"Long before it's in the papers"
January 28, 2015


“Dance restaurant” theory of water takes shape

Aug. 14, 2009
Courtesy SLAC Na­tional Ac­cel­er­a­tor Lab­o­r­a­to­ry
and World Science staff

Eve­ry­one knows wa­ter—it shapes our bod­ies and our plan­et. But de­spite this abun­dance, the mo­lec­u­lar struc­ture of wa­ter has re­mained a mys­tery. Com­pared to other li­quids, wa­ter has strange prop­er­ties that are still poorly un­der­stood.

Re­cent work at the U.S. De­part­ment of En­er­gy’s SLAC Na­tional Ac­cel­er­a­tor Lab­o­r­a­to­ry and un­ivers­i­ties in Swe­den and Ja­pan, though, is shed­ding new light on wa­ter’s mo­lec­u­lar id­iosyn­crasies, of­fer­ing in­sight in­to its odd prop­er­ties. 

This artist's de­pic­tion shows two dis­tinct struc­tures of wa­ter: in the fore­ground, tet­ra­he­dral low-density wa­ter and in the back­ground, dis­tort­ed high-density wa­ter. (Im­age cour­te­sy of Hi­ro­hi­to Oga­sawara and Ning­dong Huang, SLAC)

Re­search­ers say the mo­lec­u­lar struc­ture of wa­ter can be com­pared to a crowd­ed res­tau­rant with a dance floor, where pa­trons—the “mol­e­cules”—al­ter­nate­ly switch be­tween danc­ing ex­cit­ed­ly, and sit­ting in or­derly fash­ion at ta­bles.

Wa­ter ex­hibits 66 known anoma­lies, sci­en­tists say, in­clud­ing a strangely var­y­ing dens­ity, large heat ca­pacity and high sur­face ten­sion. 

Con­tra­ry to oth­er “nor­mal” liq­uids, which be­come dens­er, or more packed, as they get colder, wa­ter reaches its max­i­mum dens­ity at about 4 de­grees Cel­si­us. Above and be­low this tem­per­a­ture, wa­ter is less dense; this is why, for ex­am­ple, lakes freeze from the sur­face down. 

Wa­ter al­so has an un­usu­ally large ca­pacity to store heat, which sta­bi­lizes ocean tem­per­a­tures, and a high sur­face ten­sion, which al­lows in­sects to walk on wa­ter, droplets to form and trees to trans­port wa­ter to great heights. 

“Un­der­stand­ing these anoma­lies is very im­por­tant be­cause wa­ter is the ul­ti­mate ba­sis for our ex­is­tence,” said SLAC sci­ent­ist An­ders Nils­son, who is lead­ing the ex­pe­ri­men­tal ef­forts. (SLAC stands for noth­ing, but re­flects the in­sti­tu­tion’s form­er name as the Stan­ford Lin­ear Ac­cel­er­a­tor Cen­ter.) 

“Our work helps ex­plain these anoma­lies on the mo­lec­u­lar lev­el at tem­per­a­tures which are rel­e­vant to life.”

How wa­ter mol­e­cules ar­range them­selves in the sub­stance’s sol­id form, ice, was long ago es­tab­lished: the mol­e­cules form a tight “te­tra­he­dral” lat­tice, with each mol­e­cule bind­ing to four oth­ers. 

Dis­cov­er­ing the mo­lec­u­lar ar­rangement in liq­uid wa­ter, how­ev­er, is prov­ing to be much more com­plex. For over 100 years, this struc­ture has been the sub­ject of in­tense de­bate. The cur­rent text­book mod­el holds that, since ice is made up of tet­ra­he­dral struc­tures, liq­uid wa­ter should be sim­i­lar, but less struc­tured since heat cre­ates disor­der and breaks bonds. As ice melts, the sto­ry goes, the tet­ra­he­dral struc­tures loos­en their grip, break­ing apart as the tem­per­a­ture rises, but all still striv­ing to re­main as tet­ra­he­dral as pos­si­ble, re­sult­ing in a smooth dis­tri­bu­tion around dis­tort­ed, par­tially bro­ken tet­ra­he­dral struc­tures. 

Nils­son and col­leagues re­cently di­rect­ed pow­er­ful X-rays at sam­ples of liq­uid wa­ter. Their re­sults sug­gested the text­book mod­el of wa­ter at or­di­nary con­di­tions was wrong and that, un­ex­pect­edly, two dis­tinct struc­tures, ei­ther very disor­dered or very tet­ra­he­dral, ex­ist no mat­ter the tem­per­a­ture. 

