Question about fundamental chemistry of water answered

02/13/04

Water is simple, right? It is a simple, stable molecule - two hydrogen atoms strongly bonded to an oxygen atom. It is common in the universe, existing at a wide range of temperatures. As a liquid, it has interesting properties that allow it to dissolve many substances. It is basic to life, and it makes up most of your body.

However, a vigorous argument about some fundamental physical properties of this ubiquitous substance has been raging for over half a century. Now, a new finding to be published in the February 19 issue of the journal Nature may settle the dispute.

The article, by Y-Z Yue of Aalborg University in Denmark, and C.Austen Angell of the Department of Chemistry and Biochemistry at Arizona State University, is entitled "Clarifying the glass-transition behavior of water by comparison with hyperquenched inorganic glasses."

The authors argue that the currently accepted temperature at which water in the glassy state softens into a liquid ("glass transition"), is incorrect due to mistaking an "experimental artifact" for the glass transition itself. In fact, Yue and Angell argue that the amorphous solid form of water crystallizes before this softening ever happens.

Most of us assume that water's basic properties are well understood, but in many ways, they are not. While we are familiar with water either as a liquid or as a crystalline solid (ice), its most common state in the universe is as a glass, a peculiar form of matter which is solid like ice but has a disorderly arrangement of molecules like a liquid. Scientists believe that water mainly exists in the glass state in intergalactic space -- in water films on dust particles -- and that comets (sometimes affectionately called "dirty iceballs") are made of it as well.

The transition between a liquid and its crystalline solid phase is sudden, with an abrupt change in the material's heat content -- and state of order -- occurring when the material changes phase. Glasses, however, show a very different sort of behavior when they are heated, changing into a liquid gradually, showing a jump in heat capacity as the softening begins, but no jump in energy or state of order, as in melting. This jump in heat capacity defines the "glass transition."

Chemists, who form glassy water in the laboratory by splattering micro-droplets on extremely cold surfaces (a process called "hyperquenching"), were long unable to detect a glass transition for glassy water, as the material appeared to change to a crystalline solid before reaching the transition temperature. Finally, in 1987 a weak heat capacity change was thought to have been detected at 136 degrees Kelvin by "annealing" (heating and re-cooling to relax its structure) the water glass before it reached the point where it crystallizes. Since then, this has been generally accepted as the glass transition for water.

Now, Yue and Angell have shown, by examining a number of other hyperquenched inorganic glasses that have known glass transitions, that annealing the glasses causes a "shadow" of the glass transition to occur at lower temperatures than the actual transition occurs at.

"What people thought was the glass transition in water is actually just an annealing effect," said Angell. "The actual glass transition temperature cannot be seen in any experiment because, as many of us thought before, water crystallizes before the glass transition can occur."

Angell's results point to a new understanding for the phases of water. Glassy water can now be seen as remaining solid (not changing to a liquid) at a much higher temperature than before, probably because of the strong tetrahedral network of hydrogen bonds holding the water molecules in place. However, this network is disrupted when substances dissolve in the water.

This fundamental problem on water has been only one part of a flurry of recent activities in Angell's lab, including some that have potential importance for new technology. In a paper published in the October 17 issue of Science, Angell announced the development of a class of salts that, without needing a solvent to dissolve them, are liquid at room temperature and have the conductivity of aqueous solutions. These electrolytes not only closed a historical gap between aqueous solutions and other liquids but also prove to be exceptional, possibly superior, electrolytes for use in developing efficient hydrogen fuel cells.

In other, more esoteric, work Angell and colleague Srikanth Sastry, at the Nehru Research Center in India, reported in the November 27 issue of Nature Materials that a liquid-to-liquid phase transition, much discussed but not proven in connection with the anomalies of water, had been clearly observed in the liquid state of silicon. Silicon, in analogy to water, likes to have four nearest neighbors around every atom forming a network like that in glassy water.

Source: Eurekalert & others

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