October 31, 2007

‘The Name’s Bond. Hydrogen Bond.’

By Rostro, Betty Catalina

Water is one of nature's most amazing fluids-and the hydrogen bond is the key to understanding its peculiar, eccentric and mesmerizing properties. Water is all over-two-thirds of the planet is covered by it-sometimes where we need it, sometimes not. Water is, in the words of Nobel Prize winner Albert Szent-Gyorgyi, "life's matter and matrix, mother and medium."

Water seems such a simple substance, just two hydrogen atoms covalently bonded to a single oxygen atom, yet it has always exhibited abnormal behavior. While the polarity, high dielectric constant and small size of water make it an exceptional solvent, liquid water's overall random and fluctuating three-dimensional network of hydrogen bonds makes it one of the most complicated fluids in nature.

Look at some of its puzzling physical and chemical properties- its expansion on freezing, its frictional properties, surface tension, density, liquid-gas behavior, bonding arrangement, viscosity and resulting hydrophobicity and hydrophilicity, not to mention the so far mysterious structure of ice. Water's overall bond, named the hydrogen bond, is responsible for some of its most fascinating properties, but this isn't all there is to it. The structure of water-whether it is normal hydrogenbased or heavy deuterated water, its Van der Waals and hydrogen bonding abilities, its relation to polarity, electrostatics and ensuing hydrophobicity and hydrophilicity tendencies are also responsible for water's eccentricities.

Water has acquired a status suggesting there's a certain cachet in being obscure. In fact, water's behavior is so complex that it continues to amaze and mesmerize modern scientists. Because of its fundamental role in sustaining life, it has been among the most studied substances on Earth. Plato, Aristotle, Da Vinci, Archimedes, Pythagoras. Newton, Maxwell, Lord Rayleigh, Faraday and Einstein also have delved into water chemistry, each in his own way, pioneering research in the field.

Da Vinci, for example, was so captivated with the properties of water that he spent years inventing powerful devices by which to conquer its fury. The reason for such devotion was that prior to 1850, water was central to global trade, commerce and tourism and, therefore, essential to the transportation needs of society. From conquistadors to modern day explorers, water also has been essential to the rise of imperialism. Prior to the Industrial Revolution, water was also the chief source for generating power and irrigation and was essential for many milling, mining and manufacturing activities.

During World War II, heavy water, a variant of ordinary water where the hydrogen atoms are replaced with deuterium (an isotope of hydrogen that is twice as heavy due to an added neutron) became crucial to Hitler's Nazi Germany. Heavy water, which was produced via electrolysis and is a byproduct of ammonia fertilizer production, was identified by German scientists as being essential to the creation of an atomic bomb. Nazi scientists had settled on a heavy water moderated nuclear reactor that could be used to convert uranium238 into plutonium-239, from which they could potentially construct a nuclear bomb. It was for this reason that the German army devoted its increasingly stressed military resources to invading Europe's only potential heavy water facility, Norway's Vemork hydroelectric plant.

Hitler underestimated the ability of the Norwegian resistance and British intelligence to mount a counter offense to protect the plant, and the result was that the technology did not fall into the hands of the Germans. But heavy water production and heavy water outsourcing were completely halted by these operations. This battle was different in its goal from the range wars of the American West, where settlers fought and died for water rights on their land, the key to their very survival; but it is another example of how the complexity and relevance of the many fascinating properties of water have determined its important role in history, society, industry and technology, In today's world, that role is center stage.

The science of water

To understand the complexity of water, it's critical to understand the historical identification of the water molecule, its detailed molecular interaction and its resulting hydrogen bonding and the Van der Waals interactions. It was the German team of Alfred Werner and Arthur Hantzsch that, in 1910, pioneered the use of secondary valence, which would later bring to fruition the concept of hydrogen bonding in organic chemistry. In 1920 Wendell M. Latimer and Worth H, Rodebush were the first to give an accurate description of the structure of water and to initially describe the concept of hydrogen bonding, calling it a "weak bond." This approach was eventually dropped, as covalent, metallic and ionic bonding took precedence.

