Sorption hysteresis in nanoporous swelling adsorbents

Despite its ubiquitous practical impact, the physics of sorption in hygroscopic materials is still under debate. One major, but not yet understood, characteristic is the sorption-desorption hysteresis observed with such systems despite the absence of capillary effects. Here, through an experimental and theoretical approach, we show that this hysteresis is intimately linked to sorption-induced deformation, and more precisely to the existence of a compressive stress state opposing the swelling of the material. First, with the help of magnetic resonance imaging, in contrast with most previous studies, we get a detailed picture of the moisture distribution and transport through the thickness of a model cellulosic material under well-controlled humidity boundary conditions, which makes it possible to identify the actual local sorption dynamics and equilibrium. We observe that the sorption can be hampered, whereas the desorption process is instantaneous at our scale of observation. Interestingly, only sufficiently compacted samples exhibit reduced sorption, suggesting that the mechanical stress is central. Then, based on a statistical thermodynamic and poromechanical approach, we propose a model coupling adsorption and pressure that predicts our observation. This model is fully parametrized from independent tests, and it suggests that sorption is significantly affected when sample compaction exceeds a few megapascals. These developments clarify the link between adsorption hysteresis and mechanics with important implications for biopolymers sensitive to moisture (wood, textiles, paper, etc.).