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Science: Water presumptions: All wet

Posted on: Wednesday, 3 September 2003, 06:00 CDT

The headwaters of the Arkansas River near Leadville, Colo.

Below: The Tulsa skyline with Interstate 44 and the Arkansas River in the foreground.

Photos by TOM GILBERT / Tulsa World

For something comprising nearly three-quarters of the planet's surface, water is poorly understood.

DALLAS -- Of all the things you learned in high school chemistry, H20 should have been the simplest. Two atoms of hydrogen and one of oxygen, bonded together into pure, drinkable water. But H20 is an H- 2-no. Water is really H-1.5-O -- at least momentarily, scientists have found.

For something that makes up nearly two-thirds of the human body and three-quarters of the planet's surface, water is pretty poorly understood. But a rush of new discoveries is giving scientists something to drink down.

One new pair of studies reveals that, inside microscopic tubes, water flows virtually without friction and in a bizarre transitory state between liquid and vapor. Another set of studies shows how water dissolves a salt and neutralizes an acid -- two essential tasks.

It's pretty basic stuff for such a popular fluid.

First, to straighten out the confusion over water's chemical formula: Most of the time, H20 works just fine. But if you were a subatomic particle hurtling toward water molecules, you might see three hydrogen atoms for every two of oxygen, giving you the impression that water's chemical formula was H-1.5-O.

The effect lasts just a few billionths of a billionth of a second. And then everything reverts back to H20, German-led scientists reported this month in Physical Review Letters.

Blame this slippery trick on quantum effects that happen on extremely short time scales. For a fraction of a moment, protons inside the hydrogen atoms become involved with each other in such a way as to render them invisible to particles that would otherwise bounce off them.

Confused? It's far from being water's only surprise.

"One of the central questions always has been whether water is just a passive molecule floating around or whether it plays a more active role," said Gerhard Hummer, a physicist at the National Institutes of Health in Bethesda, Md.

The active concept seems to be winning. Water molecules measure just 0.3 nanometers, or millionths of a millimeter, across, and regularly squeeze through openings just a bit bigger.

Scientists use computer models to simulate how water molecules travel, single file, through tiny pores like those that feed the body's cells. The flow isn't as continuous as one might expect, a new study out of England found.

Biophysicists Oliver Beckstein and Mark Sansom created hypothetical pores measuring 0.8 nanometers long and 0.7 to 2 nanometers wide. Then simulated water molecules went coursing through the tiny pipe.

Surprisingly, the water flow didn't slow down gradually as the tube got narrower. Instead, the molecules began oscillating between a liquid and a vapor state, condensing and then evaporating.

"The balance is between the water escaping at the surface of the tube and entering the mouth of the tube," said Sansom, of the University of Oxford.

While not immediately useful, the work "expands our basic knowledge of physics at the atomic scale," wrote Helmut Grubmuller, of Germany's Max Planck Institute for Biophysical Chemistry, in a commentary accompanying Sansom's work in the Proceedings of the National Academy of Sciences.

Microscopic pumps

Understanding how water flows at the nanoscale could help scientists create the next generation of nanometer-scale pumps, motors or other devices. In another drop of nanowater research, Hummer and colleagues have shown how water might flow through a membrane made of nano-sized carbon tubes.

Such tubes are chemically very simple -- just a rolled-up grid of carbon atoms resembling molecular chicken wire. That simplicity allows researchers to model what happens to water flowing through the nanotube, said Hummer, which in turn can reveal how water passes through pores into cells.

"One might expect that when you go down to pore sizes, which are barely wide enough for a single water molecule to pass through, that the water molecules would simply be stuck in there," said Hummer. "But what happens is quite the opposite."

Water flows right through the nanotube membrane almost without friction, his team reported recently in the Proceedings of the National Academy of Sciences.

If this effect happened in the macroscopic world, a garden hose would spew the same rate of water even if it were doubled in length, Hummer explains. The fact that it happens on small scales could be relevant for building miniature fuel cells, water purifiers or devices relying on tiny pumps and channels.

Computer simulations are one way of modeling how water behaves on the nanoscale. To see what's actually going on, other scientists are using lasers to freeze-frame water in action.

In one study, scientists from Germany and Israel have captured what happens when a proton moves swiftly between an acid and a base, neutralizing each.

Acid-base reactions are important in many areas of chemistry, but until now scientists didn't know the details of how the proton moved.

In the presence of water, the proton zipped from the acid to the base within 150 millionths of a billionth of a second, found a team led by Erik Nibbering of the Max Born Institute in Berlin. But when water wasn't around, the transfer took place one-tenth as fast.

Add a pinch of salt

The work is the first to describe how important water is for the fleeting proton transfer, the researchers reported last month in Science. Another new study using superfast lasers has overturned a decades-old explanation for why water changes its viscosity, or gooeyness, when salts are dissolved in it.

Some salts make water much more viscous; scientists had thought that a charged salt particle, or ion, caused the hydrogen ends of water molecules to bond together more strongly, thus making the water flow stickily. These salts are called "structure makers"; those that reduce the viscosity are called "structure breakers."

But no one had managed to explain how structure makers and structure breakers work, said Anne Willem Omta, a graduate student at the Institute for Atomic and Molecular Physics in Amsterdam.

Omta's research, reported last month in Science, shows that they don't -- at least not in the way scientists had thought. The team, led by Huib Bakker, used lasers to analyze how water molecules respond to the presence of an ion.

"We did it just to understand what ions do to water," Omta said.

Only the water molecules immediately surrounding the ion were affected, the scientists found. The ion became coated with a rigid layer of water molecules, but water beyond that remained unchanged.

There was, the researchers realized, no "structure maker" linking the water molecules together.

"It seems these viscosity effects can be accounted for just by presence of the rigid spheres," said Omta.

And that may be a final reason to toss chemistry textbooks into a lake.

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