Association of water to polar groups; estimations by an adsorption model for ligno-cellulosic materials

Abstract
The sorption of moisture by ligno-cellulosic materials has a significant influence on their physical properties since it affects both swelling and softening. In order to estimate the adsorption behaviour, an attempt is made to calculate the number and type of water molecules (whether non-freezing bound water or freezing bound water) adsorbed by the polar groups (OH, COO−, SO3−) with counterions (H+, Na+, Ca2+) common in paper products. The analysis is based on sorption experiments using differential scanning calorimetry and thermogravimetric analysis. The main contributors to the adsorption on wood are the hydroxyl groups of the carbohydrates and the lignin. Any adsorption of freezing bound water in the studied moisture range is attributed to ionic groups present in the sample. Differences in adsorption between weak carboxylic acid and strong sulphonic acid systems are explained as being a function of their different degrees of dissociation. The values calculated for the polar groups using model substances are used in an attempt to predict the adsorption for two xylans, a lignin preparation and a sulphonated wood meal of which the composition and the degree of crystallinity are known, assuming a simple law of additivity. The calculated results agree well with the experimental results.