ON THE ADSORPTION OF WATER VAPOUR BY CELLULOSE

Abstract

In this paper an attempt is made to gather together the best available data on the adsorption of water vapour by cellulose and to draw from this data a unified picture of the phenomenon. In the first place the various theories explaining the adsorption isotherm are tested. The Langmuir isotherm is shown to fit a small portion of the curve at low relative humidities, and the polymolecular theory of Brunauer, Emmett, and Teller is found to give agreement up to the point where capillary adsorption is assumed to become predominant. It is pointed out, however, that the assumptions involved in the theory of Brunauer, Emmett, and Teller are such that an equally satisfactory agreement might be obtained by assuming that the adsorption was confined to a monomolecular layer in which, owing to repulsive forces between neighbouring atoms, the adsorbing force varied with the number of atoms. Thus it is still doubtful whether the adsorption is monomolecular or polymolecular. The investigation shows that the adsorption on cotton is similar to that on wood, except for the fact that the internal surface area available for adsorption is less by a factor of 1.86.An analysis is made of the data available for calculation of the heats of adsorption, and the values obtained from the different sources are compared. It is found that when the heats of adsorption are plotted against the relative humidity there is no appreciable difference between the values obtained with cotton and with wood. A mean value for the heats of adsorption is obtained. The relation between heats of adsorption and the latent heat of condensation is developed, and it is shown that there is an excess of internal energy in the adsorbed state over that of the liquid state.Finally, an explanation is advanced to account for the experimental fact that there is a reduction in total volume when water vapour is adsorbed on cellulose. An attempt is also made to show (i) how the water molecule may be attached to the cellulose surface by hydrogen bonds in such a way that the energy will be greater than that in liquid water, and, (ii) that, on the basis of such a picture a volume contraction is to be expected.