In a pa­per pub­lished in the journal Pro­ceed­ings of the Na­tional Acad­e­my of Sci­ences, the re­search­ers re­ported the ad­di­tion­al finding that the two types of struc­ture are spa­tially sep­a­rat­ed, with the tet­ra­he­dral struc­tures ex­isting in “clumps” made of up to about 100 mol­e­cules sur­rounded by disor­dered re­gions. The liq­uid is a fluc­tu­at­ing mix of the two struc­tures at tem­per­a­tures rang­ing from am­bi­ent to all the way up near the boil­ing point. As the tem­per­a­ture of wa­ter in­creases, few­er and few­er of these clumps ex­ist; but they are al­ways there to some de­gree, in clumps of a si­m­i­lar size. The re­search­ers al­so found that the disor­dered re­gions them­selves be­come more disor­dered as the tem­per­a­ture rises. 

“One can vis­u­al­ize this as a crowd­ed dance res­tau­rant, with some peo­ple sit­ting at large ta­bles, tak­ing up quite a bit of room—like the tet­ra­he­dral com­po­nent in wa­ter—and oth­er peo­ple on the dance floor, stand­ing close to­geth­er and mov­ing slow­er or faster de­pend­ing on the mood or ‘tem­per­a­ture’ of the res­tau­rant—like the mol­e­cules in the disor­dered re­gions can be ex­cit­ed by heat, the dancers can be ex­cit­ed and move faster with the mu­sic,” Nils­son said. “There’s an ex­change when peo­ple sit­ting de­cide to get up to dance and oth­er dancers sit down to rest. When the dance floor really gets busy, ta­bles can al­so be moved out of the way to al­low for more dancers, and when things cool back off, more ta­bles can be brought in.” 

This more de­tailed un­der­stand­ing of the mo­lec­u­lar struc­ture and dy­nam­ics of liq­uid wa­ter at am­bi­ent tem­per­a­tures mir­rors the­o­ret­i­cal work on “su­per­cooled” wa­ter: an un­usu­al state in which wa­ter has not turned in­to ice even though it is far be­low the freez­ing point. In this state, the­o­rists pos­tu­late, the liq­uid is made up of a con­tin­u­ously fluc­tu­at­ing mix of tet­ra­he­dral and more disor­dered struc­tures, with the ra­tio of the two de­pend­ing on tem­per­a­ture—just as Nils­son and his col­leagues have found to be the case with wa­ter at the am­bi­ent tem­per­a­tures im­por­tant for life. 

The new work helps ex­plain the liq­uid’s strange prop­er­ties, scient­ists said. Wa­ter’s dens­ity max­i­mum at 4 de­grees Cel­si­us can be ex­plained by the fact that the tet­ra­he­dral struc­tures are of low­er dens­ity, which does not vary sig­nif­i­cantly with tem­per­a­ture, while the more disor­dered re­gions—which are of high­er dens­ity—be­come more disor­dered and so less dense with in­creas­ing tem­per­a­ture. 

Like­wise, as wa­ter heats, the per­cent­age of mol­e­cules in the more disor­dered state in­creases, al­lowing this ex­cit­a­ble struc­ture to ab­sorb sig­nif­i­cant amounts of heat, which leads to wa­ter’s high heat ca­pacity. Wa­ter mol­e­cules’ ten­den­cy to stick to­geth­er, in link­ages called hy­dro­gen bonds, ex­plains the high sur­face ten­sion that in­sects take ad­van­tage of when walk­ing across wa­ter. 

Con­nect­ing the mo­lec­u­lar struc­ture of wa­ter with its bulk prop­er­ties in this way is im­por­tant for fields rang­ing from med­i­cine and bi­ol­o­gy to cli­mate and en­er­gy re­search.

“If we don’t un­der­stand this bas­ic life ma­te­ri­al, how can we study the more com­plex life ma­te­ri­als—like pro­tein­s—that are im­mersed in wa­ter?” asked post­doc­tor­al re­searcher Con­g­cong Huang, who con­ducted the X-ray ex­pe­ri­ments. “We must un­der­stand the sim­ple be­fore we can un­der­stand the com­plex.”

