Hydration time ratio, definition

The same water model, SPC/E at 25 °C was used by other authors too. Chowd-huri and Chandra (2001) employed 256 water molecules per ion, as well as lower ratios at increasing concentrations, and reported the average residence times of water molecules near ions in ps Na+ 18.5, K+ 7.9, and Cl 10.0. Guardia et al. (2006) also reported residence times in ps of the water molecules in the first hydration shells of ions Li+ 101, Na+ 25.0, K+ 8.2, Cs+ 6.9, F 35.5, Cl 14.0, and I 8.5, compared with 10 1 for water molecules in the bulk. These values, resulting from detailed considerations of the hydrogen bond dynamics in water and near the ions, can be compared with experimental values derived from NMR. According to Bakker (2008) these are Li+ 39, Na+ 27, K+ 15, Cl 15 (by definition the same as for K+), Br 10, and 5 ps, and for water molecules in the bulk 17 ps, calculated from the self diffusion coefficient. 

In situ thermal transitions were also described by Taylor et al., who examined the isothermal dehydration behavior of trehalose dihydrate [29]. For small particle size fractions (<45 fjLm), heating at 80°C caused loss of peak definition until, at 210 min, amorphous material was present. In contrast, a larger particle size fraction (>425 fim) converted to the crystalline anhydrous form of the material. The kinetics of this conversion was probed from the Raman data using peak height ratios with time a two-stage rearrangement was indicated. A broader consideration of pharmaceutical hydrates, including their characterization by several techniques (NMR, Raman spectroscopy, and isothermal calorimetry) can be found in the literature [30] as can a review of the use of spectroscopic techniques for the characterization of polymorphs and hydrates [31]. 

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