Features of water vapor sorption by cellulose materials

In this research, the sorption of water vapor (WV) by various cellulose samples (microcrystalline cellulose, cotton cellulose, Kraft pulp, mercerized cellulose, and rayon fibers) has been studied. The sorption isotherms of WV for various samples have a sigmoid shape. To describe such isotherms, various models have been proposed, including mono- and multimolecular adsorption, formation of hydrates and solid solutions, as well as mixed models combining various sorption mechanisms. After critical analysis of known models, the improved absorption model was proposed to explain the interaction of VW with cellulose samples. It was confirmed that crystallites are inaccessible for water, and therefore water molecules can be trapped only by polar groups of non-crystalline (amorphous) domains of cellulose materials. The developed model permitted obtaining the general absorption isotherm of water molecules in amorphous domains of cellulose. Moreover, the equation was derived, which describes both the general isotherm and isotherm of any semi-crystalline cellulose sample, when its crystallinity or amorphicity degree was pre-determined. Based on the proposed absorption model, various characteristics can be found, e.g., degrees of amorphicity and crystallinity, the maximum amount of absorbed water, the specific surface area of wet cellulose sample, the amount of capillary condensed water, etc.