Nanoscale Mechanism of Moisture-Induced Swelling in Wood Microfibril Bundles

Understanding nanoscale moisture interactions is fundamental to most applications of wood, including cellulosic nanomaterials with tailored properties. By combining X-ray scattering experiments with molecular simulations and taking advantage of computed scattering, we studied the moisture-induced changes in cellulose microfibril bundles of softwood secondary cell walls. Our models reproduced the most important experimentally observed changes in diffraction peak locations and widths and gave new insights into their interpretation. We found that changes in the packing of microfibrils dominate at moisture contents above 10–15%, whereas deformations in cellulose crystallites take place closer to the dry state. Fibrillar aggregation is a significant source of drying-related changes in the interior of the microfibrils. Our results corroborate the fundamental role of nanoscale phenomena in the swelling behavior and properties of wood-based materials and promote their utilization in nanomaterials development. Simulation-assisted scattering analysis proved an efficient tool for advancing the nanoscale characterization of cellulosic materials

Nanoscale Mechanism of Moisture-Induced Swelling in Wood Microfibril Bundles

Understanding nanoscale moisture interactions is fundamental to most applications of wood, including cellulosic nanomaterials with tailored properties. By combining X-ray scattering experiments with molecular simulations and taking advantage of computed scattering, we studied the moisture-induced changes in cellulose microfibril bundles of softwood secondary cell walls. Our models reproduced the most important experimentally observed changes in diffraction peak locations and widths and gave new insights into their interpretation. We found that changes in the packing of microfibrils dominate at moisture contents above 10–15%, whereas deformations in cellulose crystallites take place closer to the dry state. Fibrillar aggregation is a significant source of drying-related changes in the interior of the microfibrils. Our results corroborate the fundamental role of nanoscale phenomena in the swelling behavior and properties of wood-based materials and promote their utilization in nanomaterials development. Simulation-assisted scattering analysis proved an efficient tool for advancing the nanoscale characterization of cellulosic materials

How Cellulose Nanofibrils Affect Bulk, Surface, and Foam Properties of Anionic Surfactant Solutions

We study the generation and decay of aqueous foams stabilized by sodium dodecyl sulfate (SDS) in the presence of unmodified cellulose nanofibrils (CNF). Together with the rheology of aqueous suspensions containing CNF and SDS, the interfacial/colloidal interactions are determined by quartz crystal microgravimetry with dissipation monitoring, surface plasmon resonance, and isothermal titration calorimetry. The results are used to explain the properties of the air/water interface (interfacial activity and dilatational moduli determined from oscillating air bubbles) and of the bulk (steady-state flow, oscillatory shear, and capillary thinning). These properties are finally correlated to the foamability and to the foam stability. The latter was studied as a function of time by monitoring the foam volume, the liquid fraction, and the bubble size distribution. The shear-thinning effect of CNF is found to facilitate foam formation at SDS concentrations above the critical micelle concentration (cSDS ≥ cmc). Compared with foams stabilized by pure SDS, the presence of CNF enhances the viscosity and elasticity of the continuous phase as well as of the air/water interface. The CNF-containing foams have higher liquid fractions, larger initial bubble sizes, and better stability. Due to charge screening effects caused by sodium counter ions and depletion attraction caused by SDS micelles, especially at high SDS concentrations, CNF forms aggregates in the Plateau borders and nodes of the foam, thus slowing down liquid drainage and bubble growth and improving foam stability. Overall, our findings advance the understanding of the role of CNF in foam generation and stabilization

High Internal Phase Oil-in-Water Pickering Emulsions Stabilized by Chitin Nanofibrils: 3D Structuring and Solid Foam