Nevertheless, the topic of hydrogen bonding would later peak the interest of Linus Pauling who, in 1935, used the term in order to account for the residual entropy of ice. By 1945 Henry S. Frank and Marjorie W. Evans were working on a theory that would thoroughly describe the hydrophobic effect. In 1958 Irving M. Klotz developed the theory of bonding, which is present between two nonpolar molecules. It was Walter Kauzmann who, in 1959, coined the expression of hydrophobic bond, which he described as existing between an aqueous solution of nonpolar protein groups. Kauzmann would additionally affirm that chemical bonds differ from the weak Van der Waals forces that often are present in water. By 1968 scientists were exploring the spontaneous tendency of nonpolar groups to adhere in water; such work would further increase the understanding of the hydrophobic interaction. The computational power of the early 1970s led to a new era of discovery. Water chemistry continues to be a hotly debated topic, as scientists even today continue to dispute the long-ranged hydrophobic interaction and its relation to water.

It's the hydrogen bond!

Water's puzzling behavior results from its hydrogen atoms, which are constantly exchanging due to protonation and deprotonation processes that are catalyzed by both acids and bases. The oxygen atom in water attracts electrons more strongly than the hydrogen atom, which yields a positive charge on the hydrogen and negative charge on the oxygen, producing a net dipole moment. So as the hydrogen atom is covalently attached to the oxygen atom, it also is attracted to the neighboring oxygen of another water molecule. This dipole attraction gives way to the hydrogen bonding that is felt by the hydrogen atom, which is further attracted to the strong forces pulling it toward the two oxygen atoms. The resulting hydrogen bond is 90% electrostatic in nature and only 10% covalent, with the bond strength being dependent on the bond length and angle.

The breaking of only a single hydrogen bond would weaken the network of surrounding atoms and further restrict the hydrogen bonding potential of adjacent water molecules, resulting in fewer and weaker hydrogen bonds. However, the making of a hydrogen bond would reinforce the hydrogen-bonded network and mutually strengthen the long-range interactions of water, resulting in a synergistic and directive hydrogen bonding network that can fluctuate based on the reorientation of a single molecule. This is why liquid water can, in effect, "sense" information about solutes that are over several nanometers (nm) in length. For surfaces, such sensing can extend to up to tens of nanometers. The polarization effects and resonant intermolecular vibrational O-H energy transfers, which affect neighboring molecules, also further reinforce water's sensing abilities.

Water chemistry's added complexity

The introduction of nanotechnology and picotechnology, which is defined as the ability to manipulate matter with nanometer or subnanometer accuracy, has taken water chemistry to another level of complexity. The overall importance of water to tribology and lubrication science, ceramics, microelectromechanical systems (MEMS), emerging nanotechnologies, microfluidics, colloid science and hydraulics has led to studies that further shed light on the surface chemistry of confined water, the hydrophobic interaction and its role in lubricity.

It has been known for some time that the extraordinary "stickiness" of water is due to its hydrogen bonding and electrostatic nature. We could, in fact, blame the hydrogens and their sole electrons for spending most of their time inside the water molecule, constantly searching for the more negative oxygen atoms. This makes the outer hydrogens of water more positively charged, resulting in oxygen atoms that are always seeking out the positively charged hydrogens. This constant fluctuation of hydrogen and oxygen atoms results in temporary hydrogen bonds that make water an extraordinarily sticky fluid.

When surfaces meet, water molecules begin to engage in a tug of war with their neighbors. Each water molecule begins to pull up, down, left and right, and this results in the water molecules feeling no net force. When the surface presents itself, lack of liquid above the surface makes water lose its ability to engage in the tug of war. The surface molecules instead begin to pull themselves back into the liquid, minimizing the exposed area. Nevertheless, the water molecules also begin to maximize their surface molecular density, i.e., the number of water molecules per unit area. In doing so they produce a 'stretched skin' effect. While this takes some amount of work, the exposed surface further encourages more molecules to be dragged in from the liquid. The end result is surface tension, where the surface layer of water causes the exposed surface to behave like an elastic sheet. Surface tension then paves the way to capillarity, which is the ability of a substance to draw another substance into itself. Capillarity occurs when the adhesive intermolecular forces between liquid water and a solute become stronger than the cohesive intermolecular forces resting within the liquid. Both of these forces will then begin to play important roles in modulating the behavior of liquid water with the newly exposed surface.

Hydrophobicity and nanobubbles

This is where hydrophobicity kicks in-even though liquid water has been studied for many decades, its behavior with hydrophobic (water-hating, e.g., teflon, paraffin wax) surfaces has remained vague. Water's hydrophobic interaction has long been implicated in generating a long-range hydrophobic attraction that can extend to distances that are from 20 nanometers to 100 nanometers in length. Professor Phil Attard from the University of Sydney in Australia has concluded that the long-range hydrophobic interaction present in liquid water arises due to submicroscopic bubbles that are present on hydrophobic surfaces.

