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From the Cover: Observation of the density minimum in deeply supercooled confined water.
by Liu D, Zhang Y, Chen CC, Mou CY, Poole PH, Chen SHProceedings of the National Academy of Sciences of the United States of America.
Article Abstract:
Small angle neutron scattering (SANS) is used to measure the density of heavy water contained in 1D cylindrical pores of mesoporous silica material MCM-41-S-15, with pores of diameter of 15 +/- 1 A. In these pores the homogenous nucleation process of bulk water at 235 K does not occur, and the liquid can be supercooled down to at least 160 K. The analysis of SANS data allows us to determine the absolute value of the density of D(2)O as a function of temperature. We observe a density minimum at 210 +/- 5 K with a value of 1.041 +/- 0.003 g/cm(3). We show that the results are consistent with the predictions of molecular dynamics simulations of supercooled bulk water. Here we present an experimental report of the existence of the density minimum in supercooled water, which has not been described previously.
Observation of the density minimum in deeply supercooled confined water
By: Anonymous - Sun 6/17/2007 PMBy 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.
Source: http://shpyrko.wordpress.com/2007/06/16/obser...