The surface of water comprises the first few layers of molecules that sit on top of a lake, a glass of Evian, or even a droplet in the atmosphere. It has a chemical personality that’s quite different from the bulk liquid it covers. Make that a complex, possibly multiple, personality whose nature is still vexing scientists despite a recent flurry of theoretical and experimental research.
The question of what sits at the surface of water is more than academic. Water is arguably the most important substance on Earth and perhaps elsewhere in the universe too. It’s also one of the most intractable to study, a polar jumble of H2O molecules loosely knit together by constantly making and breaking hydrogen bonds.
The chemistry resulting from water’s interactions with proteins in biological systems, or with chemicals in the atmosphere, is naturally assumed to be affected by the nature of the interface between water and whatever it touches, be it air or a hydrophilic protein pocket.
For more than 100 years, scientists have amassed evidence from experiments on the bulk properties of water showing that gas bubbles or oil drops in water migrate toward a positively charged electrode, implying that hydroxide ions adsorb to the surface of the water at the bubble or drop interface. In 2004, James K. Beattie, a chemistry professor at the University of Sydney, in Australia, and coworkers continued this line of exploration, measuring the surface charge from adsorbed hydroxide ions on emulsions drops. They found that the charge density was very high, supporting the hypothesis that water surfaces are basic.
Also in 2004, Gregory Voth, then a University of Chicago chemistry professor, and colleagues stirred things up when they reported molecular dynamics simulations of acidic water that indicated that hydronium ions prefer to collect on the surface in greater concentrations than in the bulk liquid. This would make the surface acidic, contradicting the earlier findings on surface basicity.
This led to volleys of papers with contradicting titles, such as “The Surface of Neat Water is Basic” (Faraday Discuss. 2009, 141, 31) and “Water Surface is Acidic” (Proc. Natl. Acad. Sci. USA 2007, 104, 7342).
“It’s mind-boggling that we don’t fully understand the surface of water,” comments Pavel Jungwirth, a theoretical chemist at the Academy of Sciences of the Czech Republic, in Prague.
Jungwirth, along with the late Victoria Buch, of Hebrew University of Jerusalem, and their colleagues, created another splash with computer simulations of neutral water. They also concluded that hydronium ions weakly favor the water surface and that hydroxide ions do not.
But Pacific Northwest National Laboratory chemist Liem Dang recently reported simulations indicating that hydroxide ions gravitate toward water’s surface (J. Phys. Chem. A 2009, 113, 6356).
Experimentalists have also brought spectroscopy into play to help characterize the species on water’s surface. For example, chemistry professor Heather Allen of Ohio State University and coworkers have shown spectroscopic signatures of both hydronium and hydroxide ions on the surface of water.
University of California, Berkeley, chemistry professor and water expert Richard Saykally comments that the two sets of findings—some showing that the water surface is basic and others that it is acidic—are both credible. “The results from both camps are pretty compelling,” he says.
A number of issues make this a thorny scientific problem. For the experimentalists, pure water is extremely difficult to obtain. Even minute contamination from a few molecules of soap or hydrocarbons from scientists’ hands, or even laboratory air, can muddy the waters.
And for the modelers, even the exponential increases in computer power in recent years can’t begin to explicitly describe the dynamics of billions of water molecules in a dilute solution.
Beattie, along with Angus Gray-Weale, chemistry professor at Monash University, in Australia, recently proposed a mechanism by which hydroxide ions can collect at the water surface. They suggest that hydroxide ions inhibit fluctuations in bulk water more than any other monovalent ion—a condition bulk water doesn’t like. Therefore, hydroxide is naturally driven to the surface (Phys. Chem. Chem. Phys. 2009, 11, 10994). “There is no longer a controversy,” Beattie says.
But the picture is still far from clear, and resolving it may take a lot more experiments and calculations, both Saykally and Voth say.
Advancing experimental techniques provide more clues, but not definitive answers.
For example, Saykally and colleagues reported experiments hinting that hydronium ions gravitate toward the surface (Chem. Phys. Lett. 2008, 458, 255). UC Berkeley emeritus physics professor Yuen-Ron Shen and coworkers recently carried out sophisticated spectroscopic experiments that support both hydronium and hydroxide ions at the surface (Proc. Natl. Acad. Sci. USA 2009, 106, 15148). And spectroscopist Bernd Winter from the BESSY synchrotron facility, in Berlin, showed that hydroxide does not accumulate at the surface (Chem. Phys. Lett. 2009, 474, 241).
Most recently, Agustin Colussi, a research scientist at California Institute of Technology, and coworkers exposed water surfaces to trimethylamine, a compound that quickly picks up any available protons. From the relatively few protons that were extracted in this way, Colussi calculated that protons (in the form of hydronium ions) don’t populate the surface of water until the bulk solution itself is chock full of protons—acidified to a pH below 4. This supports the surface-of-neutral-water-is-basic camp (J. Phys. Chem. Lett. 2010, 1, 1599).
Saykally thinks that concentration effects, such as ion pairing, probably account for the disparate observations. In solutions with millimolar concentrations of ions, hydroxide or basic conditions dominate the surface, he suggests. But when ionic concentrations venture into the molar region, hydronium ions prefer the surface.
Voth points out that the extrapolation of his group’s findings to neutral water via computer simulation is extremely challenging given the low ion concentration and the fact that “surface” is difficult to define. A more complicated picture may be emerging. It could be, he suggests, that the first two or three molecular layers of water carry more hydronium ions, but in the layer just below, down to about 10–20 Å, negatively charged species may dominate under normal pH conditions.
And it’s becoming apparent that whatever the nature of water’s surface, the effect is likely mild and may not prove to be as big a deal for biological or atmospheric chemistry as previously thought, Jungwirth says. “It’s a very interesting problem, but in terms of real nature, the effect may be minor,” he says.