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A SPECIALBREWInvestigatorsstillcan'tcompletelyexplainthestrangemolecularworkingsof water.

By ChristopherJ. Mundy,ShawnM. Kathmann,andGregoryK. Schenter

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s children,we haveall lain in the grass andlooked up at the clouds. Sometimes theyseemed to take on the shape of an animal, afavorite plaything, a familiar face. For manyof us, such daydreaming segued into a deepercuriosity. What are the clouds? we wondered.

What are they made of?From an adult perspective, the answer seems obvious:

water. Stand among the clouds on a mountaintop, andyou can feel their moisture. Watch the plump whiteclouds of a sunny day transform into dark, dauntingbehemoths, and before long, sheets of water come pour-ing down. The common wisdom that clouds presagethe weather is grounded in a less well known fact: theunique properties of water-in particular, its capacityto transport enormous quantities of energy-are whatgive the weather its variability, its energy, and its oc-casional violence.

Of course, our relationship with water goes far beyondthe weather. We have fun with it whenever we go skiingor skating, boating, fishing, or swimming. The pleasureof a cold glass of thirst-quenching water on a hot summerday has a more serious basis, though. Without water, a

IJ 321 NATURAL HISTORY November 2007

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human being can live only a few days. Every organism ismade up mostly of water, and the substance covers nearlythree-quarters of the Earth's surface.

Yet this commonplace, familiar, and essential stuff oflife is also quite peculiar, as substances go. For example,if the water molecule (HzO) acted in bulk like othersmall molecules-oxygen (Oz), carbon monoxide (CO),nitrogen (Nz)-it would be a gas under the conditionsprevailing on Earth. Instead, water occurs in all threestates of matter: solid, liquid, and gas. Furthermore,water reaches its maximum density in its liquid form,at 39.2 degrees Fahrenheit (four degrees Celsius), just afew degrees above the freezing point. Thus water staysat the surface as it starts to freeze, and ice floats-a rareproperty shared by very few other substances. If its naturewere otherwise, all temperate-zone lakes, ponds, rivers,and even oceans would eventually freeze solid from thebottom up, and life as we know it could not exist. Instead,a floating skin of ice cocoons the life in the liquid waterbeneath a layer of insulation, enabling it to persist underthe frozen surface.

Another unusual and related property of ice is that, fora given temperature, increasing the pressure decreases

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Strange properties of water: (a)Cohesiveness enables water to travel upward from roots toleaves, against gravity. (b) High surface tension makes liquid water behave as if coated with aninvisible film, which explains why insects can walk on it. (c) Water exists in all three phases-gas,liquid, and solid-at temperatures and pressures that are common on Earth. This familiar propertyis actually quite unusual. (d) Ice floats on liquid water; unlike most substances, water is most densein its liquid phase. Lakes and even oceans would otherwise freeze solid in winter. (e) Water'sabundance and heat capacity are, in part, responsible for the moderation of global temperaturefluctuations and the gradual change of the seasons. (f) Water can dissolve a variety of substances,including acids, bases, and salts, earning it the moniker "universal solvent."

the melting point. (Ordinary solids remain solid underpressure.) Even though these and other unusual bulk prop-erties of water have been described in detail, a completepicture of how and why water acts the way it does is

Anyone who has visited the San Francisco Bay area has

still lacking. It is not possible, for instance, to completely experienced local climate moderation. The city ofpredict the properties of materials that incorporate water San Francisco maintains a mild climate year-round,in their structure, either physically or chemically, or to but just a few miles inland, where hills guard the bay,design and tune their responses to various conditions. temperatures can soar to 90 degrees F (32.2 degrees C) inPerhaps the key to achieving that level of understand- the summer and plummet to near freezing in the winter.ing and control is The reason for thisto study water on k contrast, as most

a molecular scale: ~ .,; y '" tS residents are wellhow water mol- 9-,. V 'Q9.Q ~ aware, is that theecules arrange V. ocean moderates

