Water is water, right? Of course it is, but it turns out that the "skin" of even the tiniest droplet of water is quite different than what lies beneath. In the last five years or so, several lines of evidence have hinted that the zone where the water molecules meet the air might be as acid as sour milk (a pH of about 4.5), even if the water within was neutral, with a pH of 7. But there has been no way to test that directly—until now.
Postdoctoral Scholar Shinichi Enami and Senior Research Associate Agustín J. Colussi, from Professor Michael R. Hoffmann's group at Caltech's W.M. Keck Laboratories, have devised a way to determine the acidity of the outermost nanometer of water directly. (A nanometer is a billionth of a meter, or about a hundred-thousandth the thickness of a human hair and three times the diameter of a water molecule.) The scientists shot tiny jets of water into a cloud of gaseous trimethylamine. Water's acidity is due to a surplus of positively charged hydrogen ions (H+), which ride on ordinary water molecules to form hydronium ions (H3O+). The trimethylamine molecules hitting the water's surface would avidly strip the hydrogen ions from any exposed hydroniums, becoming positively charged trimethylammonium ions that could easily be measured in a mass spectrometer.
The researchers were astonished at how few hydronium ions the trimethylamine molecules found—essentially none, until the bulk liquid had been acidified to a pH of 3.8, about the same as orange juice. This means that the surface of pure water is effectively alkaline, a finding that could have a huge impact on our understanding of how atmospheric gases such as carbon dioxide are absorbed into the ocean—a process that is controlled by surface acidity. "Nothing gets into the bulk without crossing the interface," says Colussi. "We will have to rethink the mechanism by which CO2 is absorbed."
The findings appeared in the May 3 issue of the Journal of Physical Chemistry Letters. The work was supported by a grant from the National Science Foundation.