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Molecular Hydrate Crystals - Formation, Transformation and Properties

Jennifer Swift (Georgetown University)

Molecular hydrates are frequently encountered in a variety of contexts such as pharmaceuticals, energetic materials, food additives and even in precipitates resulting from crystal deposition diseases. This talk will focus on the growth and tunable physical properties of two very different molecular hydrates - uric acid and thymine.

Crystalline uric acid is a component of human kidney stones, where it is most often identified in either an anhydrous (UA) or a dihydrate (UAD) phase. UAD is metastable, and transforms both in air and in aqueous solution to UA. The physical properties of UAD and its transformation rate to UA are extremely sensitive to the initial growth conditions under which it was prepared. A combination of in situ AFM, X-ray diffraction, optical microscopy and and thermal analysis studies on UAD suggest that differences in growth mechanism and/or microstructure can have a very large impact on their single crystal properties including thermal stability and dehydration rates. Crystallization of uric acid solutions containing other molecular components show a variety of unusual behaviors.

Thymine, one of the four nucleobases of DNA, crystallizes from aqueous solution in either an anhydrous (T) or monohydrate (TMH) phase. Our re-investigation of the TMH structure shows that it undergoes a low temperature symmetry change resulting from the freezing out different water positions. At higher temperatures, TMH crystals dehydrate and eventually sublime to anhydrous phases which have not been previously reported. TMH crystals grown from solutions containing molecular dopants readily absorb and trap small concentrations (<1%) of these dopants in the matrix. Although pure and doped TMH crystals look identical by XRD, both the low and high temperature phase transitions are shifted, in some cases by 20 °C or more. Efforts to better understand the water mobility in TMH crystals grown under different conditions will be addressed.

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