Solving the tangle of water's abnormal properties

For centuries, scientists have attempted to understand the chemical properties of water, the most ubiquitous substance on earth. Now two Princeton scientists are leading the way in this area of research in the 21st century.

Composing over two-thirds of the earth's surface, water plays an important part in nearly every human activity. Life on earth depends on it and nearly every industrial project we undertake must take it into account.

But despite its importance, basic questions about water remain — namely the anomalies it exhibits at near-freezing temperatures. With this in mind, chemical engineers Jeffrey Errington and Pablo Debenedetti developed a new quantitative model for understanding the chemical properties of water.

Using a computer program, Errington and Debenedetti created models of water molecules interacting with each other. Each molecule was programmed to follow the basic rules of motion first discovered by Isaac Newton. Just like in the real world, however, the computers showed that simple rules did not necessarily lead to simple behavior patterns.

The goal of the research was to explain the properties that water has in specific ranges of temperature and pressure, such as the effect of temperature on the volume of water, Debenedetti said. For most liquids, a decrease in temperature leads a liquid to shrink. Water, however, expands when cooled from temperatures just above freezing.

Scientists believe that much of water's unusual character comes from its molecular structure — an oxygen atom surrounded by two hydrogen atoms. Each of these three atoms has protons and neutrons in a nucleus, with electrons spinning around it.

In a water molecule, the electrons of the hydrogen atoms are drawn to the oxygen atom, creating a negative charge around the oxygen atom and positive charges around the hydrogen atoms. Because opposite charges attract, water molecules tend to organize in certain shapes whenever they are brought together.

The positively charged hydrogen atoms in one water molecule are attracted to the negatively charged oxygen atoms in surrounding molecules. These weak hydrogen bonds that extend throughout a large number of water molecules are called hydrogen-bond networks.

Hydrogen-bond networks give solid ice its famous crystalized structure. Scientists believe that these networks also account for the behavior of liquid water, Errington explained. "People in the past had for years predicted that there were things going on with structural changes in the hydrogen-bond networks that explain water's anomalous properties," he said.

"The significance of our work is that we were able to relate what happens on a molecular level to what one can observe macroscopically in lab," Errington said.

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In a laboratory, to keep track of the exact position and velocity of each molecule in a drop of water is impossible. Errington and Debenedetti's computer simulation, however, tracks the movements of about 250 water molecules for a simulated nanosecond.

Although the scale of the computer simulations was small in comparison to those in lab experiments, performing the work on a computer meant they could extract complete, exact information about every single molecule in the experiment.

Debenedetti and Errington knew that the work their work was meaningful because their small amount of simulated water behaved just like real water does, Debenedetti explained. Once they saw that their model matched real processes, they were able to use the numbers the computer produced to create equations describing liquid water's behavior in numerical terms.

Although scientists believe that hydrogen networks cause liquid water's anomalous properties, they had not been able to put this belief into numbers. Debenedetti explained that his work provides these numbers. "In my group we like to ask simple but precise questions," he said. "We're developing quantitative tools."

The worked focused on measuring two kinds of order, Debenedetti said. One, called translational order, concerns how far apart the water molecules are from each other. In ice, the water has solidified into a crystal lattice in which all of the molecules are the same distance apart.

When water is cooled to a freezing temperature, the molecules move around less and start to organize into a crystal shape. As this happens, the distances between molecules become more uniform. Comparing the distances between different molecules in the model thus gave Debenedetti and Errington one way to measure how organized the system was.

A second way of measuring the organization of the water was looking at the orientation of the molecules, Debenedetti explained. In ice, the molecules are lined up in a predictable way. The two researchers compared how the molecules in the computer program were lined with how real molecules in ice — water's most organized state — are aligned. The more similar the molecules in the model were to ice, the more ordered they could be said to be.

Errington and Debenedetti found that, at certain temperatures, the amount of order in water actually decreases as the pressure goes up. This trend distinguishes water from most liquids, which tend to become more ordered when compressed. This explains many of the unusual characteristics seen in large amounts of freezing water at the temperatures and pressures Errington and Debenedetti studied.

Their work has a number of possible applications to real world problems, Debenedetti explained. For example, environmental scientists often need to anticipate how quickly a pollutant will dissipate in water. The equations Errington and Debenedetti have written are important tools in trying to make such a prediction.

In the pharmaceuticals industry, special compounds are designed to keep proteins from being damaged. A better understanding of water's properties could lead to better protein storage methods, Debenedetti said.