The investigation of swollen process in gelatin gel by using positron annihilation spectroscopy

The change of free volume hole radius and bound state of water in the biopolymer gel swollen with water were investigated by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC). In this study, gelatin with multiple helix structure of polypeptide (MW similar to 100,000) was used as biopolymer. To examine into bound state of water in gelatin gel, the weight percentage of freezing and non-freezing water against total water in gelatin gels were calculated from fusion enthalpy of DSC curves. Accompanied with the increase of gelatin concentration, the percentage of non-freezing water increased while that of freezing water decreased, and above 60 %(w/w) of gelatin concentration, percentage of non-freezing water reached approximately 100 %(w/w). Since the change in bound state of water results from network structure of gelatin gel, free volume hole radius of gelatin gels with different concentrations were characterized from positron annihilation lifetime spectra using PATFIT-88 program. The lifetime of o-Ps became longer or the size of free volume hole increased accompanied with the increase of water content. It is considered that the hydrophilic sites, carboxyl groups in gelatin molecules, locate inside of the helix and hydrophobic groups locate outside of the helix. This means that once the water molecules penetrate into the gelatin network, they are attracted by carboxyl groups inside the helix structure and the hydrogen bonds are formed between water and carboxyl groups. The water molecules localized inside the helix expand the helix surface outward by the thermal motion, leading to the increase of free volume hole radius. This idea assumes localization of positrons in the vicinity of the hydrophilic group and the influence of local potential on lifetimes of positron and positronium. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.