Wood Moisture-Induced Swelling at the Cellular Scale—Ab Intra

Elucidation of temperature-induced water structuring on cellulose surfaces for environmental and energy sustainability

Optimizing drying energy in the forest products industry is critical for integrating lignocellulosic feedstocks across all manufacturing sectors. Despite substantial efforts to reduce thermal energy consumption during drying, further enhancements are possible. Cellulose, the main component of forest products, is Earth's most abundant biopolymer and a promising renewable feedstock. This study employs all-atom molecular dynamics (MD) simulations to explore the structural dynamics of a small Iβ-cellulose microcrystallite and surrounding water layers during drying. Molecular and atomistic profiles revealed localized water near the cellulose surface, with water structuring extending beyond 8 Å into the water bulk, influencing solvent-accessible surface area and solvation energy. With increasing temperature, there was a ∼20 % reduction in the cellulose surface available for interaction with water molecules, and a ∼22 % reduction in solvation energy. The number of hydrogen bonds increased with thicker water layers, facilitated by a "bridging" effect. Electrostatic interactions dominated the intermolecular interactions at all temperatures, creating an energetic barrier that hinders water removal, slowing the drying processes. Understanding temperature-dependent cellulose-water interactions at the molecular level will help in designing novel strategies to address drying energy consumption, advancing the adoption of lignocellulosics as viable manufacturing feedstocks.

Micro- and nano-hybrid cellulose fibers prepared by straightforward and high-efficiency hot water soaking-assisted colloid grinding for high-performance cellulose paper

Micro- and nano-hybrid cellulose fiber (MNCF) stands out as a versatile cellulosic nanomaterial with promising applications in various fields owing to its excellent intrinsic nature and outstanding characteristics. However, the inefficiency in preparing MNCF, attributed to a complex multi-step processing, hinders its widespread adoption. In this study, a straightforward and highly efficient method for MNCF preparation was developed via a hot water soaking-assisted colloid grinding strategy. Active water molecules in hot water facilitating stronger transverse shrinkage and longitudinal expansion in fiber crystallized region, and thus improving the fibrillation degree of cellulose fibers. As a result, MNCFs with a mean diameter of 37.5 ± 22.2 nm and high concentration (2 wt%) were successfully achieved though pure mechanical method. The micro and nano-hybrid structure leads to the corresponding resulting cellulose paper with micro- and nano-hybrid structure possesses a compact stacking and fewer defects, leading to extraordinary mechanical properties including tensile strength of 204.5 MPa, Young's modulus of 6.3 GPa and elongation of 10.1 %. This work achieves significant progress towards straightforward and highly efficient production of MNCFs, offering an appreciable prospect for the development of multifunctional MNCF-based materials.

Effect of Moisture Content on Ion Diffusion and Glass Transition Temperature in Wood Cell Walls

Understanding and controlling the diffusion of ions and chemicals within the secondary plant cell walls are pivotal in various applications of biomasses. Recent studies have shown that inorganic ion diffusion through secondary cell walls is controlled by a moisture-induced glass transition in amorphous polysaccharides, including amorphous cellulose and hemicelluloses. Understanding the diffusion of ions in these structures has been the subject of numerous recent experiments; however, a deep understanding of the underlying mechanisms of interactions between ion atoms and water/hemicellulose molecules is still lacking. This study uses molecular dynamics simulations to elucidate the diffusion mechanisms of potassium and chloride ions in the cell walls under varying moisture content. The results reveal that a higher moisture content leads to the formation of solvent layers around the ions and reduces the charge interaction between the functional groups of wood polymers and ions. Hence, a higher moisture content results in an improved diffusion rate of ions within the domain. The simulation results also show that higher moisture content lowers the glass transition temperature, promoting diffusion of ions in the system. In contrast, increases in the ion concentration increase the glass transition temperature of the system and degrade the diffusion of ions in the system.

