Donning his regular work attire—jeans and a Hawaiian shirt—Richard Saykally tells me in four words the answer to a question I had often pondered in the shower: Why is water wet?
“Strong tetrahedral hydrogen bonding,” he said. The reply didn’t provide the instant illumination I was hoping for, but then, water is not simple. Saykally’s research group at the University of California, Berkeley (where he is a professor of chemistry) studies water with an exotic-sounding list of apparatuses, including cavity ringdown spectroscopes, terahertz lasers, and supersonic beams.
His goal is to develop a “universal water force field,” a computer model of water that could predict the behavior of water in any circumstance, down to the atomic scale. I was properly impressed by this ambition, but not particularly intimidated: Saykally made sure of that by offering more than once to play me a ditty on his harmonica.
The video plays at the top of the screen.
Why is water wet?
When my daughters were very little we had an interesting revelation on that subject. I was actually giving both of my daughters a bath when they were very young and my youngest daughter said, “Daddy? Why is water wet?” And the proper answer is: strong tetrahedral hydrogen bonding, which they then related to their teachers for years afterward whenever the subject of water came, they’d say, “Strong tetrahedral hydrogen bonding!” But that’s the correct answer. That’s what makes water wet.
What does a water cluster look like?
A water cluster is an arrangement of two or more water molecules. So they adopt various structures. Two water molecules doesn’t really have much of a shape; three water molecules makes a three-membered ring; four makes a squarish-looking ring; five makes a pentagon; and when you get to six water molecules, the morphology changes from being cyclic planar to being a three-dimensional cage; and thereafter seven, eight, nine and so on look like three-dimensional cages. The water eight—the eight-fold cluster—looks like a distorted cube, and then all larger clusters build on that cubic shape. These are the most stable forms that you would then find at very close to the absolute zero of temperature.
Is another form of liquid water possible?
This is currently the most hotly debated subject about water. It has been postulated for quite some time that in the deeply supercool region of water—that is, when water is cooled below its freezing point—that there may exist two different types of liquid. Ordinary liquid water we would call the low-density form and it’s proposed that there is a high-density form of water and that there’s a phase transition between these two types in the super cold region. And this debate has come up a number of times but right now it’s being ferociously debated. Actually, one of my colleagues in this department—a very famous theoretical chemist—and his former student are at the forefront of this, and it has not been resolved yet.
Why does water lose density as it turns into ice?
When water freezes into ordinary ice, which is the kind that makes the ice cubes that float in our highballs, this happens at what we would call zero degrees centigrade, at atmospheric pressure. When water freezes into ice it creates a very open structure. That form of ice comprises arrays of six membered rings that are stacked on top of one another to make channels and most of that ice is actually empty space. When you melt the ice to make liquid water, you break about 10 percent of the hydrogen bonds in the ice and it becomes much more disordered and compact, so the liquid being more disordered is denser than the ice. When the ice freezes, it makes this very open network and the density drops by an order of 10 percent. But that’s only true for the familiar form of ice that we call ice 1h, for hexagonal. There are actually 16 crystalline forms of ice. All the other forms are actually denser than liquid water. Only one of the 16 forms is actually less than.
Why are there 17 different kinds of ice?
Only the familiar form of ice that we call ice one is less dense than the liquid. All the other forms are denser than the liquid and they form at high pressures. When you squeeze the lattice of ice 1h, you force it into more compact arrangements. Like I said, the crystal structure of ice 1h has a great deal of empty space in it, so when you squeeze on it by applying high pressures, you force it into more compact structures; well you fill in that empty space more. And the harder you squeeze, you form more and more compact and dense structures until you reach what we call a close-packed limit, which hasn’t really been reached yet. So as technology evolves to apply higher and higher pressures, you can collapse ice to denser and denser forms. So I don’t think we’re done yet. There are 16 crystalline forms and as technology evolves, we’ll probably be able to generate another six or eight. In addition to the 16 crystalline forms of ice, there are also amorphous or glassy forms of ice that are by definition disordered, and there are a whole family of those. It used to be believed that there were two types of amorphous ice, but now we realize there are actually many, of varying density.
How is the surface of water different from bulk water?
At the surface of water there’s a different hydrogen-bonding arrangement. In the bulk water, every water molecule makes approximately four hydrogen bonds with other water molecules at tetrahedral angles; not perfect, as in the case of ice 1h. So it’s a disordered tetrahedral network. But at the surface when water molecules terminate the bulk, there are necessarily fewer hydrogen bonds. So the average number of hydrogen bonds for water molecules on the surface is perhaps two and a half or something like that. So there are dangling O-H (oxygen-hydrogen) bonds at the surface of water and this makes the surface layer behave differently than the bulk. So you have the outermost layer of liquid density, as we would call it, defining the surface and then you become more ordered as you move from that outermost layer of liquid density into the true bulk. So the surface layer has fewer hydrogen bonds; it’s more mobile and has different bonding properties.
