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

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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.

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