1—MOISTURE RELATIONS OF CELLULOSE. III. SORPTION HYSTERESIS AND THE EFFECT OF TEMPERATURE

Abstract

Adsorption–desorption cycles of water vapour on ‘stabilized’ standard cellulose from dryness to saturation have been determined at 20°C, 30°C and 40°C. The 4O°C and 30°C adsorption isotherms are closer together than the 30°C and 20°C ones. The hysteresis loop at low relative humidity is considerably wider at 40°C than at the lower temperatures. Isosteric evaluation of the heats of sorption by means of the Clausiuss–Clapeyron equation indicates that the sorbing surface of cellulose at a given moisture content increases above a certain optimum temperature, between 30°C and 40°C, probably owing to the weakening of some semicrystalline inter-chain linkages. Isosteric heats of adsorption are higher than the calorimetric values only below that optimum temperature, because of the energy absorbed in the swelling of cellulose when wetted; this effect is masked above the optimum temperature by the increase in sorbing surface. Examination of phase changes in the adsorbed film suggests a bigger monolayer capacity on desorption than on adsorption. These results support Urquhart's theory of hysteresis that the sorbing surface is in a more active state on desorption than on adsorption. A consideration of the free energy of adsorption, as applied to the chain molecular structure of cellulose, indicates that hysteresis is increased at lower humidities above the optimum temperature.