Why is there intense debate about what ions do at the surface of water?
That has been one of the most controversial subjects involving water for decades because the behavior of ions at the surface of water has profound implications in biology and other areas of science; so it’s an important subject in a practical sense. The behavior of ions in water has classically been described through what we would call the dielectric continuum theory and this is in most of the textbooks on water until recently and this says that there should be no ions at the surface of water. But, we need to be a little more specific; let’s call it the air-water interface or the interface of water with the hydrophobic domains of proteins. In those cases there should be no ions at all at those interfaces because of a phenomenon called image-charge repulsion that emerges in this dielectric continuum theory.
But that theory is outdated and over the years experiments have accumulated that clearly demonstrated that some ions prefer to be at the surface rather than in the bulk. My group has established a number of different ions as cases where that is obeyed. Those ions prefer the surface and we’ve numerically verified the energies and forces with which they are drawn to the surface. So this is in violation of the textbook description of ions at the surface of water.
Why is the rate of water evaporation so hard to measure?
It has been very hard to measure over the years because it’s a surface phenomenon, very subject to contamination problems; and perhaps most importantly, evaporation of water is a very rare event. If you’re a water molecule in a glass of water, or even at the surface of water in a glass of water, the probability of you evaporating is very low. It’s a very rare event when a water molecule leaves the surface and therefore it’s extremely difficult to model that phenomenon by computer simulations. And the experiments are very problematic because contamination of the surface is a very big problem.
And the other problem is that most of the experiments that have addressed that observe simultaneous evaporation and condensation because in those experiments, there’s a layer of water vapor and contact with the liquid water, and so you get condensation of the vapor to the liquid at the same time you’re having the liquid evaporate into the gas phase and it’s very difficult to separate those two processes. So what my group did, to try to separate those processes, is use liquid microjet technology where we would make a microjet of water, that was perhaps 10 microns in diameter, in a vacuum system and then we could arrange the conditions to look at evaporation without having any condensation obscure our results.
So those are our recent experiments and our results agree quite well with theoretical calculations that have been done by David Chandler’s group, where they were able to transcend this limitation on being able to simulate very rare events because of this beautiful rare event methodology that the Chandler group has developed called transition path sampling. In that methodology, they’re able to directly observe the details of how a water molecule evaporates even though it’s a very rare event and they show in their very recent paper that a water molecule evaporates from the surface when it collides with another liquid molecule in such a way [as to] to give it enough kinetic energy to escape the surface tension, let’s call it, of the surface, and it does so where the surface has a capillary wave, as we call it. There will be an anomalously large fluctuation in the surface topology. So it’s like a wave breaks away from the liquid and when that wave breaks away, it strains the hydrogen bonds in surface water molecule and weakens it enough that the molecule can escape.
What do you think of California’s current drought?
Well, I think this needs to be taken very seriously. I’ve actually spent quite a bit of time in the last month thinking about this and educating myself on the drought situation and how some of the technology that’s being proposed to mitigate it can be implemented. So first of all there’s a lot of misinformation floating around. We hear that this is the worst drought in the history of California. We have to qualify that. In the written history since the government of California has been formed, this is probably true. But in the natural history of California, we know there have been far, far worst droughts. Let’s see, it was how many years ago … Centuries ago, there’s evidence from tree rings that have been recently studied by fossil experts that show that there have actually been 150-year droughts not all that far back in California’s natural history—let’s say 500 years or something like that ago. I forgot the exact dates. But there has been a record of far worse droughts than what we’re experiencing now. It is fully possible that this could turn into a 50-year drought or 100-year drought, which would be devastating, unless we have reliable sources of water that don’t rely on precipitation.
So desalinization seems to be the wisest course of action for coastal areas like California, where we have an ocean very nearby. If we can figure out how to cheaply desalinate ocean water and do it in a manner that doesn’t add a lot of carbon dioxide to our atmosphere, this would be a very big step forward for the long-term wellbeing of California. And I actually just came from spending 10 days in San Diego where the largest desalinization project in the western hemisphere is nearing completion at Carlsbad, north of San Diego. There’s a $1 billion desalinization plant that’s scheduled to come into operation in a couple of months and I’ve gotten very interested in the physics and chemistry of these desalinization plants. And right now, desalinization is very expensive and very energy demanding and it won’t really be an environmentally acceptable way to produce fresh water unless we can make it much, much more efficient and less polluting.
