Transport mechanisms in nanoscale amorphous solid water films

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McClure, Sean Michael

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Amorphous solid water (ASW) is a disordered, glassy form of water, which upon heating above T~135-140 K, crystallizes to cubic ice. Despite much interest and research, many properties of this glassy water phase remain contentious. Particular controversy surrounds the location of its glass transition temperature (Tg) and details regarding relaxation dynamics (viscosity, diffusivity) as water is heated and/or cooled through Tg. In this dissertation, transport mechanisms in layered, isotopically-labeled (H2 18O, H2 16O) nanoscale ASW films (~ 10-100 nm) are investigated via temperature programmed desorption (TPD) techniques. ASW films are known to fracture during crystallization, presumably due to stresses generated within the film during the nucleation and growth process. This fracturing produces pathways for vapor-phase transport of desorbing molecules within the film. Translational motion of H2O is, in all cases, observed coincident with crystallization (and hence film fracture) during heating and desorption of layered ASW films. Comparison with a bulk diffusion model illustrates that the observed intermixing is inconsistent with a bulk diffusion mechanism. Through the use of hydrophobic, diffusion "barrier" layers (CCl4, CHCl3) I have been able to demonstrate that bulk diffusion in ASW is likely very small prior to crack/fracture formation. Transport properties of dilute, glassy nitric acid films (0 - 2.2 mol % HNO3) are also investigated. Results demonstrate that the presence of dilute amounts of HNO3 dramatically reduce crystallization-induced film fracture. Intermixing experiments using structured films of dilute HNO3/H2 16O and labeled water (H2 18O) demonstrate that water intermixing during crystallization is substantially reduced. Combined, the experimental results suggest that the intermixing observed in thin ASW films during crystallization is due to a porosity-mediated transport mechanism. This implies that the ASW bulk self-diffusivity near crystallization is not 'fragile' in nature, in contrast with previous results. Instead, these results suggest that ASW is likely either (i) a glass or (ii) a strong liquid prior to T~160 K. The latter case (ii) would require a change in water dynamics from its known 'fragile' behavior at higher temperatures (T~231-273 K) to 'strong' behavior at low temperatures (T<160 K).