themselves, how Q J..: 1 -0 n large temperaturethey interact, and -. Q' a. ...Q..j n W ~f"J. fluctuations. Thehow they dance 6- ~ Q. JIIf \J same effect, onwith other kinds a global scale. isof molecules. We . . a factor in keep-Ice (left) consIsts of a collectIon of hydrogen-bonded water molecules .and our colleagues arranged in a pattern of hexagonal rings: oxygen atoms make up the vertices, Ing seasonal tem-in the growing andhydrogenbondsthe edges. In liquid water (center), the water molecules perature changesfield of molecular are linked to their neighbors by three to four hydrogen bonds, which continu- gradual ratherscience hope that ally break and re-form. Water vapor (right), an important constituent ofthe than abrupt.by understanding atmosphere, consists of weakly interacting water molecules. What governsexactly what happens at very small scales (around 10-tO the ocean's moderating effect is the large quantity andmeter, or a billionth of a meter), we can zoom out by a heat capacity of water. Heat capacity is the amount offactor of a billion or so to understand and predict phe- heat energy that must be absorbed or released to raise ornomena on a human scale. lower the temperature by a given amount. For example,

But we don't stop there. Because water is fundamental to it takes four times as much energy to warm a given mass

all life on Earth, we alsowant to zoom out by another factorof 10 million to study its properties on a global sf:ale.

November 2007 KATURAL HISTORY 133

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of water by one degree Celsius as it does to warm thesame mass of dry air by that amount. The heat capacityof water acts as a buffer, or perhaps a heavy flywheel, onclimate, smoothing out what would otherwise be sharpchanges in temperature.

Heat capacity is a good example of a macroscopicproperty of water that can be explained by what takesplace at a molecular level. The chemical formula H20,instantly recognized round the world, indicates that thewater molecule is a bound system of three atoms, two ofhydrogen and one of oxygen. When you add heat (a formof energy) to a macroscopic sample of water molecules,the molecules increase their average speed and collidemore often. The temperature of the sample is simply ameasure of their average speed. Any energy added to orsubtracted from the energy stored as such "translational"motion-movement from one place to another-changesthe temperature.

But molecules can absorb and store energy in other ways,too. A water molecule can spin or rotate like a top, or it canwiggle and vibrate. Both rotational and vibrational motioncan store energy. Yet they can't completely explain liquidwater's observed heat capacity. In general, the more waysmolecules can absorb heat energy without increasing theaverage speed of their translational motion-that is, thegreater the molecules' capacity to act as heat sinks withoutraising the temperature of a substance-the greater theheat capacity of that substance.

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you might think that no matter how little energy a

molecule may have stored, the energy would ~tillspread more or less evenly among all the pOSSIble

kinds of motions. In other words, the energy would bepartitioned among all possible "degrees of freedom."Because virtually all molecules made up of three atoms

341 NATURAL HISTORY November 2007

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have the same number of degrees of freedom, their heatcapacity, too, should not differ substantially. And sureenough, the heat capacity of the water molecule is aboutthe same as that of other triatomic molecules.

Although the conclusion to the preceding argumentagrees with the observed result, the argument itself isfaulty. In the world of atoms and molecules, energy cannotbe absorbed by the various kinds of molecular motions inarbitrarily small amounts. Rather, atoms and moleculesare subject to the laws of quantum mechanics. In thequantum world, all changes in the energy stored by anatom or molecule are quantized. Each kind of molecularmotion-translational, rotational, and vibrational-canabsorb energy only in discrete chunks, whose sizesdepend on the details of a particular molecule and thekind of motion involved. Expose a water molecule to theright-size chunk of incident energy (from the Sun, forinstance), and the molecule will suddenly rotate or vibratefaster. Expose it to the wrong-size chunk, and nothingwill happen; the molecule will simply "disregard" thepassing energy. As it happens, the amount of energyavailable at the normal range of temperatures on Earthis only enough to generate translations and rotationsof a single water molecule, but not enough to generatevibrations. Thus, on its own, the single water moleculeisn't enough to explain the heat capacity ofliquid water.In fact, the heat capacity of water vapor is smaller thanthat ofliquid water by more than a factor of two! So thepuzzle reemerges: how can the heat capacity of water beexplained from a molecular point of view?

