CHARACTERIZATION OF SORBED WATER-MOLECULES ON NEUTRAL AND IONIC POLYSACCHARIDES

Ionic and neutral polysaccharides with well-defined structures were ch osen to investigate the mechanism of water sorption at different relat ive humidities. From an experimental point of view, the freezing water was determined by DSC when the total sorbed water was obtained from t hermogravimetry. The isotherms of sorption and enthalpies of interacti on were determined using the combination of a microbalance and a micro calorimeter. It is shown that freezing water appears for P/P-0 > 0.85 especially with the neutral polymers. The differential molar enthalpy of interaction is higher for P/P-0 < 0.85 corresponding to the fixatio n of two water molecules forming double H-bonds; this result is confir med by molecular modelling; saturation is obtained experimentally for 4 water molecules interacting per glucose unit. On ionic polymers, the water retention increases especially over P/P-0 similar to 0.8 and th e enthalpy of interaction is higher for the first water molecules sorb ed. For P/P-0 congruent to 0.8, the numbers of bound water molecules f ound are 2 per glucopyranosyl unit for neutral polysaccharides, 5 for glucuronan and 9-10 for carboxymethylcellulose (CMC) of D (S) over bar = 2 and hyaluronan (HA).