37—MOISTURE RELATIONS OF CELLULOSE. IV—THE EFFECT OF STABILIZATION OF COTTON CELLULOSE ON ITS SORPTIVITY AT DIFFERENT TEMPERATURES, AND ITS BEARING ON ISOSTERIC HEATS OF SORPTION

Abstract

Adsorption–desorption cycles of water vapour from dryness to saturation are determined on unstabilized cellulose at 15°C, 30°C and 4O°C, and on stabilized cellulose from 0°C to 50°C. As well as bringing about rcproducibility of the sorption isotherms, which is particularly lacking on desorption paths on unstabilized cellulose, stabilization causes the same displacement of the isotherms whether it is achieved using water vapour at 18°C or liquid water at 60°C. Stabilization lowers the 15°C isotherms, particularly at higher humidities, and especially on the desorption path. At 40°C, the decrease in sorptivity is apparent only at high humidities, and the adsorption and desorption isotherms cross over the corresponding ones on unstabilized cellulose at lower humidities. Stabilization thus seems to lower the temperature above which the adsorption isotherms become closer together. This ‘optimum’ temperature is about 30°C for unstabilized cellulose and about 15°C for stabilized. The hysteresis loop on stabilized cellulose at lower humidities widens progressively with rise of temperature above the optimum temperature, supporting the explanation of hysteresis given previously¹². The displacement of the isotherms with stabilization is explained in terms of a difference in the variation with temperature of the binding energy of the chain molecules in the non-crystalline regions of cellulose. This affects the sorbing capacity at lower humidities, while at higher humidities the effect of the size of interchain spaces, which apparently is decreased by stabilization, predomiriates. Isosteric heats of adsorption are too low, especially above the optimum temperature, and those of desorption are quite erroneous, resulting from the inapplicability of the Clausius–Clapeyron equation, since the sorbing surface increases with temperature for the same regain.