Using an atomic force microscope, Dr. Attard's team was able to image glass surfaces that were immersed in water. As shown in Figure 1, their images revealed closely packed and interconnected irregular networks of pancakeshaped nanobubbles that were 30 nm in height and whose radius of curvature ranged from 100 nm to 300 nm. "The nanobubbles appear on hydrophobic surfaces in water. They are due to dissolved gas coming out of the water," Attard said. "It may be that they are enhanced if the surface has a higher temperature than the solution. It is possible that water supersaturated with CO2 displays nanobubbles more readily than atmospheric gases, but this is unclear. The hydrophobic surfaces appear to act as nucleation sites for the nanobubbles, but they do not have to be rough." Another idiosyncrasy is that the nanobubbles were almost completely covering the hydrophobic glass surface, and they would rapidly reform even after they were disrupted.

The fact that the nanobubbles are pancake-shaped meant that the pressure inside the bubbles would be related to the curvature of the bubble's surface, Attard's team concluded. Small spherical bubbles would have a higher curvature and experience a higher pressure; this would cause the trapped gases to rapidly dissolve into the surrounding water, leading to a loss of structure. A pancake-shaped nanobubble, on the other hand, is more flattened, resulting in a lower curvature that endows it with a reduced pressure. This generates a nanobubble structure whose lifetime can be in the neighborhood of hours and a morphology that would be dependent on pH. The electric double-layer repulsion that is present between neighboring nanobubbles would further serve to stabilize the nanobubbles.

Yet it seems that the hydrophobicity of the surface is also relevant. "Hydrophobicity is important, and the higher the contact angle the more likely it is that nanobubbles will appear," Attard said. "Electrostatics appear important. The nanobubbles appear to have the same surface charge as the macroscopic air-water interface. The nanobubble surface charge is sensitive to pH, being zero at around pH 3, (point of zero charge) and being negative at higher pH, (due to preferential adsorption of OH- ions). It has been shown that the radius of curvature of the nanobubbles is a maximum at the point of zero charge."

Attard also believes that the formation of these nanobubbles is brought about by the depletion of water density in the region. The growth and bridging of these nanobubbles would in turn generate a bridging meniscus that could be strong-enough to pull two surfaces together. The role of hydrogen bonding in the formation of nanobubbles remains a mystery. "It is unknown if hydrogen bonding plays a role. Nanobubbles have been found in water and also in water- ethanol solutions. Few studies have been done in other liquids," Attard said.

But why the nanobubbles? It has always been known that as liquid water begins to interact with a hydrophobic surface, the reaction is rather unfavorable-the water molecules will attempt to get away from the undesirable material. A lower fluid density at the interface of a hydrophobic surface forces liquid water into forming a low- density gas-like layer, which results from capillary condensation and ensuing evaporation at the surface. This helps to nucleate the bubble formation, which is responsible for generating the long- range attraction that is present between hydrophobic surfaces. The depletion of water density at the hydrophobic interface region gives way to the partial drying of the surface by the nanobubbles.

Some researchers have been reluctant to accept Attard's explanation, instead suggesting that bubble formation could result from nucleation that originated with the atomic force microscope. However, a less invasive study involving neutron reflectivity of deuterated heavy water in contact with a deuterated hydrophobic polystyrene surface also showed uniform nanobubbles that were 50 nm to 120 nm wide and 18 nm high. Other X-ray and neutron reflectivity studies also have observed these nanobubble inclusions. These studies have produced a consistent picture that begins to describe water's ability to form a gas-like layer of nanobubbles on a hydrophobic surface.

The nanobubble phenomenon also would help explain how water can render hydrophobic surfaces either surprisingly slick or unusually sticky, according to Attard. "Nanobubbles have two different effects: They increase the adhesion between hydrophobic surfaces, and this leads to an increase in friction for a given net load. The second effect concerns the flow of liquid or lubricant past a nanobubble-covered surface. In this case, stick boundary conditions are replaced by slip boundary conditions, and so the fluid flows more easily. So if acting as a lubricant, two bodies shear past each other more easily. Or, if considering the flow of liquid in a channel, the fluidsolid 'friction' is reduced."