The solution is to look beyond the properties of a singlewater molecule and consider the interactions amongthe vast number of molecules in a bulk sample. Begin

by considering the interaction of two water molecules. Eachmolecule isshaped like a tetrahedron, with the oxygen atomat its center. Each of the two hydrogen atoms lies at oneof the four corners of the tetrahedron, and each one actsas a center of positive electric charge. When it bonds withhydrogen, the oxygen atom acquires two complementary

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A cloud begins forming (a) as water molecules gatheraround dust (orange) and sulfuric acid (green) impuri-ties in the air. Attracted by intermolecular forces, moremolecules join the cluster (b). Eventually the clusterreaches a critical size-about 50 molecules-of less

than 1 nanometer (c). Until this stage, the collection ofmolecules is highly unstable and may break up. Gettingover the energy barrier of the critical cluster constitutesa phase transformation from gas to liquid. The growthprocess takes off (d) eventually resulting in condensationnuclei, made up of particles that differ in size by a factorof ten or even a thousand. A cloud droplet (e) ultimatelyemerges, made of 10'5molecules; it may eventually growinto a raindrop (f) of 1020molecules.

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centers of negative electronic charge, which cluster at theopposite two corners of the tetrahedron.

When the two water molecules are brought together,opposite charges attract. One of the sites of positivecharge-a hydrogen atom-in one molecule becomesattracted to one of the negatively charged sites associ-ated with the oxygen atom in the second molecule. Thisattractive interaction is known as a hydrogen bond. Inliquid water, each molecule often forms four hydrogenbonds. Two of them link the two hydrogen atoms withthe oxygen atoms of two other water molecules [seeillustrationon page 33]. The other two hydrogen bondslink the oxygen atom with hydrogen atoms of two morewater molecules. Those bonds give rise to a stable net-work of tetrahedral water molecules. In the liquid thenetwork extends only locally, and the hydrogen bondscontinually break and re-form. But in ice, the networkof tetrahedrons extends over a long range and becomesa relatively unchanging lattice.

Within a network of tetrahedrons, the number of waysincident energy can create rotations, twists, vibrations,and suchlike significantly rises. Each new mode of motionprovides an additional degree of freedom, and so the heatcapacity of the network far exceeds the heat capacity ofa single constituent molecule. Note, though, that many

other molecules, including other triatomic moleculessuch as carbon disulfide (CS2), also form linked networkswhose heat capacity far exceeds the heat capacity of oneof their constituent molecules.

In fact, although it is not widely appreciated, the heatcapacity of water, even within a linked network havingmany degrees of freedom, is not unusually large-pro-vided the heat capacities are stated in units of energy permolecule or per mole, which is 6.02 X 1023molecules ofthe substance. On that basis, the heat capacity of wateris about the same as that of other triatomic molecules.In the appropriate units, for instance, the heat capacityof water is 75.3, whereas the heat capacity of carbondisulfide is 75.7.

Only when heat capacity is measured in the amount ofenergy per unit mass does the heat capacity of water lookanomalously large. The reason is that the molecular massof water is small compared with that of other triatomicmolecules. Expressed in those units, the heat capacity ofH20 is more than four times' that of CS2.

The study of the various configurations of the hydrogenbond has made it possible for molecular scientists to explaina number of other anomalies of water. For example, in ice,the hydrogen bonds tend to be slightly longer than theyare in the liquid phase, resulting in a larger volume and

November 2007 NATURAL HISTORY 135

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thus the lower density of ice than of liquid H20. If iceis compressed, the hydrogen bonds shorten and becomemore like those in the liquid.

Stable hydrogen bonds also lead to the strong cohesiveforces underlying the unusually high surface tension ofwater. Insects such as water striders take advantage of thesurface tension when they skip across a pond-as if itssurface were made of clear, flexible plastic. And the highcohesiveness and surface tension of water explain how

interaction, the larger the target area onto which moremolecules and other nearby droplets can be pulled.