Chitin nanofibrils (NCh, ∼10 nm lateral size) were produced under conditions that were less severe compared to those for other biomass-derived nanomaterials and used to formulate high internal phase Pickering emulsions (HIPPEs). Pre-emulsification followed by continuous oil feeding facilitated a “scaffold” with high elasticity, which arrested droplet mobility and coarsening, achieving edible oil-in-water emulsions with internal phase volume fraction as high as 88%. The high stabilization ability of rodlike NCh originated from the restricted coarsening, droplet breakage and coalescence upon emulsion formation. This was the result of (a) irreversible adsorption at the interface (wettability measurements by the captive bubble method) and (b) structuring in highly interconnected fibrillar networks in the continuous phase (rheology, cryo-SEM, and fluorescent microscopies). Because the surface energy of NCh can be tailored by pH (protonation of surface amino groups), emulsion formation was found to be pH-dependent. Emulsions produced at pH from 3 to 5 were most stable (at least for 3 weeks). Although at a higher pH NCh was dispersible and the three-phase contact angle indicated better interfacial wettability to the oil phase, the lower interdroplet repulsion caused coarsening at high oil loading. We further show the existence of a trade-off between NCh axial aspect and minimum NCh concentration to stabilize 88% oil-in-water HIPPEs: only 0.038 wt % (based on emulsion mass) NCh of high axial aspect was required compared to 0.064 wt % for the shorter one. The as-produced HIPPEs were easily textured by taking advantage of their elastic behavior and resilience to compositional changes. Hence, chitin-based HIPPEs were demonstrated as emulgel inks suitable for 3D printing (millimeter definition) via direct ink writing, e.g., for edible functional foods and ultralight solid foams displaying highly interconnected pores and for potential cell culturing applications

Changing the Structural and Mechanical Anisotropy of Foam-Formed Cellulose Materials by Affecting Bubble–Fiber Interaction with Surfactant

Cellulose fiber materials suitable for filtering, insulation, protective, and hygiene applications can be formed using aqueous foam as a carrier phase. The subtle fiber–bubble interaction provides a tool which can be utilized to alter both structural and mechanical material properties. Earlier model surface studies have only indicated clear surface-bubble adhesion when both the surface hydrophobicity and surface tension of the solution are high enough. In this work, we first show that for silica model surfaces these basic mechanisms are similar for both nonionic polyethylene glycol sorbitan monolaurate (Tween 20) and anionic sodium dodecyl sulfate (SDS) surfactants. In the second step, thick nonwoven materials were foam formed from hydrophilic or hydrophobic viscose fibers using small amounts of cellulose microfibers (CMFs) to form a bonding agent. There was a clear variation in structure and strength properties between the samples made using different fibers and surfactants. The partial alignment and layering of fibers in the wet foam led to anisotropy in the mechanical properties of the formed samples. Using SDS, the fiber alignment was very strong for hydrophilic fibers but was reduced for hydrophobic fibers because of stronger coupling to bubbles during structure forming, impacting the microscale fiber network. For nonionic Tween 20, in addition to surfactant adsorption on the fibers, the ethoxylated surfactant headgroups are suggested to form bridges between CMFs and other fibers, restricting fiber movements during formation. For hydrophilic fibers, this showed up as a lower in-plane compression modulus but higher transverse strength for Tween 20 compared with SDS surfactant. For hydrophobic fibers, the sensitivity of the mechanical properties on surfactant type was even stronger

Changing the Structural and Mechanical Anisotropy of Foam-Formed Cellulose Materials by Affecting Bubble–Fiber Interaction with Surfactant

Cellulose fiber materials suitable for filtering, insulation, protective, and hygiene applications can be formed using aqueous foam as a carrier phase. The subtle fiber–bubble interaction provides a tool which can be utilized to alter both structural and mechanical material properties. Earlier model surface studies have only indicated clear surface-bubble adhesion when both the surface hydrophobicity and surface tension of the solution are high enough. In this work, we first show that for silica model surfaces these basic mechanisms are similar for both nonionic polyethylene glycol sorbitan monolaurate (Tween 20) and anionic sodium dodecyl sulfate (SDS) surfactants. In the second step, thick nonwoven materials were foam formed from hydrophilic or hydrophobic viscose fibers using small amounts of cellulose microfibers (CMFs) to form a bonding agent. There was a clear variation in structure and strength properties between the samples made using different fibers and surfactants. The partial alignment and layering of fibers in the wet foam led to anisotropy in the mechanical properties of the formed samples. Using SDS, the fiber alignment was very strong for hydrophilic fibers but was reduced for hydrophobic fibers because of stronger coupling to bubbles during structure forming, impacting the microscale fiber network. For nonionic Tween 20, in addition to surfactant adsorption on the fibers, the ethoxylated surfactant headgroups are suggested to form bridges between CMFs and other fibers, restricting fiber movements during formation. For hydrophilic fibers, this showed up as a lower in-plane compression modulus but higher transverse strength for Tween 20 compared with SDS surfactant. For hydrophobic fibers, the sensitivity of the mechanical properties on surfactant type was even stronger