Some colleagues of mine and I put together a short proposal during my time in San Diego with the title of, “Towards Green, Efficient Desalinization.” The technology that people are thinking about right now is using what we call carbon nanotubes as a way to filter the salt out of seawater. It’s possible that that can be done with much less energy input because the resistance to pushing water through these tubes may be much lower than with current technology, but this must be established through fundamental laboratory science that I’m proposing to do and other people are proposing to do. We need to study the behavior of ions at the interface of water, our previous subject, with these carbon membranes, and it’s possible that the nature of that interface is such that with proper geometry, water can flow through tubes of pure carbon with very low resistance so that you could use much lower pressures to force the seawater through the desalinating membranes. That’s a very exciting prospect. And then that would greatly mitigate the energy consumption.
And then there are ways to think about how to sequester the carbon dioxide produced by, let’s say, combusting natural gas as a means of producing electricity, to sequester the carbon dioxide produced in that combustion in deep aquifers of very salty water that’s the product of desalinization. You get very concentrated salt brines that cause a problem in disposal. So if one could actually use those brines to store the carbon dioxide that would be a big advance too. People are thinking about all those directions; and at the same time, hoping California isn’t embarking on 100-year drought!
What is a water dimer and why is it important in understanding our atmosphere?
A water dimer is a cluster of two water molecules where one water molecule donates a hydrogen bond to the other. It’s very important in a theoretical sense because it is the prototype of a hydrogen bond. In a practical sense, there has been a lot of discussion about the potential role of this water dimer in the atmosphere. There are some important reactions in the atmosphere—for example, the formation of acid rain—that would proceed much faster if there were indeed water dimers present in the atmosphere. For example, the reaction of sulfur trioxide SO3 with a water molecule to make sulphuric acid, and subsequently acid rain, would require the collision of three gaseous molecules. But if instead, an SO3 molecule could collide with a water dimer, it would greatly speed up the reactions and the subsequent formation of acid rain.
And also, from the point of view of absorption of sunlight, the water dimer absorbs in a different part of the electromagnetic spectrum than just a water monomer, a single water molecule, and could potentially play an important role in global warming. So there’s been a lot of interest in ascertaining: Are there appreciable concentrations of water dimers in the atmosphere, and if so where would they be most probably located? The answer seems to be that water dimers can form effectively if the relative humidity is high and that happens in the equator regions. So it seems that as wet air from the tropics around the equator rises, water dimers can form in the atmosphere quite effectively and whether then they can be transported to other regions of the atmosphere is a current question.
Is it just a coincidence that water is essential for life on Earth?
No, it’s something intrinsic about water in that the strong tetrahedral hydrogen bond network that water makes is a very flexible environment for chemical processes to happen. It has the right properties to dissolve many ions; it has the right properties to cause what we call hydrophobic materials to fold up in special ways; and it would be hard to design a liquid that is that versatile that can adopt so many different configurations in the liquid and so on. It’s really quite special.
What has water taught us about the hydrogen bond?
The nature of the hydrogen bond itself has been vigorously debated for decades. It was originally thought that the hydrogen bond was a manifestation of what we call the dipole moment of water molecules—that there’s a positive end and a negative end to each water molecule and the hydrogen bond occurs when those two dipoles interact in an attractive fashion. But as sophistication of both experiment and theory evolved, it led to a more complex description based on quantum theory where we know now that indeed the major source of the attraction between two water molecules that comprises its hydrogen bond is this dipole-dipole interaction as it’s called, but there are others. There’s also something called induction where this dipole of one water molecule distorts the electron cloud of the other one and that adds some attraction to it. There’s also something called dispersion, which is a strictly quantum mechanical effect where the electron clouds of the two molecules interact in an attractive fashion. And then the fourth component is repulsion—that as you bring any two objects, any two molecules or atoms, close enough together, their electron clouds start to overlap and it becomes very repulsive, and that limits how close you can bring two water molecules together. So now we understand that the hydrogen bond is really a sum of those four different interactions that we call electrostatics, induction, dispersion, and repulsion.
Why did you invent a new laser to study water?
Two water molecules will vibrate relative to one another by the stretching motion or the bending motion of that hydrogen bond and those frequencies occur in the far infrared region of the spectrum—or in the terahertz region, as it’s called. It’s the same region of the spectrum. So the most direct probe of a hydrogen bond is to actually look at the stretching and bending vibrations of that hydrogen bond itself and that happens in the far infrared or terahertz region of the spectrum. So we developed technology based on far infrared lasers to be able to look at, to be able to measure those motions in water molecules and that’s what led to our many studies of water clusters.
What is the “universal water force field?”