* * *

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Everyone knows water—it shapes our bodies and our planet. But despite this abundance, the molecular structure of water has remained a mystery, with the substance exhibiting many strange properties that are still poorly understood. Recent work at the Department of Energy’s SLAC National Accelerator Laboratory and universities in Sweden and Japan, though, is shedding new light on water’s molecular idiosyncrasies, offering insight into its strange bulk properties. Researchers say the molecular structure of water can be compared to a crowded restaurant with a dance floor, where patrons—the “molecules”—alternately switch between dancing excitedly, and sitting in orderly fashion at tables. Water exhibits 66 known anomalies, scientists say, including a strangely varying density, large heat capacity and high surface tension. Contrary to other “normal” liquids, which become denser, or more packed, as they get colder, water reaches its maximum density at about 4 degrees Celsius. Above and below this temperature, water is less dense; this is why, for example, lakes freeze from the surface down. Water also has an unusually large capacity to store heat, which stabilizes ocean temperatures, and a high surface tension, which allows insects to walk on water, droplets to form and trees to transport water to great heights. “Understanding these anomalies is very important because water is the ultimate basis for our existence,” said SLAC scientist Anders Nilsson, who is leading the experimental efforts. (SLAC stands for nothing, but reflects the institution’s former name as the Stanford Linear Accelerator Center.) “Our work helps explain these anomalies on the molecular level at temperatures which are relevant to life.” How water molecules arrange themselves in the substance’s solid form, ice, was long ago established: the molecules form a tight “tetrahedral” lattice, with each molecule binding to four others. Discovering the molecular arrangement in liquid water, however, is proving to be much more complex. For over 100 years, this structure has been the subject of intense debate. The current textbook model holds that, since ice is made up of tetrahedral structures, liquid water should be similar, but less structured since heat creates disorder and breaks bonds. As ice melts, the story goes, the tetrahedral structures loosen their grip, breaking apart as the temperature rises, but all still striving to remain as tetrahedral as possible, resulting in a smooth distribution around distorted, partially broken tetrahedral structures. Nilsson and colleagues recently directed powerful X-rays at samples of liquid water. Their results suggested the textbook model of water at ordinary conditions was wrong and that, unexpectedly, two distinct structures, either very disordered or very tetrahedral, exist no matter the temperature. In a paper published in the Proceedings of the National Academy of Sciences, the researchers revealed the additional discovery that the two types of structure are spatially separated, with the tetrahedral structures existing in “clumps” made of up to about 100 molecules surrounded by disordered regions. The liquid is a fluctuating mix of the two structures at temperatures ranging from ambient to all the way up near the boiling point. As the temperature of water increases, fewer and fewer of these clumps exist; but they are always there to some degree, in clumps of a similar size. The researchers also found that the disordered regions themselves become more disordered as the temperature rises. “One can visualize this as a crowded dance restaurant, with some people sitting at large tables, taking up quite a bit of room—like the tetrahedral component in water—and other people on the dance floor, standing close together and moving slower or faster depending on the mood or ‘temperature’ of the restaurant—like the molecules in the disordered regions can be excited by heat, the dancers can be excited and move faster with the music,” Nilsson said. “There’s an exchange when people sitting decide to get up to dance and other dancers sit down to rest. When the dance floor really gets busy, tables can also be moved out of the way to allow for more dancers, and when things cool back off, more tables can be brought in.” This more detailed understanding of the molecular structure and dynamics of liquid water at ambient temperatures mirrors theoretical work on “supercooled” water: an unusual state in which water has not turned into ice even though it is far below the freezing point. In this state, theorists postulate, the liquid is made up of a continuously fluctuating mix of tetrahedral and more disordered structures, with the ratio of the two depending on temperature—just as Nilsson and his colleagues have found to be the case with water at the ambient temperatures important for life. “Previously, hardly anyone thought that such fluctuations leading to distinct local structures existed at ambient temperatures,” Nilsson said. “But that’s precisely what we found.” This new work explains, in part, the liquid’s strange properties. Water’s density maximum at 4 degrees Celsius can be explained by the fact that the tetrahedral structures are of lower density, which does not vary significantly with temperature, while the more disordered regions—which are of higher density—become more disordered and so less dense with increasing temperature. Likewise, as water heats, the percentage of molecules in the more disordered state increases, allowing this excitable structure to absorb significant amounts of heat, which leads to water’s high heat capacity. Water molecules’ tendency to stick together, in linkages called hydrogen bonds, explains the high surface tension that insects take advantage of when walking across water. Connecting the molecular structure of water with its bulk properties in this way is important for fields ranging from medicine and biology to climate and energy research. “If we don’t understand this basic life material, how can we study the more complex life materials—like proteins—that are immersed in water?” asked postdoctoral researcher Congcong Huang, who conducted the X-ray scattering experiments. “We must understand the simple before we can understand the complex.”