Water Mobility in Poly(ethy1ene glycol)–, Poly(vinylpyrro1idone)–, and Gelatin–Water Systems, as Indicated by Dielectric Relaxation Time, Spin–Latice Relaxation Time, and Water Activity

Abstract

The mobility of water molecules present in poly(ethylene glycol) (PEG)—, poly(vinylpyrrolidone) (PVP)—, and gelatin—water systems was determined by dielectric relaxation and17O NMR spectroscopy. Water activity was also measured. Dielectric relaxation spectra indicate that all the polymer systems studied contained water exhibiting a dispersion at a frequency > 109Hz; in other words, water with high mobility close to that of bulk water. The dielectric relaxation time of the highly mobile water increased as polymer concentration increased. The PVP— and gelatin—water systems also contained water exhibiting a dispersion at a frequency < 109Hz, which can be considered to be “bound water” with a restricted mobility because of its association with polymer molecules. Dielectric relaxation spectroscopy was used to determine water mobility separately for the populations of highly mobile water and bound water, whereas NMR relaxation spectroscopy was used to determine the average mobility of both populations. The spin—lattice relaxation time of water in these polymer—water systems showed a deviation from the isotropic two‐state model. Dielectric relaxation data indicate that this deviation can be ascribed to variations in the relaxation time of highly mobile water caused by a change in polymer concentration. The dielectric relaxation time of highly mobile water in the gelatin system did not change with a change in polymer concentration to the extent that it did in the PEG and PVP systems. This result is consistent with a slight change in water activity of the gelatin system with increasing polymer concentration. Although water activity was useful as a measure of the mobility of highly mobile water, information on the dynamics of bound water cannot be obtained from water activity.