This is what I was telling you is the ultimate object of our research in studying water clusters, both theoretically from our experiments and with quantum chemistry; [it’s] to produce the perfect model for water. We want to combine all the information available from studies of water clusters with our terahertz laser spectroscopy, from quantum chemical calculations, and from condensed phase measurements—we want to put all that information together and make a computer model of water that will answer any question you ask. Any question that is in principle answerable could then be answered by a computer calculation if you had the perfect water model. And that perfect water model is what we have been calling the universal first principles model of water.
What predictions could you make with the universal model of water?
If we had the perfect water model and we had a great deal of computer time, we could do simulations that would test this idea of, “are there two kinds of liquid water connected by a first-order phase transition.” That sort of thing could be done. We could do computer calculations of the surface of water and accurately determine what the surface looks like and how that surface changes as we bring the surface of water into contact with the hydrophobic domain of a protein, for example. Any question at all that you would have about water, that is in principle answerable, could be addressed by a computer calculation using the perfect water model.
The reason we can’t do that now is because, like I said, there are 100 or more models—computer models for water—and they all do some things well. None of them do everything well and in particular these models were developed for room temperature water or in a narrow temperature range, so when you take these computer models for water developed at room temperature and you apply them in the supercool region to the study of, “are there two kinds of liquids in the supercool region,” the first thing that comes to mind is [that] this water model is not capable of giving reliable results in that very low temperature range. It wasn’t produced with that in mind. So if we had a universal first principles model it would work at all temperatures, all pressures, etcetera.
What is it about water that makes it ripe for pseudoscientific speculation?
Well, since we live on a water planet and water is very much a part of every human being’s daily life, it has been recognized from early on that water is essential and it has these unusual properties. So if you go back to the Greeks, the Greek formulation of chemistry was that there were four elements: earth, air, fire, and water, right? And in fact, there were several competing philosophies. It’s only recently in science where we actually are making careful measurements of things that we claim to be correct. Modern science works on the basis of, you make a prediction from your theory or your laws of chemistry and physics and you test it against experiment. That wasn’t the case and so all of these pseudosciences have evolved based on this early idea of water being so essential an element. So homeopathy evolved out of that sort of thinking.
Even in the modern context, one of the interesting debates is, is there anything unique about so-called structured water? There are companies that sell bottled, structured water and they make claims that the structured water somehow penetrates your cell walls more effectively and has all kinds of health benefits and all this. There’s no scientific basis to that at all. You can’t make structured water. Doesn’t make any sense because the hydrogen bond in water lives for a few picoseconds—10-12 seconds—and these hydrogen bond structures of water are rearranging very rapidly so you don’t have water clusters existing as isolated entities in water despite a lot of these claims. But still you can go to the store and find bottled water that’s supposed to have these magic structural properties and so on.
Who inspires you?
Well, my own personal hero in science has been Charles Townes. Charles Townes passed away recently and was a very famous physicist here at U.C. Berkeley. Charles Townes was a co-inventor of the laser who got the Nobel Prize in 1950 … I forgot the dates but he got the Nobel Prize for inventing the laser. He discovered the first molecules in space and most recently, in collaboration with his post-doc Reinhard Genzel established the first characterization of a black hole—a detailed characterization of the black hole that exists in the center of our galaxy—he’s just a fantastic scientist. And one of the most exciting things for me to come to Berkeley, which I did in 1979, was to be able to interact with Charles Townes, who had been a hero of mine since I went to graduate school. One of the first things that happened to me when I joined the research group of (Robert) Claude Woods at the University of Wisconsin in graduate school is [that] he handed me the book from Charles Townes called, Microwave Spectroscopy, and he says, “Read this, this is the Bible.” And so Charles Townes has always been a great hero of mine and I think I have great choice in heroes.
What would you be if you weren’t a scientist?
If I weren’t a scientist? Well, the story is, I grew up in the very north of Wisconsin in a town of like 100 people; and if you grow up in Wisconsin, you are necessarily a great fan of the Green Bay Packers football team. So in my early days, I aspired to become a Green Bay Packer football player and I was torn between number 66, Ray Nitschke, who is the middle linebacker and considered the toughest linebacker in football; or being number 31, Jim Taylor, famous fullback for the Green Bay Packers. I wanted to be a Green Bay Packer but the sad news is, God did not cooperate very well in that. As I was in my high school sort of era, I wanted to become a rock star and played in rock bands all my life. So if I weren’t a scientist, hmmm … Oh, the other thing that happened when I was an undergrad, I, through luck of the draw, became a chemistry major and really liked introductory, or freshman chemistry, but then came organic chemistry and after a year and a half of organic chemistry, I became an English major. But I worked my way back to chemistry. So you know maybe … I love to write. I write a little poetry and I write stories and stuff just for fun. I might be a writer. Or maybe a rock star. But I can’t sing.
Brian Gallagher is the assistant research editor at Nautilus.