Expectations of a rapidly increasing heat capacity for water at low temperatures are confirmed by differential scanning calorimetry. Cp is found to rise very rapidly from 18 to 29 cal mol-1 deg-1 between -10° and the homogeneous nucleation temperature, -40°. Proton magnetic resonance chemical shift measurements suggest this behavior is associated with a cooperative acceleration in hydrogen bond strength or formation rate at low temperatures. It is shown that in light of these results the existence of vitreous waters which soften and crystallize near 150 K is paradoxical. To resolve the paradox the existence below the homogeneous nucleation temperature of a λ-type transition similar to that encountered in liquid sulfur is postulated. Its origin is tentatively accounted for in terms of the cooperative bond lattice model. It is apparent that the positive volume change associated with hydrogen bond formation plays a dominant role in determining the observed constant pressure behavior on cooling, the positive volume change itself being a consequence of the geometry of formation of the four coordinate random network.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry