细胞壁层级重构：锂基低共熔溶剂（Li-DES）构筑结构稳定的高吸湿杨木

Cell-wall-level restructuring: engineering highly hygroscopic and structurally stable poplar wood via lithium-based deep eutectic solvent (Li-DES)

摘要

【目的】速生材的高孔隙率与亲水基团赋予其开发为大气水收集材料的潜力，但其天然吸湿性不足及湿胀引起的结构失稳，严重制约了集水效率与服役寿命。针对这一矛盾，本研究提出利用锂基低共熔溶剂（Li-DES）对杨木进行改性，同步实现木材细胞壁的高吸湿性能与结构稳定化。【方法】采用 LiCl 分别与乳酸、甘油和尿素复配，制备具有酸碱梯度的 Li-DES 浸渍体系。将杨木试样置于 Li-DES 中进行真空−加压处理，随后干燥至绝干。采用 DVS 测定试材在不同相对湿度下的平衡含水率（EMC）及（57 ± 3）% 相对湿度下的体积湿胀率，以此评价处理材的吸湿性与尺寸稳定性。利用 SEM-EDS 观察微观形貌并分析 LiCl 在细胞壁中的分布，结合氮气吸附测试表征孔隙结构演变；通过经典湿化学分析法测定纤维素、半纤维素及木质素的含量，并借助 ATR-FT-IR 解析官能团的变化，从而阐明物理化学结构对吸湿性能的调控机制。基于 XRD 和 XPS 分别表征处理材的结晶结构和配位结构，从分子层面揭示尺寸稳定机制。【结果】Li-DES 处理成功实现了 Li⁺在细胞壁层面的均匀负载（增重率 60.26% ~ 74.84%）。在 57% 相对湿度下，处理材的 EMC 提升至未处理材的 4 倍左右，显示出优异的高吸湿性；同时湿胀率显著降低，其中 LiCl/LA 体系的抗胀率高达 80.99%，实现吸湿性与尺寸稳定性的协同提升。微观分析表明，细胞壁层面重构主要体现在 3 方面。（1）孔隙结构演变：处理材的孔体积明显增加。（2）结晶区变化：（200）晶面间距缩小（衍射角偏移 0.20° ~ 0.61°），相对结晶度由 54.12% 提升至 54.85% ~ 63.63%（相对增幅 1.35% ~ 17.57%）。（3）非晶区配位交联：XPS 证实木材含氧官能团与 Li⁺形成了金属配位键，构建了分子尺度的交联网络。前者拓宽了水分的物理通路和储存空间，后两者共同增强了木材抵抗水分引发形变的能力。【结论】本研究证实，Li-DES 改性有效解决了天然木材吸湿性与尺寸稳定性难以兼顾的矛盾。化学组分负载为吸湿性提升提供了物质基础，同时主导了细胞壁的层级重构：多级孔隙结构的增加丰富了水分吸着的物理环境；并通过结晶区致密化与非晶区配位交联的双路径调控赋予材料结构稳定性。该工作突破了传统改性中性能此消彼长的局限，为木材的湿膨胀调控提供了基于酸碱/配位的新策略，也为速生材在大气水收集领域的应用奠定了科学基础。

Abstract

[Objective] While the high porosity and abundant hydrophilic groups of fast-growing wood 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. [Method] LiCl was systematically formulated with lactic acid, glycerol, and urea 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, DVS was employed to determine the equilibrium moisture content (EMC) at various relative humidities, and volumetric swelling was measured at a relative humidity of (57 ± 3)%. Furthermore, SEM-EDS was utilized to observe the micromorphology and map the distribution of LiCl within the cell walls, complemented by nitrogen adsorption test 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 weight percent gains ranging from 60.26% to 74.84%. 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) Changes in crystalline regions: the d-spacing of the (200) crystal planes narrowed, with the diffraction peak shifting toward higher angles by 0.20° ~ 0.61°, while the relative crystallinity increased from 54.12% to 54.85% ~ 63.63%, corresponding to a relative increase of 1.35% ~ 17.57%. (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 wood in atmospheric water harvesting applications.

关键词

锂基低共熔溶剂 / 生物基吸湿材料 / 细胞壁重构 / 湿胀调控 / 尺寸稳定性 / 孔隙结构 / 大气水收集

Key words

lithium-based deep eutectic solvents / bio-based hygroscopic materials / cell wall restructuring / swelling control / dimensional stability / pore structure / atmospheric water harvesting

参考文献

[1] Xu J, Li J, Penttilä P, et al. Wood—water interaction: fundamental understanding, advanced characterization and functional frontiers[J].Progress in Materials Science, 2026: 101746.

