On the origin of the heat capacity feature of annealed ices and ice clathrates, and interpreting water’s diffusivity in terms of the entropy

Abstract

In order to investigate the origins of (i) the spontaneous temperature rise on annealing pure hexagonal and cubic ices and ice clathrates and (ii) the sigmoid-shape increase in the heat capacity, C p , on heating the annealed samples, the enthalpy and entropy decrease on annealing of pure cubic ice and one ice clathrate have been determined from their C p data. This decrease is found to be much higher than that expected from orientational relaxation of H 2O molecules and that calculated from the spontaneous decrease in the Bjerrum or orientational defects concentrations. On this basis and the known observation that dopants not only decrease the relaxation time of the ices and ice clathrates but also modify the shape of the C p -increase feature, it is concluded that the spontaneous temperature rise on annealing occurs when some of the H 2O molecules achieve a preferential orientation, which is equivalent to their partial proton ordering. The sigmoid-shape C p -feature observed on heating the annealed samples is therefore due to the time- and temperature-dependent recovery of their random orientations or complete proton disorder. This is fundamentally different from structural relaxation in glasses whose entropy of disorder itself increases on heating. In a second aspect of the study, the known temperature dependence of the self-diffusion coefficient of water is used to calculate two fundamental quantities of the configurational entropy theory: (i) the size of the cooperatively rearranging regions, which is 4.7 molecules at 150 K, and (ii) the temperature-invariant energy, which is 7.4 kJ mol −1. These seem plausible and similar to those observed for other liquids. Finally, it is pointed out that a comparison of bulk water’s dielectric properties with those of the ions afflicted, two-molecule thick water layer between the platelets of sodium vermiculite clay [R. Bergman, J. Sweson, Nature 403 (2000) 283], is inappropriate on fundamental grounds.