Thermodynamics, Kinetics and y-Irradiation of Metastable Solid Forms of Water.

Research project P 13930 Metastable Solid Forms of Water Andreas HALLBRUCKNER 11.10.1999 Scientific interest in water in its gaseous, liquid, and solid state is still vivid as our understanding of this most important substance is far from being complete. Apart from continous studies on the most abundant forms of water in nature - water vapor, liquid water, and hexagonal ice - research on the less common` crystalline and various amorphous states of water is steadily increasing. Evidenced by recent spacecraft-based observations water is most likely a major component in astrophysical environments, like planetary rings, comets, and interstellar clouds. Due to exceptional temperature and pressure conditions in outer space solid water is likely to appear predominantly in the amorphous state, which is also documented by IR spectra. On the occasion of a temperature increase some of this amorphous solid water will probably be transformed into cubic ice. Knowledge of formation conditions, range of stability, and other physical properties of the several different forms of solid water is therefore highly desirable in order to enable deduction of the structural state of water from environmental conditions where direct investigation is prevented for practical reasons. But there is also lively discussion on several other basic issues in context with water, like the relationship between liquid, supercooled and glassy water, existence of polymorphism in the amorphous state, new crystalline ice phases, existence of viscous liquid water at temperatures far below the `critical` temperature of -43C, etc. In the course of the present application we want to investigate in detail samples of cubic ice, which have been prepared by different methods: i) directly by deposition of water vapor or liquid water within the appropriate temperature range, which has to be determined in the present study ii) by heating of vapor deposited amorphous solid water (ASW), hyperquenched glassy water (HGW), and of pressure amorphized crystalline ices. Samples prepared that way will be investigated by X-ray diffraction and Differential Scanning Calorimetry (DSC). On account of the experimental setup developed by us we are able to do complementary X-ray and DSC measurements on the same batch of sample, which allows to correlate structural changes during heating with observed thermal effects. Formation conditions of variously prepared samples of cubic ice will be investigated, as well as thermodynamics and kinetics of phase transformations during heating. In particular high temperature limits of stability of the differently prepared cubic ice samples are of major interest in cryobiology as occuring crystalline ice in biological specimens is preferred to be formed in the cubic state which causes significant less freeze-damage than the hexagonal form. The projected studies will also include exploration of temperature limits of cubic and amorphous ice formation from liquid water and water vapor. Existence of a glass-liquid transition in ASW and HGW might be proofed unambigously without the necessity of previous extensive annealing procedures, by depositing water vapor and liquid water at temperatures considerably above 77 K in order to avoid the marked enthalphy relaxation effects which otherwise mask the relatively weak glass transition. In cooperation with Prof. Plonka in Lodz, Poland, we will further do electron spin resonance (ESR) studies on radicals formed in cubic ice of different prior history upon gamma-irradiation, as preliminary studies have shown unexpected differences in radical-formation and -decay depending on the way the cubic samples were formed. That is, directly formed cubic ice samples resembled ordinary hexagonal ice, whereas cubic ice samples formed by heating of ASW or HGW were similar to amorphous ices regarding radical formation and -decay. ESR studies on variously prepared cubic ice samples after different thermal treatments will possibly. reveal the formation and decomposition mechanisms of the different radicals formed.