Diffusion in Intact Secondary Cell Wall Models of Plants at Different Equilibrium Moisture Content

Secondary plant cell walls are composed of carbohydrate and lignin polymers, and collectively represent a significant renewable resource. Leveraging these resources depends in part on a mechanistic understanding for diffusive processes within plant cell walls. Common wood protection treatments and biomass conversion processes to create biorefinery feedstocks feature ion or solvent diffusion within the cell wall. X-ray fluorescence microscopy experiments have determined that ionic diffusion rates are dependent on cell wall hydration as well as the ionic species through non-linear relationships. In this work, we use classical molecular dynamics simulations to map the diffusion behavior of different plant cell wall components (cellulose, hemicellulose, lignin), ions (Na⁺, K⁺, Cu²⁺, Cl^-) and water within a model for an intact plant cell wall at various hydration states (3-30 wt% water). From these simulations, we analyze the contacts between different plant cell wall components with each other and their interaction with the ions. Generally, diffusion increases with increasing hydration, with lignin and hemicellulose components increasing diffusion by an order of magnitude over the tested hydration range. Ion diffusion depends on charge. Positively charged cations preferentially interact with hemicellulose components, which include negatively charged carboxylates. As a result, positive ions diffuse more slowly than negatively charged ions. Measured diffusion coefficients are largely observed to best fit piecewise linear trends, with an inflection point between 10 and 15% hydration. These observations shed light onto the molecular mechanisms for diffusive processes within secondary plant cell walls at atomic resolution.

Moisture-Related Shrinkage Behavior of Wood at Macroscale and Cellular Level

Due to wood moisture sensitivity, shrinkage cracks tend to present wooden structures. These failures are caused by moisture-related shrinkage behavior. In order to avoid it, it is necessary to have a better understanding of shrinkage behavior. In this respect, studying the dimension changes in wood at different scales is of utmost significance for a better understanding of the shrinkage properties. Herein, the shrinkage behavior of Masson pines (Pinus massoniana) wood was investigated at macroscopic and cellular levels during moisture loss via digital image correlation using VIC-3D and digital microscopic systems, respectively. According to the full-field strain maps, shrinkage strain near the external face was higher than that at the internal face, which increased susceptibility to cracking at the external face of lumber. Additionally, the anisotropic shrinkage of wood was explored. The shrinkage ratio at the end of drying was about 5.5% in the tangential (T) direction and 3.5% in the radial (R) direction. However, at a cellular level, the shrinkage ratios in the T and R directions of earlywood tracheids were 7.13% and 2.46%, whereas the corresponding values for latewood tracheid were 9.27% and 5.52%, respectively. Furthermore, the maximum T/R shrinkage ratio at the macroscopic level (1.7) was found to be similar to the value of latewood tracheid (1.72). The earlywood showed high anisotropic, its T/R shrinkage ratio was 2.75. The macroscopic shrinkage was the result of the interaction of the tracheids of earlywood and latewood and was mainly dominated by latewood tracheids.

Wood Modification as a Tool to Understand Moisture in Wood

Moisture plays a central role in the performance of wood products because it affects important material properties such as the resistance to decomposition, the mechanical properties, and the dimensions. To improve wood performance, a wide range of wood modification techniques that alter the wood chemistry in various ways have been described in the literature. Typically, these modifications aim to improve resistance to decomposition, dimensional stability, or, to introduce novel functionalities in the wood. However, wood modification techniques can also be an important tool to improve our understanding of the interactions between wood and moisture. In this review, we describe current knowledge gaps in our understanding of moisture in wood and how modification has been and could be used to clarify some of these gaps. This review shows that introducing specific chemical changes, and even controlling the distribution of these, in combination with the variety of experimental methods available for characterization of moisture in wood, could give novel insights into the interaction between moisture and wood. Such insights could further contribute to applications in several related fields of research such as how to enhance the resistance to decomposition, how to improve the performance of moisture-induced wooden actuators, or how to improve the utilization of wood biomass with challenging swelling anisotropy.

Bundling of cellulose microfibrils in native and polyethylene glycol-containing wood cell walls revealed by small-angle neutron scattering

Wood and other plant-based resources provide abundant, renewable raw materials for a variety of applications. Nevertheless, their utilization would greatly benefit from more efficient and accurate methods to characterize the detailed nanoscale architecture of plant cell walls. Non-invasive techniques such as neutron and X-ray scattering hold a promise for elucidating the hierarchical cell wall structure and any changes in its morphology, but their use is hindered by challenges in interpreting the experimental data. We used small-angle neutron scattering in combination with contrast variation by poly(ethylene glycol) (PEG) to identify the scattering contribution from cellulose microfibril bundles in native wood cell walls. Using this method, mean diameters for the microfibril bundles from 12 to 19 nm were determined, without the necessity of cutting, drying or freezing the cell wall. The packing distance of the individual microfibrils inside the bundles can be obtained from the same data. This finding opens up possibilities for further utilization of small-angle scattering in characterizing the plant cell wall nanostructure and its response to chemical, physical and biological modifications or even in situ treatments. Moreover, our results give new insights into the interaction between PEG and the wood nanostructure, which may be helpful for preservation of archaeological woods.