Thermal Behavior and Structure of Low-temperature Water Confined in Sephadex G15 Gel by Differential Scanning Calorimetry and X-ray Diffraction Method

The thermal behavior and structure of water confined in Sephadex 015 gel were investigated over a temperature range of 298 - 173 K at hydration levels, h (= mass of water/mass of dry 015 gel), of 0.24 - 1.38 by differential scanning calorimetry (DSC) and an X-ray diffraction (XRD) method, respectively. The ice-melting peaks on the DSC curves were deconvoluted to estimate the amounts of three states of water in 015 with h: free water, freezable bound water, and unfrozen water. The X-ray radial distribution functions of unfrozen water at h = 0.24 revealed that the hydrogen-bonded structure of water is largely distorted, due to hydrogen bonding with the surface hydroxyl groups of gel substrates, compared with those of freezable bound water at h = 0.47 and bulk water. A plausible separation mechanism of solutes in gel chromatography was considered from a structural point of view of confined water.