Sorption kinetics of salt-embedded bulk and porous hydrogels: A mass transfer analysis

• A liquid transport model for the bulk hydrogel is established. • The dominant sorption mechanisms of the bulk and porous hydrogels are clarified. • The sorption kinetics for the bulk and porous hydrogels in dynamic scenarios were analyzed. Salt-embedded hydrogels have emerged as promising materials for flexible electronics, atmospheric water harvesting, and evaporative cooling applications. However, a comprehensive understanding of the sorption kinetics of hydrogels with varying structures remains limited, hindering further optimization and practical applications. In this work, we utilized a liquid transport model and a two-concentration model to analyze the sorption properties and dominant mechanisms of salt-embedded bulk and porous hydrogels. Specifically, the bulk hydrogel exhibits slow sorption kinetics due to surface interfacial sorption coupled with liquid diffusion, whereas the porous hydrogel shows fast sorption kinetics because of the gas-liquid coupled transport and internal interfacial sorption. Additionally, we analyzed the critical factors affecting sorption kinetics, including relative humidity, temperature, porosity, thickness, salt content, and shear modulus. Finally, we designed targeted application scenarios and demonstrated that the bulk hydrogel can maintain stable water content for flexible material uses, while the porous hydrogel shows potential for atmospheric water capture. This work provides theoretical guidance for the design and application of salt-embedded hydrogels.