Ammonia sorption isotherms of wood and cotton cellulose

Abstract

Sorption isotherms of ammonia were measured on cellulosic materials, such as beech and birch wood, as well as on cotton cellulose, the object being to obtain information on the nature of interaction between the wood and the ammonia by means of application of various sorption theories. As a result several analogies between the sorption of ammonia vapour and water vapour could be observed.—The isotherms displayed the typical S-shape and developed a hysteresis along the adsorption and desorption lines. The sorbate films reached a thickness of 4–6 molecular layers. The lowering of enthalpy of the ammonia vapour appeared to be the propelling force of the sorption process, as in the case with the uptake of water vapour.—In contrast to water isotherms, however, ammonia isotherms cannot be reproduced with the same specimen. Each sorption cycle brought a loss of substance and a reduction of the fibre saturation capacities of the adsorbent. In ammonia vapour the fibre saturation points were found at sorbate concentrations twice as high as in water vapour. The hysteresis between the adsorption and desorption processes also appeared with ammonia vapour; however, the continuous changes of the adsorbent, caused by chemical interaction with ammonia, produced heavy displacements. The specific surface area of the samples in ammonia was approximately twice the size of that in water, but the extensions varied greatly during the sorption cycles.