Effect of Hydrophobic Surface Modification on the Water Vapor Sorption Properties of Cellulose Paper

Water adsorption by cellulosic materials plays a vital role in material stability, processing, and performance. These interactions are governed by cellulose's native hydrophilicity, arising from a high density of hydroxyl groups. To investigate the influence of hydrophobic modification on water vapor sorption in a porous cellulose substrate, hydroxyl groups were systematically substituted with linear alkyl chains (C4-C12) using a green, gas-phase, surface-selective esterification process. Contrary to the expectation of a monotonic decrease in water affinity with increasing hydrophobicity, water vapor sorption exhibited a pronounced nonmonotonic dependence on the degree of hydroxyl substitution (DS). Initial modification reduced water vapor uptake; however, beyond a critical level of esterification, further modification caused vapor sorption to increase and ultimately return to levels comparable to those of unmodified cellulose. This behavioral transition is correlated with modification-induced changes in near-surface morphology observed by SEM. These structural changes increase surface porosity, granting water vapor access to the interior, unmodified cellulose below the hydrophobic surface layer. Collectively, these findings highlight the coupled roles of surface chemistry, morphology, and mass transport in governing moisture interactions in modified polysaccharides and underscore the importance of controlling functionalization extent to achieve effective moisture barrier performance in porous, hygroscopic materials.