a number of molecules of water surround another in a specific formation. That formation creates a quick electrical field that pulls a proton from the central molecule. Less than a billionth of a second later, the formation breaks and the proton either falls back to the central molecule, or its path is cut off and it roams as a free proton. Though this occurrence is extremely rare for any one molecule, there are countless molecules in a single glass of water, so the process happens constantly.
To understand the way the protons are stripped away in the first place, the team used a proven algorithm to model the process, but the real hurdle lay in the rarity of the event itself. "If we just set up the algorithm and let it run, we would have waited many times the age of the universe for something to happen," says Dellago. Devising a way to "zoom in" on just the single moment when the stripping took place demanded the integration of a second complex algorithm, which in turn demanded ultra-high speed computers. "The result is that we now have the first model of why water has the pH it does," says Dellago.
Dellago hopes the new combination of techniques can be used to uncover new ways to understand and control chemical reactions.
Phillip Geissler of the University of California, is the first author of the paper.