[2] Chang X, Mujawar Mubarak N, Ali Mazari S, et al. A review on the properties and applications of chitosan, cellulose and deep eutectic solvent in green chemistry [J].Journal of Industrial and Engineering Chemistry, 2021, 104: 362-380.

[3] Liu X, Li M C, Lu Y, et al. Cellulose nanofiber—coated delignified wood as an efficient filter for microplastic removal [J].Progress in Natural Science: Materials International, 2024, 34(1): 162-171.

[4] Ran Y, Li J, Zhang S, et al. Super—stable modified wood for enhanced autonomous indoor humidity regulation [J].Chemical Engineering Journal, 2024, 488: 150814.

[5] Khalid A, Tahir S, Khalid A R, et al. Breaking new grounds: metal salts based—deep eutectic solvents and their applications: a comprehensive review [J].Green Chemistry, 2024, 26(5): 2421-2453.

[6] Davies P A, Knowles P R. Seawater bitterns as a source of liquid desiccant for use in solar—cooled greenhouses [J].Desalination, 2006, 196(1−3): 266-279.

[7] Meng X, Zhou J, Jin X, et al. High—strength, high—swelling—resistant, high—sensitivity hydrogel sensor prepared with wood that retains lignin [J].Biomacromolecules, 2024, 25(3): 1696-1708.

[8] Zhang K, Zong L, Tan Y, et al. Improve the flame retardancy of cellulose fibers by grafting zinc ion [J].Carbohydrate Polymers, 2016, 136: 121-127.

[9] Sibikina O V, Iozep A A, Moskvin A V. Polysaccharide complexes with metal cations: structure and application (a review) [J].Pharmaceutical Chemistry Journal, 2009, 43(6): 341-345.

[10] Macromolecule Academy. Physical properties of macromolecules[M]. Tokyo: Kyoritsu Press,1958.

[11] Barrett E P, Joyner L G, Halenda P P. The determination of pore volume and area distributions in porous substances. I. computations from nitrogen isotherms [J].Journal of the American Chemical Society, 1951, 73(1): 373-380.

[12] Horváth G, Kawazoe K. Method for the calculation of effective pore size distribution in molecular sieve carbon [J].Journal of Chemical Engineering of Japan, 1983, 16(6): 470-475.

[13] Wang H, Jia Y, Wang X, et al. Physical—chemical properties of nickel analogs ionic liquid based on choline chloride [J].Journal of Thermal Analysis and Calorimetry, 2014, 115(2): 1779-1785.

[14] Cao J, Wang C, Shi L, et al. Water based—deep eutectic solvent for ultrasound—assisted liquid—liquid microextraction of parabens in edible oil [J].Food Chemistry, 2022, 383: 132586.

[15] Xu L, Yin J, He J, et al. Completely inorganic deep eutectic solvents for efficient and recyclable liquid—liquid interface catalysis [J].Advanced Materials, 2024, 36(29): 2313853.

[16] Han K S, Yu Z, Wang H, et al. Origin of unusual acidity and Li+ diffusivity in a series of water—in—salt electrolytes [J].The Journal of Physical Chemistry B, 2020, 124(25): 5284-5291.

[17] Gilson R, Durrant M C. Estimation of the pKa values of water ligands in transition metal complexes using density functional theory with polarized continuum model solvent corrections [J].Dalton Transactions, 2009(46): 10223-10230.

[18] Deng Z, Liu W, Ruan G, et al. Hygroscopic behavior, dimensional stability, and chemical structure of bamboo after thermal treatments under dry and wet conditions [J].Holzforschung, 2025, 79(12): 733-747.

[19] Chen H, Sun C, Hu Y, et al. Reaction characteristics of metal—salt coordinated deep eutectic solvents during lignocellulosic pretreatment [J].Journal of Environmental Chemical Engineering, 2023, 11(2): 109531.

[20] El Oudiani A, Chaabouni Y, Msahli S, et al. Crystal transition from cellulose I to cellulose II in NaOH treated Agave americana L. fibre [J].Carbohydrate Polymers, 2011, 86(3): 1221-1229.

[21] Ren Z, Guo R, Zhou X, et al. Effect of amorphous cellulose on the deformation behavior of cellulose composites: Molecular dynamics simulation [J].RSC Advances, 2021, 11(33): 19967-19977.

[22] Pombeiro A J L, Guedes da Silva M F C, Lemos M A N D A. Electron—transfer induced isomerizations of coordination compounds [J].Coordination Chemistry Reviews, 2001, 219: 53-80.

[23] Dominic J, David T, Vanaja A, et al. Supercapacitor performance study of lithium chloride doped polyaniline [J].Applied Surface Science, 2018, 460: 40-47.

[24] Pearson R G. Hard and soft acids and bases [J].Journal of the American Chemical Society, 1963, 85(22): 3533-3539.

基金资助

北京市自然科学基金项目(6242021)