Diffusive properties of water in Artemia cysts as determined from quasi-elastic neutron scattering spectra

Results have been obtained on the quasi-elastic spectra of neutrons scattered from pure water, a 20% agarose gel (hydration four grams H2O per gram of dry solid) and cysts of the brine shrimp Artemia for hydrations between 0.10 and 1.2 grams H2O per gram of dry solids. The spectra were interpreted using a two-component model that included contributions from the covalently bonded protons and the hydration water, and a mobile water fraction. The mobile fraction was described by a jump-diffusion correlation function for the translation motion and a simple diffusive orientational correlation function. The results for the line widths gamma (Q2) for pure water were in good agreement with previous measurements. The agarose results were consistent with NMR measurements that show a slightly reduced translational diffusion for the mobile water fraction. The Artemia results show that the translational diffusion coefficient of the mobile water fraction was greatly reduced from that of pure water. The line width was determined mainly by the rotational motion, which was also substantially reduced from the pure water value as determined from dielectric relaxation studies. The translational and rotational diffusion parameters were consistent with the NMR measurements of diffusion and relaxation. Values for the hydration fraction and the mean square thermal displacement [u2] as determined from the Q-dependence of the line areas were also obtained.