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February 18, 2005
When Water Meets a Solid Surface

The interaction of water with the surfaces of solid materials is ubiquitous. In the primeval days of our planet such interactions may have given rise to the creation of prebiotic organic matter. Today, water-solid interfaces play a critical role in a great many environmental, biophysical, and even technological processes. Yet as vital as these interfaces are, our knowledge of what takes place on the molecular level at the point where water meets solid has been limited. That situation, however, is about to change.

The interaction of water with solid objects touches on almost all aspects of our daily lives, encompassing a huge list of environmental, biophysical, and chemical processes. Yet our understanding of what takes place on the molecular level where water and solid objects meet is far from complete.

A team of scientists at Berkeley Lab and the University of California at Berkeley has developed a new technique that provides the most detailed molecular level pictures ever of water-solid interfaces. They have already used this technique, called phase-sensitive sum-frequency vibrational spectroscopy, to record the first direct observations of the interface between water and crystalline quartz — with surprising results.

"We have discovered the existence of intermixed regions of ice-like and liquid-like water structures at the water-quartz interface, with different net polar orientations corresponding to excess water molecules that have either their oxygen or hydrogen ends pointing towards the surface," says Victor Ostroverkhov, a scientist with Berkeley Lab's Earth Sciences Division and one of three coauthors of a paper on this research appearing in Physical Review Letters. The discovery of these intermixed regions holds importance for a wide range of applications, from the practical, such as improved water purification and corrosion-resistant technologies, to the theoretical.

"Our direct observations of water structuring on a solid surface should be challenging to theorists who create molecular level simulations of water on interfaces," Ostroverkhov explains. "Theorists are always on the lookout for experimental tests that can provide a more complete picture of water interfaces."

Ostroverkhov's coauthors were Glenn Waychunas of Berkeley Lab's Earth Sciences Division and Yuen Ron Shen of the Materials Sciences Division. Shen and Ostroverkhov also hold appointments with UC Berkeley's Physics Department.

Probing the interface

The study of any interface where two surfaces meet is a tricky business: probing technologies, by their very nature, tend to measure not only the interface region but a substantial volume of material away from the interface as well. As a result, interface information may be only a small part of what is actually measured.

This tricky business is made even more complicated when the interface entails a solid material and liquid water. The weak attraction that connects one water molecule to another — the hydrogen bond — makes water unlike any other substance. For example, water is a liquid at room temperature, when by virtue of its molecular weight it ought to be a gas. And while most other materials become denser when they make the transition from the liquid to the solid phase, water becomes less dense.

A number of surface-sensitive probing techniques have been developed in recent years, but among the best is sum-frequency generation (SFG) spectroscopy, pioneered by Shen. For most materials, SFG takes place mainly at surfaces and interfaces; SFG spectroscopy capitalizes on the nonlinear optical properties of these interfaces, whereby exposing the interface to light of certain frequencies causes it to emit light at a different frequency.

  In sum-frequency generation (SFG) spectroscopy, an interface is simultaneously lit with two beams of light, one at a visible light frequency (green) and one at an infrared frequency (red). This causes molecules at the interface to emit light at the sum frequency of the two incident beams (blue). Scientists use the SFG signal to characterize the light-emitting molecules.

In the case of SFG vibrational spectroscopy, a surface or interface is exposed to a mix of two beams of light, one from an infrared laser and one from a visible light laser. This mixture of incident light beams causes molecular groups at the surface or interface to vibrate and reradiate light at the sum frequency of the two incident beams.

"Using SFG spectroscopy, we probe no more than a few monolayers of molecules at a surface or interface," says Ostroverkhov. "The signal generated in the bulk of the material is negligible."

Phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) is the next generation of SFG spectroscopy, with the capability of retaining phase information. This enables researchers to determine the net polar orientation of the interface molecules or molecular groups being studied.

"When you scan the frequency of the output signal, you get resonance peaks that can be assigned to specific molecular vibrations. But when you only measure the magnitude of the SFG signal, you automatically lose phase information," says Ostroverkhov. "With our technique, the output signal is mixed with a reference signal of a known and fixed phase. When you monitor the two signals together, you can determine whether they are in or out of phase, depending on whether there is constructive or destructive interference between the two signals."

Water molecules take a position

With their PS-SFVS technique, Ostroverkhov and his colleagues were able to directly observe the orientation and reorientation of various water molecules in the intermixed regions of ice-like and liquid-like water structures at the water-quartz interface.

Hydrogen bonds are weak, constantly forming and breaking to create partially ordered structures in liquid water; in ice, each water molecule bonds rigidly to four others. At a water-solid interface, even though the temperature and pressure are that of liquid water, some molecules are nevertheless ordered like ice.

This phenomenon has been the source of scientific speculation but has never before experimentally confirmed. The Berkeley study shows that, while the intensity spectrum of water molecules may look similar for different water-solid interfaces, the orientation and ordering of these molecules can be very different, and that means the interfacial properties can be very different.

For example, a single molecule of water, which consists of two hydrogen atoms joined to a single atom of oxygen, is V-shaped. In their study of the water-quartz interface, Ostroverkhov and his colleagues found that, depending on the acidity (pH) of the water solution, the hydrogen-atoms portion of the V could either point up or down from the interface.

"As the acidity of the water solution is varied, water molecules associated with different surface sites respond differently," Ostroverkhov says. "Some readily reverse their orientation with respect to the surface, while others show little change over a wide range of pH conditions."

Given the abundance of quartz and silicate minerals in the environment, the findings of the Berkeley researchers are important for understanding mechanisms behind such phenomena as soil formation and weathering, and how chemical nutrients and contaminants get transported through soils. But Ostroverkhov and his colleagues expect their findings for water-quartz interfaces to hold true for other oxygen-containing solid-water interfaces as well, which makes the importance of this work even more far-ranging.

In the area of water purification, for instance, the information gained from this study could help in designing improved filtration membranes based on various oxide or ceramic materials.

Berkeley Lab scientists (from left) Ron Shen, Victor Ostroverkhov, and Glenn Waychunas developed the new technique of phase-sensitive sum-frequency vibrational spectroscopy to provide a detailed molecular-level picture of the water-solid interface. (Photo Roy Kaltschmidt)

"In the reverse-osmosis membranes developed to separate water from salt ions and various impurities, the pores of the membrane materials are so small that only water molecules are able to pass through, while salt ions and large organic molecules are rejected," says Ostroverkhov. "In such constrained geometries, the behavior of water within several molecular monolayers from the solid surface is essential for defining the functionality and efficiency of the membrane."

Ostroverkhov and his colleagues plan to use their PS-SFVS technique on a wide number of water-solid interfaces, including those that are hydrophobic. For hydrophobic interfaces, scientists would like to know what constitutes a "perfect" water-repellent surface.

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