Observation of the density minimum in deeply supercooled confined water

Q vs. T SANS data on supercooled waterThis week’s item is the recent PNAS paper Liu et al., “Observation of the density minimum in deeply supercooled confined water” PNAS 104, 9547 (2007), by a collaboration lead by S. H. Chen at MIT.

By using Small Angle Neutron Scatterig (SANS) they are able to study water confined in mesoporous silica channels and find a density minimum at -63 deg C.

The density *maximum* of water at 4 deg C is a well known anomaly – one of many, many anomalies of water. The 4 deg maximum has some interesting consequences for living organisms – the denser 4 C water “sinks” to the bottom and makes it possible for fish and other water organisms to survive the winters, since bottoms of lakes and ponds don’t freeze out even in the harshest winters and remain at “balmy” 4 C.

Now SANS measurements find the opposite phenomena – a density *minimum*, at -63 C. But since bulk water in equilibrium freezes at 0 C, the researchers had to play some tricks to produce metastable, “supercooled” water. In absence of nucleation centers one could supercool bulk water by some 20-30 degrees – in fact this is the state in which water can exist in atmospheric clouds, in a form of supercooled droplets. But to go further than that one has to confine water to nanoscopic cylindrical channels, as was done in this work.

Several other previous studies on this topic by Chen group at MIT:

Chen et al., The violation of the Stokes–Einstein relation in supercooled water, Proc. Nat. Acad. Sci. (2006)

Liu et al., Pressure Dependence of Fragile-to-Strong Transition and a Possible Second Critical Point in Supercooled Confined Water Phys. Rev. Lett. 95, 117802 (2005).

And the same June 5 issue of PNAS contains a paper by Eugene Stanley’s group at BU on Relation between the Widom line and the breakdown of the Stokes–Einstein relation in supercooled water.

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