Objective While the high porosity and abundant hydrophilic groups of fast-growing timber present promising potential for atmospheric water harvesting applications, its inherently inadequate hygroscopicity and swelling-induced dimensional instability severely limit both collection efficiency and service life. To resolve this dilemma, this study proposes a modification approach for poplar wood via a lithium-based deep eutectic solvent (Li-DES). This strategy aims to simultaneously achieve the high hygroscopic performance and structural stabilization of the wood cell wall, thereby elucidating the underlying interrelationship among solvent acid-base characteristics, cell wall structural evolution, and macroscopic properties.

Method Lithium chloride (LiCl) was systematically formulated with lactic acid (LA), glycerol (Gly), and urea (Ur) to prepare Li-DES impregnation systems exhibiting an acid-base gradient. Poplar wood specimens were subjected to a vacuum-pressure impregnation treatment in these solvents and subsequently dried to an oven-dry state. To evaluate the hygroscopicity and dimensional stability of the treated wood, dynamic vapor sorption (DVS) was employed to determine the equilibrium moisture content (EMC) at various relative humidities, alongside the swelling rate under air-dried conditions. Furthermore, SEM-EDS was utilized to observe the micromorphology and map the distribution of Li within the cell walls, complemented by nitrogen adsorption to characterize the evolution of the pore structure. Classical wet chemical analyses quantified the cellulose, hemicellulose, and lignin contents, while ATR-FT-IR was applied to decipher functional group alterations, thereby elucidating how physicochemical structures govern hygroscopic performance. Finally, XRD and XPS were conducted to characterize the crystalline and coordination structures, respectively, unveiling the dimensional stabilization mechanism at the molecular level.

Result The Li-DES treatment successfully achieved uniform loading of Li⁺ throughout the cell walls, with a weight percent gain (WPG) exceeding 60%. At a relative humidity of 57%, the EMC of the treated wood surged to approximately four-times that of the pristine wood, demonstrating exceptional hygroscopicity. Concurrently, the swelling rate was markedly reduced; notably, the LiCl/LA system exhibited an anti-swelling efficiency (ASE) of 80.99%, thereby realizing a synergistic enhancement in both moisture absorption and dimensional stability. Microscopic analyses reveal that the cell-wall-level restructuring predominantly manifests in three dimensions: (1) Pore structure evolution: the pore volume of the treated wood was significantly expanded; (2) Densification of crystalline regions: the d-spacing of the (200) crystal planes narrowed, accompanied by a relative increase in crystallinity ranging from 1.35% to 17.57%; and (3) Coordination cross-linking in amorphous regions: XPS corroborated the formation of metal coordination bonds between the oxygen-containing functional groups of the wood and Li⁺, establishing a molecular-scale cross-linked network. The former broadens the physical pathways and storage space for moisture, while the latter two synergistically fortify the wood’s resistance against moisture-induced deformation.

Conclusion This study demonstrates that the Li-DES modification effectively resolves the longstanding dilemma of balancing hygroscopicity and dimensional stability in natural wood. The loading of chemical components provides the material basis for enhanced moisture absorption while concurrently driving the hierarchical restructuring of the cell wall. Specifically, the expansion of the multi-scale pore structure enriches the physical environment for moisture sorption, whereas the dual-pathway regulation—comprising densification within crystalline regions and coordination cross-linking in amorphous domains—endows the material with robust structural stability. By overcoming the performance trade-offs prevalent in traditional modifications, this work offers a novel acid-base/coordination-based strategy for regulating moisture-induced swelling in wood, thereby laying a solid scientific foundation for the utilization of fast-growing timber in atmospheric water harvesting applications.