Water sorption in a dextran gel

The state of adsorbed water in a dextran gel has been investigated by near-infrared gravimetric-adsoprtion techniques. Water-vapor adsorption (desorption) isotherms at three temperatures are reported. The calculated sorption heats are found to be markedly temperature-dependent as well as dependent on the coverage. The near-infrared spectrum (4650-9000 cm-minus 1) is reported, together with tentative assignments. The H2O combination (v+delta) band at 5184 cm-minus 1 has been examined as a function of relative humidity. The line-shapes of this band have been analyzed by a recently established, Fourier-inversion technique, and information on the microdynamics of the absorbed water molecules has been resolved on the picosecond time-scale. At low and intermediate degrees of hydration, reorientational jumps take place with periods from four to six times longer than those for free water. The onset of saturation is then accompanied by the sudden removal of the reorientational jumps. A comparison of microdynamical and thermodynamic data indicates the hydration mechanism to be highly cooperative at all relative humidities.