Thermal behavior, structure, and dynamic properties of water confinedin polymer gel Sephadex G15

Abstract

The thermal behavior, structure, and dynamic properties of water confinedin polymer gel Sephadex G15 were investigatedover a temperature range of 298-173K as a function of hydration level (g H2O/g dry gel) by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and quasi-elastic neutron scattering (QENS) and neutron spin echo (NSE) measurements. The DSC data showed that there are three states of water in G15 gel: free water, freezable bound water, and unfrozen water. The X-ray radial distribution functions revealed that the tetrahedrallike hydrogen bonded network of confined water is largely distorted, compared with the bulk water. The QENS data showedthat the water molecules perform local motions characterizedby a diffusion coefficient of (0.65±0.05)×10-5 cm2s-1 within a sphere with a radius of 4.4 Å. The Arrhenius plots of the relaxation time of water gave an activation energy of 44.2±1.5 kJmol-1, which is somewhat larger than the value of 15-36 kJmol-1 of capillary condensed water confined in mesoporous silica MCM-41 C10 (pore diameter 21 Å). The present findings suggest that water molecules in G15 gels are strongly boundto the hydroxyl groups of the gel matrix.