Glass transition dynamics in water and other tetrahedral liquids: 'order-disorder' transitions versus 'normal' glass transitions

We review some aspects of the confusion concerning the glass transition in water, and then show that it must be of a quite different character from that in other molecular liquids, and in fact is the kinetically controlled part of a classical order-disorder transition. ( This is the conclusion reached in a review of the low temperature phenomenology of amorphous water currently being published in Science by the present author. Material that would normally appear in the present abbreviated paper will appear in the Science article to which the interested reader is referred.) We do this using a combination of (i) thermodynamic reasoning for 'bulk' water ( based on known properties of supercooled water and nearly glassy water), and ( ii) direct measurements on nanoscopic ( non-crystallizing) water. Both require the heat capacity to be sharply peaked near 220 K and thus to imply the existence of a 'strong-to-fragile' transition during heating. Both require the excess heat capacity to drop to near-vanishing values in the vicinity of 130-150 K. The similarity to order-disorder transitions in crystalline solids is noted, the relation to the second critical point scenario for water is discussed, and the modelling of the anomaly by current theories is considered. Finally we argue that water, with its fragile-to-strong liquid transition below the melting point, links ( lies in between) the extremes of classical network liquids ( where this transition occurs only above the experimentally accessible range) and fragile molecular liquids, where the fragile-to-strong transition is pushed beneath the glass temperature.