Attard's research also supports Ric Pashley's work at the Australian National University. Pashley found that the removal of minute quantities of dissolved oxygen and nitrogen gases, which are naturally found in water, dramatically increase the ability of water to mix with non-polar hydrophobic solutes such as oil. Pashley's team noted that at room temperature, about 20 milliliters of atmospheric gas is present in every liter of water Since these gas molecules are not accepted by the water's hydrogen bonding network, they tend to accumulate on hydrophobic surfaces. As cavitation tends to nucleate the formation of a negative pressure between the gases and hydrophobic surfaces, the gas cavities begin to produce nanobubbles, which tend to act like glue.

Nanobubbles, therefore, may hold the key to a number of practical applications. The fact that hydrophobic surfaces are covered with nanobubbles suggests that their surface chemistry is important. Altering the liquid-vapor interface of water by using either physical adsorption or a chemical reaction could in fact fine-tune the gas film formation that is responsible for generating the bubbles. This could then alter the surface chemistry and extent of nanobubble formation. "In the processing of colloids, the long- range attraction between hydrophobic colloid particles is due to nanobubbles bridging between them. This causes the colloidal dispersion to flocculate (destabilize, coalesce). This must be controlled to achieve the outcome that is desired," Attard said.

Another application where nanobubbles may be problematic is in the motion of particles in liquids. Nanobubbles would reduce the drag that hydrophobic objects or surfaces feel when liquid water flows over them, resulting in the finite slip lengths that are present at the fluid-fluid interface. The interfacial slip would, in turn, control the flow of liquids that are adjacent to surfaces or which is present in hydrophobic capillaries. This drag reduction would benefit microfluidic applications, which have a large surface- to-volume ratio and whose interfacial properties govern the dynamics of fluid flow.

"In microfluidics, the flow of fluid in small channels is crucial, and so the above discussion of slip is very important." Allard said. "In froth flotation, whereby hydrophobic mineral ores are separated from gangue material by bubbling gas through the mixture and collecting the froth with the attached ore particles, nanobubbles attached to the ore particles are responsible for their attachment to the macroscopic bubbles and froth. So again understanding and controlling the nanobubbles is important to mineral separation."

Another peculiarity-confinement

When it comes to confinement, water exhibits yet another level of complexity. Confined water is present in MEMS, nanoelectromechanical systems (NEMS) and microfluidics, which are composed of tiny moving parts that are often made out of silicon. These materials experience such high levels of friction that they can easily suffer from excessive wear, Often this wear will completely destroy the intricate structures that are housed within these systems. Using liquid water as a lubricant in such nanoscaled devices often proves to be futile, as water ceases to be wet and slippery and instead behaves like solid glue.

Professor Joost W.M. Frenken, from the Kamerlingh Onnes Laboratory at Leiden University, was able to use a friction force microscope to show that at high humidity water vapor condenses in the tiny region between two surfaces. The small capillary condensate of water, which was located between a tungsten hydrophilic tip and a hydrophobic graphite surface, formed ice at room temperature. A schematic of this is shown in Figure 2. Frenken's team noted that the static stress and regular structure of the ice-like condensate had sticky glue-like qualities. This glue-like behavior would produce many of the capillary adhesion and static frictional forces that are encountered in a variety of applications ranging from geophysics, nanotechnology, microfluidics and protein science. "The dependence of the lubricating properties of water on the dimensions has been the subject of considerable debate in the recent literature," Frenken says. "Some reports have claimed to see that the viscosity of water remained very close to that of regular water, even when it was confined to narrow gaps with a width of just a few molecular diameters (1 nm). However, other reports indicate that the water behaves increasingly viscously when it is confined more and more, the viscosity for nanoscale confined water being close to that of tar! You can imagine that tar would not be the best lubricant."

Frenken adds, "Our work shows that the truth is closer to the latter description (tar) than the former (regular water). In fact, the water appears to solidify. There is ice, bridging the nanometer gap between the two surfaces that are supposed to be lubricated. Now, you might associate ice with the slipperiness of ice-skating, but that would be the wrong picture here. The ice is actually freezing the surfaces together, thus acting like a glue rather than a lubricant. Moving the surfaces over m each other can only be done by breaking the ice."

This effect is significant in MEMS or NEMS devices, which are composed of structural elements that have capillary-like structures that are in close proximity or close to touching one another, When such capillary-like structures are exposed to air, the water molecules present in the air could end up undergoing capillary condensation into an ordered, solid-like, hydrogen-bonded network- in other words into an ice-like structure at room temperature.

"Capillary condensation is the mechanism by which you naturally get water everywhere where surfaces are brought sufficiently close to each other in an atmosphere that is not completely dry. The water molecules in the air (water vapor) prefer to condense in the small confinement between two nearby surfaces. For typical values of the humidity, this only happens if the surfaces are really close to each other, e.g., 1 nm. What we have shown is that once the water is there, it is immediately converted to ice," states Frenken.

