Dielectric Study of the Hydration Process in Biological Materials
The sorption of water vapour by biological macromolecules is generally assumed to involve the binding of H2O molecules to specific hydrophilic sites at lower relative humidities, followed by condensation of multimolecular adsorption as the humidity increases. Several methods have been applied for the investigation and detailed study of the structure, mobility, extent and modes of binding of water molecules in various systems. Among them the most commonly used are IR and Raman spectroscopy (Luck, 1985), differential scanning calorimetry (Berlin et al., 1970), NMR spectroscopy (Kuntz and Kautzmann, 1974, Mathur de Vré, 1979), neutron scattering (Lehmann, 1984), sorption and desorption methods (Pethig, 1979) and dielectric methods (Bone and Pethig, 1982, Pethig and Kell, 1987, Grant et al. 1978, Kent and Meyer 1984). All of them yield some insight into the problem. One common feature observed in nearly all cases is that the relaxation times for reorientation and the diffusion constants of water molecules sorbed in various biological systems are much lower than the values observed for free water, while the enthalpy of vaporisation of the water sorbed is by about 100 cal g−1 higher than the value of liquid water (Berlin et al., 1970, Pethig, 1979, Grant et al., 1978). This behaviour suggests that the water molecules contributing to the first hydration layer exhibit restricted motion due to a significant decrease in the translational and rotational modes of motion caused by macromolecular-water interaction. Moreover, the dynamics of the material itself (relaxation and conductivity mechanisms) is strongly influenced by the presence of sorbed water.
KeywordsMicrocrystalline Cellulose Static Permittivity Native Cellulose Dielectric Study Sorbed Water
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