As the water cluster grows, it remains highly unstableuntil it reaches a certain critical size-the critical cluster-which is about fifty molecules. Precritical clusters canbreak up at any time into single water molecules. As itapproaches the critical size, though, the cluster is alsoclimbing to the "top" of an energy "hill." If it reaches thetop and attains the critical size, it can then "roll down"

Initsvaporform,wateristhemostimportantgreenhousegas,soit playsa majorroleintheclimateofourplanet.

long columns of watery suspensions can be drawn throughextensive networks of blood vessels and even into treecanopies several hundred feet above the forest floor.

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Water is the third most abundant chemical com-

pound in the Earth's atmosphere, after nitrogenand oxygen. It is present there both as a vapor, or

gas, in which the water molecules dart about randomlyand independent of one another, and as an aerosol, amist of tiny liquid droplets or solid ice crystals that aresuspended in air because they're too fine to fall to earthas rain or snow. In its vapor form, water is the most im-portant greenhouse gas, so it plays a major role in theclimate of our planet.

But when water takes the form of an aerosol, it is crucialto cloud formation and to the reflection and absorptionof radiation. Water aerosols act as condensation nuclei forclouds-after all, clouds themselves are made up of rela-tively large aqueous aerosols. And water aerosols transformradiation in ways that, in turn, feed the factors that shapecloud development. Reaching a better understanding ofhow water aerosols affect climate has become increasinglyimportant in the past several decades.

Water aerosols enter the atmosphere when waves breakin the ocean or when vapor turns to liquid. The latterprocess, condensation, is in essence a battle between en-tropy and energy, order and disorder. As water moleculescondense into their liquid state, they gain order but losekinetic energy. The kinetic energy given up by the phasechange is dumped as heat into the surrounding air, givingrise to a pocket of thermal instability that will drive yetanother change in the weather as it equilibrates.

The physical process of condensation is "seeded," ornucleated, around tiny molecular impurities or perhapsa dust particle in the air. Once an "embryo" of the newliquid phase forms, more molecules tend to gather aroundit and glom onto it, attracted by intermolecular forces [seeillustrationonpages34-35]. The larger the surfacearea ofthe growing .cluster, or the stronger the intermolecular

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the far side of the energy hill and undergo spontaneous,runaway growth. From fifty molecules, the cluster growsand agglomerates to the size of a condensation nucleus(108molecules), then to a cloud droplet (1015molecules),and finally to a raindrop (1020molecules).

In 1998, members of our research team were among thefirst to measure the chemical identity of the nucleated par-ticlesand to show that the chemical interactions among themhave a profound influence on the aerosol formation rate. Wemodeled the rate of evaporation of molecules from clusters,developed a molecular simulation strategy to compute therelevant kinetics, and applied the strategy to water. We foundthat the molecular interactions between water and the initialnucleating particles-whether dust, sea salt, sulfuric acid,ions, or some other substance-may significantly affect therate of aerosol formation. That rate affects the distribution,duration, and precipitation processesin clouds, and thus theirtendency to reflect, transmit, or absorb the Sun's radiantheat. All those properties in turn influence the reflectivityof the Earth and thus the global climate.

Atmospheric scientists have yet to determine the exactnature of that influence. One possibility is that if clusterdroplets grow more quickly, more clouds may form,helping moderate global warming by providing morecloud cover. On the other hand, faster droplet growthcould accelerate the production of rain, causing clouds todissipate sooner. That would lead to a less cloudy world,and faster warming.

As we molecular scientists learn more about water,we are continually reminded that we have merely"scratched the surface" of its secrets. The mecha-

nisms of its impact on life are still something of a mystery.Coaxing Mother Nature to reveal further secrets aboutwater will require the full interdisciplinary sophistica-tion of today's scientific toolbox. But since water is thewellspring oflife, we owe it to ourselves-and everyoneelse-to explore all we can about its strange and intrigu-ing properties. 0