Curiously enough, it seems that surface chemistry also may play a role in influencing the formation of the ice-like condensate. As Frenken explains: "The substrates (both sides) are really important, because they determine how eager the surfaces are to becoming covered with a layer or with droplets of water in humid air and, therefore, at what distance a capillary water bridge will form between them. Another aspect that will be important is the degree to which the surfaces are imposing this crystallinity that I mentioned above on the nearest layers of water molecules. In our experiment, one of the surfaces was (probably) hydrophilic, namely the tungsten tip must have been covered by an oxide, which is known to 'like' water. The other surface, the graphite, is hydrophobic.

"If you put water droplets on graphite, they ball up into hemispheres on the surface, making the contact area between the water and the graphite relatively small. If both surfaces would have been hydrophilic, the effect would have started at a somewhat larger distance. On the other hand, if we would have had two completely hydrophobic surfaces (droplets balling up into complete spheres with minimum contact area to the substrate), the capillary condensation effect would have been suppressed very much. Based on this, we expect that the latter combination (hydrophobic surfaces) should be best for reaching low friction in atmospheres with a non-zero humidity."

Yet, why would the water turn into ice at room temperature? Frenken believes that, "The reason for the water to turn into ice is probably that the surfaces force the water molecules to line up into layers. The layered arrangement is not natural for liquid water, but it is, of course, typical for the regular structure of ice. This is making it slightly more favorable for the molecules to continue in the form of an ice lattice rather than a disordered liquid arrangement.

"This ordering effect happens almost always where a liquid is in contact with a solid wall. However, the order is noticeable only over a distance of a few molecular layers. That means that the vast majority of the water will usually be liquid. This changes, of course, in our present situation of confined water. The two 'walls', i.e., the two surfaces that are almost in contact have a gap between them that can accommodate only a few layers of water molecules. No wonder that under these circumstances, where all water molecules are close to both walls, the water is solid throughout. This is an enthalpic effect, although it is impossible to discuss these matters only in terms of enthalpies and neglecting the role of entropy, so close to the solid-liquid transition."

This ice-like structure could result in a strong capillary force that could pull the touching or nearly touching structures into intimate contact, producing the large adhesion and stiction forces or static friction forces that quite often lead to the failure of MEMS and NEMS devices. Says Franken: "As described above, the ice forms a solid bridge between the two surfaces, a hard glue. The only way to move the surfaces parallel to each other is to break the ice. Since the ice is reforming/repairing itself the whole time, the breaking is not a one-time affair, say, at the beginning of the sliding, but one has to keep on breaking ice in order to keep moving. Hence, the significant increase in the friction force experienced. This effect should be gone as soon as the water no longer solidifies."

"Although we have not yet been able to repeat this experiment with full control over the distance between the surfaces, in search for the distance dependence of the ice-effect, we can estimate that the effect should be gone at a distance of just a few nanometers," Franken adds. "It is a truly nanoscale effect. But it happens everywhere, because when materials touch each other, they locally get that close to each other."

In order to avoid the friction and wear that can be suffered at the expense of the nanoscale water and its ensuing ice-like condensate, such structures would need to be lubricated with a liquid that could remain slippery down to the nanoscale. The friction loops and elastic behavior of the ice-like-condensate are shown in Figure 3.

While the nanoscale glue-like behavior of water can be troublesome for some applications, there are other situations where it can be of benefit. In laboratory settings, capillary condensation is often used to pick up light objects by simply touching a small object with a thin wire. Such effects are commonly employed in macromolecular protein crystallography where cat whiskers, nylon loops or small capillaries often are used to mount or manipulate small-protein, nucleic acid or small-molecule crystals that are microns in size. In the semiconductor industry, self-alignment of patterned wafers can be achieved by using capillary forces that are present at the water-air interface. Micro-fabrication techniques also make use of capillary forces in order to self-assemble nanoparticles and nanowires into complex nanostructures.

In optics and photonics, novel optical switches can be generated from complex photonic structures. When these structures are composed of porous silicon whose pore size falls in the tens of nanometers (which is the order of the diameter that is needed for vapor condensation to occur at room temperature), a finer optical fine- tuning is achieved. The nanosized periodicity of the porous structure will alter an organic solvent's gas-liquid equilibrium leading to the formation of a capillary-condensate. As the organic condensate begins to fill the pores, it causes a refractive-index distribution inside the material. And this gives rise to an optical bistability that significantly improves the photonic material's ability to interact with light, resulting in an enhanced optical fine-tuning.

