Physics Colloquium by Michael Brown April 26

Michael Brown of the University of Washington will present a colloquium, "The Multiple Faces of Water from Low Temperature to High Pressure" at 3: 45 p.m. Friday at Bigelow Physics Building, Room 217.

That properties of pure water can best be rationalized as a mixture of distinct fluid phases is not a new idea. The motivations for this concept and the contributions of recent high-pressure work in our group are the focus in this talk.

Our measurements of thermodynamic and transport properties of water and aqueous electrolytic solutions extend to high pressures over a range of temperatures. Determinations of thermal diffusivities, viscosities, and sound speeds to 10 GPaand to 800 kelvins are possible in diamond anvil cells. We make highly accurate (100 parts-per-million) sound speed measurements to 0.7 GPafor temperatures from 250 K to 360 K. From such data, we construct Gibbs energy representations that can be differentiated to give accurate thermodynamic properties.

The emerging view based on the interpretation of the high-pressure data reinforces the long-standing notion that water has multiple “faces” depending on pressure, temperature, and solute concentration. At low pressures for temperatures below ambient, water favors a local four-fold coordination associated with hydrogen-bonded molecules. With increasing temperature, pressure, or solute concentration, the local coordination of water molecules increases. Above 1 GPawater behaves more as a “simple” (nearly close-packed) fluid. In the intermediate pressure regime (near or below 1 GPa), water has a strong temperature dependence of local coordination, increasing with decreasing temperature below ambient. The complex electrolytic chemistry of aqueous solutions found near ambient pressures is quenched by high pressure. Above 1 GPa, electrolytic solutions are essentially “ideal” with volumes of mixing equal to intrinsic ion sizes.

A more speculative idea follows. The presence of several amorphous ice phases in the far supercooledregime and the anomalous properties of water at low pressure and low temperature have previously provided evidence for a hypothetical lower critical point. Water might separate into two distinct fluids in a deeply metastable regime of temperature. The existence of a high pressure (very dense) amorphous ice phase and the behavior observed in our new high-pressure equation of state for water can be interpreted as requiring an additional (higher pressure) critical point.  

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