Experimental and analytical studies of molecular dynamics and phase transitions in polycrystalline ice at environmentally relevant temperatures
Open AccessAbstract of DissertationExperimental and analytical studies of molecular dynamics and phase transitions in polycrystalline ice at environmentally relevant temperaturesIce plays an important role in a variety of natural settings. Polycrystalline ice, for example, might serve as a dynamic substrate for a variety of chemical reactions and potentially release greenhouse gases and other pollutants as a result of biochemical reactions in thawing permafrost. Despite its significance for the Earth environment and beyond, laboratory studies of polycrystalline ice at temperatures near its melting point are scare. This is due to primarily the failure of standard analytical techniques under the condition when ice is in dynamic equilibrium with its vapor phase, and may undergo a variety of interfacial phase transition. Using the unique quasi-adiabatic fast scanning calorimetry (FSC) technique, I was able to measure accurately the effective heat capacity of nanometer and micrometer thick ice films undergoing rapid sublimation, premelting, and melting at temperatures from - 50 to 25 o C. By combining an advance analysis of the FSC thermogram with a set of calibration experiments and procedures, I have determined the absolute vaporization rate of ice in vacuum with unprecedented accuracy. I used this data to demonstrate that the mass accommodation coefficient (MAC) of water molecules on ice surface is equal 1.05 ± 0.5, and is virtually independent of temperature in the range from -50 to oC. I provide a detailed discussion on the significance and implications of this fundamental discovery in the context of past experimental, computational, and theoretical studies of MAC on aqueous surfaces. Combining my accurate determination of the vaporization rate of ice at temperatures near its melting point with an original quantitative secondary analysis (QSA) of the data from past fast near-isothermal desorption experiments, I have determined the excess Gibbs free energy of the aqueous interphase at grain boundaries in polycrystalline ice at temperatures between -50 and -5 o C. Unlike that of supercooled bulk liquid water, the excess free energy of confined grain boundary H2O phase (GB water) is virtually independent of temperature. Furthermore, the excess energy is significantly higher than that of supercooled water near ice melting point. Based on these fundamental observations, I conjecture that the GB water in pure polycrystalline ice is essentially in an immobile, glass-like state at temperatures up to -5 o C. I validate this conclusion with a set of FSC experiments with pure polycrystalline ice, and ice doped with acetic acid molecules, which are strongly partitioned at grain boundaries. My FSC experiments confirm that pure polycrystalline ice does not manifests any endothermic transitions consistent with liquid-like molecular dynamics at temperatures between 136 and 268 K. Yet such transitions are observed near 240 K in polycrystalline ice, when grain boundaries are saturated with guest chemical species. In addition to providing tangible accurate data on fundamental thermodynamic parameters of the grain boundary phase, my FSC experiments also reveal and emphasize the impact of impurities on molecular dynamics at grain boundaries. Based on the experiments and analysis described in this Dissertation, I also proposed several future experiments, which will provide currently unavailable data for developing future model of physics and chemistry of naturally occurring polycrystalline ice formations.
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