Water is well known for its ability to dissolve substances. Indeed, this talent makes it indispensable to all living things. Yet exactly how water attains its life-giving pH is a question that has puzzled scientists for years. Now, however, researchers have developed the first model of the process, which they published last Friday in the journal Science.
Chemists have long known that water molecules in liquid water themselves ionize, or split, producing hydronium and hydroxide ions. The pH reflects the number of protons that are stripped from water molecules during this so-called autoionization. But because of the fleeting nature of this reaction, researchers have never been able to catch the molecules in the act of splitting. This, in turn, has prevented them from understanding how it occurs.
So with the help of high-speed computers and complex algorithms, Phillip Geisseler of the University of California at Berkeley and his colleagues simulated the split. They discovered that the reaction takes place when by chance some water molecules surround another one in a particular formation. This arrangement creates an electrical field that yanks a proton from the molecule in the middle. Within a billionth of a second, the formation disbands and the proton either falls back to the central molecule or is blocked from returning, in which case it becomes a free-ranging proton.
Whether or not future experiments will bear out the predictions made by the new model remains to be seen. But scientists say the work could lead to a better understanding and greater control of other chemical reactions, such as those used to create medicines.
