SHEAR VISCOSITY AND SELF-DIFFUSION EVIDENCE FOR HIGH-CONCENTRATIONS OF HYDROGEN-BONDED CLATHRATE-LIKE STRUCTURES IN VERY HIGHLY SUPERCOOLEDLIQUID WATER

Activation energies from shear viscosity and self-diffusion of highly supercooled liquid water rise very rapidly with decreasing temperature at 1 atm and approach approximate to 70 kJ/mol, or more, at 228 K. Th ese high values may result because the flow of H2O molecules is impede d by passage through hydrogen-bonded pentagonal rings whose opening ra dius is approximate to 0.4 Angstrom, smaller than the van der Waals ra dius of H2O. Two principal effects may contribute to the overall energ etics: (1) breakage of hydrogen bonds around a given H2O molecule; and then, only if no opportunity exists for free diffusion through hexago nal or larger rings in the immediate vicinity of the freed H2O molecul e, (2) impeded, highly endothermic diffusion of that H2O molecule thro ugh a hydrogen-bonded pentagon, by forced ring expansion, followed by rebinding of the H2O. Large negative activation volumes are also indic ated from 273 to 228 K and 1 atm. These negative volumes may arise fro m activated filling of voids in bulky, clathrate-like, polyhedral stru ctures. A model for the diffusional process is described.