Within membrane science, wetting and capillarity induce special lipid phases that are found in highly hydrophobic integral membrane proteins. This generates a long-ranged lipid-mediated joining force that acts between proteins and results in increased protein organization. Water's glue-like behavior is commonly encountered poolside, when thousands of small capillary bridges form between the skin and a wet swimsuit, causing the fabric to stick to the skin and making it difficult to change out of a wet suit. Wet feet, on the other hand, evidence capillary effects that help create high levels of friction with a dry floor, and this can prevent a fall. However, on a wet floor, for sure a slip would occur.

A new molecular understanding

So you see a new stir is being caused by water. After all, it is the simple water molecule whose almost universal and common presence often deflects attention from its eccentric properties. Since water chemistry has always been central to a number of industrial processes, it is not surprising that the investigation of its properties can be traced back to hundreds of years. Yet modern studies have started to unlock the complexity of water's hydrogen bonding networks, their relation to surface chemistry and interaction with hydrophobic solutes. As a new molecular understanding of the water molecule is emerging, it is shedding new light on its peculiar behavior. For one, it is obvious that physical conditions of temperature and pressure, chemical reactivity from ions and solutes and that interface and confinement will significantly alter water's structural and molecular features. It is, therefore, prudent to examine the behavior of liquid water under a wider range of conditions. As such, a new picture of the complexity of the simple water molecule has emerged, and this has resulted in a new understanding of its role in lubricity and its long-ranged hydrophobic attraction.

Yet fundamental to this insight was the discovery of the nanobubbles, their relation to surface chemistry, capillarity and ensuing ice-like condensate. So the next time you encounter water, thank Mr. Bond, the Hydrogen Bond, for its special role in causing networks of water molecules to link together. For it's the hydrogen bond that begins to explain the inherent yet peculiar behavior that is so often associated with the simple water molecule.

Water is, in the words of Nobel Prize winner Albert Szent- Gyorgyi, life's matter and matrix, mother and medium.'

Plato, Aristotle, Da Vinci, Archimedes, Pythagoras, Newton, Maxwell, Lord Rayleigh, Faraday and Einstein also have delved into water chemistry, each in his own way, pioneering research in the field.

When surfaces meet, water molecules begin to engage in a tug of war with their neighbors. Each water molecule begins to pull up, down, left and right, and this results in the water molecules feeling no net force.

Height profile of nanobubbles (shown in white), which are irregular in dupe, highly interconnected and form a network that almost completely covers the underlying hydrophobic surface.

To understand the complexity of water, it's critical to understand the historical identification of the water molecule, its detailed molecular interaction and its resulting hydrogen bonding and the Van der Waals interactions.

The breaking of only a single hydrogen bond would weaken the network of surrounding atoms and further restrict the hydrogen bonding potential of adjacent water molecules, resulting in fewer and weaker hydrogen bonds.

Schematic of the hydrophilk microscope tip, the hydrophobic graphite substrate and die water-ke condensate in between.

The introduction of nanotechnology and picotechnology, which is defined as the ability to manipulate matter with nanometer or subnanometer accuracy, has taken water chemistry to another level of complexity.

Measurements of the lateral force as a function of the sideways displacement. Panels (a) and (b) are in low relative humidity (RH), where no capillary condensation is present, and as such evidenced the friction of the tip that resulted from imaging the graphite surface. The upper left panels shows a saw teeth curve, which corresponds to the tip imaging approximately four carbon atoms for each nm of sliding. This stop-and-go motion is referred to as "stick- and-slip motion." Panels (c) and (d) were at relatively high humidity and evidenced a lack of the stick-and-slip motion, which signified that the ice like condensate was intervening between the tip and the surface measurements. The curves from the lower panels showed higher lateral forces and evidenced the elasticity of the ice- like condensate, which meant that the condensate behaved much like solid ice.

Water's puzzling behavior results from its hydrogen atoms, which are constantly exchanging due to protonation and deprotonation processes that are catalyzed by both adds and bases.

Betty Catalina Rostra is a free-lance scientific writer and doctoral student in the department of physics and astronomy at Rice University in Houston. You can reach her at [email protected]

Copyright Society of Tribologists and Lubrication Engineers Oct 2007

(c) 2007 Lubrication Engineering. Provided by ProQuest Information and Learning. All rights Reserved.