Helicoidal self-ordering of cellulose microfibrils in aqueous suspension
In many skeletal support systems of plants and animals, cellulose, chitin, and collagen occur in the form of microfibrils ordered in a chiral nematic fashion (helicoids). However, these structures remain poorly understood due to the many constituents present in biological tissues. Here we report an in vitro system that attracts by its simplicity. Only one chemical component, cellulose, is present in the form of fibrillar fragments dispersed in water. Above a critical concentration the colloidal dispersion separates spontaneously into a chiral nematic liquid crystalline phase. On drying this phase solidifies into regularly twisted fibrillar layers that mimic the structural organization of helicoids in nature.
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Thermodynamics of interactions between cellulose nanocrystals and monovalent counterions
2024, Carbohydrate PolymersAlkali and quaternary ammonium cations interact with negatively charged cellulose nanocrystals (CNCs) bearing sulfated or carboxylated functional groups. As these are some of the most commonly occurring cations CNC encounter in applications, the thermodynamic parameters of these CNC-counterion interactions were evaluated with isothermal titration calorimetry (ITC). Whereas the adsorption of monovalent counterions onto CNCs was thermodynamically favourable at all evaluated conditions as indicated by a negative Gibbs free energy, the enthalpic and entropic contributions to the CNC-ion interactions were found to be strongly dependent on the hydration characteristics of the counterion and could be correlated with the potential barrier to water exchange of the respective ions. The adsorption of chaotropic cations onto the surface was exothermic, while the interactions with kosmotropic cations were endothermic and completely entropy-driven. The interactions of CNCs with more bulky quaternary ammonium counterions were more complex, and the mechanism of interaction shifted from electrostatic interactions with surface charged groups of CNCs towards adsorption of alkyl chains onto the CNC hydrophobic planes when the alkyl chain length increased.
Effects of different salt ions on the structure and rheological behavior of sulfated cellulose nanocrystal hydrogel
2024, Food HydrocolloidsCellulose nanocrystal (CNC) hydrogels are increasingly used in various applications. Here, CNC hydrogels were formed by adding four salt ions (NaCl, KCl, MgCl2, and CaCl2) to sulfated CNC suspensions. NaCl and KCl are monovalent salt ions and MgCl2 and CaCl2 are divalent salt ions; each pair has similar cation charges but different cation radii. This study compared the effects of four salt ions on the structure and rheological behavior of CNC hydrogels. At the same ion concentration, compared with NaCl and KCl, the addition of MgCl2 and CaCl2 significantly reduced the distance between the CNC particles, forming pronounced aggregates and denser network structures. Furthermore, the G′ and G″ of the CNC hydrogels with salt ions followed the order NaCl < KCl < MgCl2 < CaCl2 in the small-amplitude oscillatory shear (SAOS) test, and more viscoelastic hydrogels exhibited relatively lower deformability in the creep-recovery test. Under large-amplitude oscillatory shear (LAOS) conditions, the CNC hydrogels with salt ions exhibited LAOS type III behavior, and the change in yield stress followed the same order as the SAOS results. Among them, CNC hydrogel with CaCl2 exhibited stronger nonlinear response and enhanced strain-stiffening behavior (higher I3/I1 and e3/e1) to resist deformation before 20% strain, as well as an enhanced softening effect (higher |v3/v1|) as the structure was destroyed and rearranged at strains ranging from 20% to 100%. Such CNC hydrogels with different salt ions, which can regulate structural and rheological properties, provide basis for designing hydrogels and meet potential requirements in food and biomedicine fields.
On rheological properties of disc-shaped cellulose nanocrystals
2024, Carbohydrate PolymersThe rheological properties of a substance depend greatly on its morphology, and rod-shaped cellulose nanocrystals (RCNCs) and cellulose nanofibrils (CNFs) have been extensively studied for their rheological properties. Nevertheless, the rheological properties of disc-shaped cellulose nanocrystals (DCNCs) with crystalline allomorph II derived from mercerized cellulose remain unknown yet. This work investigated the DCNCs' rheological properties in depth using steady-shear and oscillation measurements. At the same concentration, DCNC's suspension viscosity is lower than that of RCNC; RCNC has an instinct viscosity of 258.2, while DCNC has 187.9. Comparing RCNC suspensions with cellulose nanorods, DCNC has a lower aspect ratio and exhibits a distinct steady shear behavior. Under polarized film, DCNC suspension cannot self-assemble into chiral or liquid crystal phases, and with increasing concentrations, the system transitions from an isotropic phase to a gel phase. Oscillation sweeps demonstrate that the gel transition occurs at 7 %–8 %. Based on thixotropic recovery sweep outcomes, the high-stress oscillations enhance the network structure of DCNC suspensions, which is significantly different from that of RCNC suspensions. Results demonstrate the unique properties of DCNC, highlighting its application as a rheological modifier.
Orthotropic organization of a cellulose nanocrystal suspension realized via the combined action of frontal ultrafiltration and ultrasound as revealed by in situ SAXS
2024, Journal of Colloid and Interface ScienceRodlike cellulose nanocrystals (CNCs) exhibit significant potential as building blocks for creating uniform, sustainable materials. However, a critical hurdle lies in the need to enhance existing or devise novel processing that provides improved control over the alignment and arrangement of CNCs across a wide spatial range. Specifically, the challenge is to achieve orthotropic organization in a single-step processing, which entails creating non-uniform CNC orientations to generate spatial variations in anisotropy.
A novel processing method combining frontal ultrafiltration (FU) and ultrasound (US) has been developed. A dedicated channel-cell was designed to simultaneously generate (1) a vertical acoustic force thanks to a vibrating blade at the top and (2) a transmembrane pressure force at the bottom. Time-resolved in situ small-angle X-ray scattering permitted to probe the dynamical structural organization/orientation of CNCs during the processing.
For the first time, a typical three-layer orthotropic structure that resembles the articular cartilage organization was achieved in one step during the FU/US process: a first layer composed of CNCs having their director aligned parallel to the horizontal membrane surface, a second intermediate isotropic layer, and a third layer of CNCs with their director vertically oriented along the direction of US wave propagation direction.
Centrifuge fractionation during purification of cellulose nanocrystals after acid hydrolysis and consequences on their chiral self-assembly
2024, Carbohydrate PolymersThe inherent colloidal dispersity (due to length, aspect ratio, surface charge heterogeneity) of CNCs, when produced using the typical traditional sulfuric acid hydrolysis route, presents a great challenge when interpreting colloidal properties and linking the CNC film nanostructure to the helicoidal self-assembly mechanism during drying. Indeed, further improvement of this CNC preparation route is required to yield films with better control over the CNC pitch and optical properties. Here we present a modified CNC-preparation protocol, by fractionating and harvesting CNCs with different average surface charges, rod lengths, aspect ratios, already during the centrifugation steps after hydrolysis. This enables faster CNC fractionation, because it is performed in a high ionic strength aqueous medium. By comparing dry films from the three CNC fractions, discrepancies in the CNC self-assembly and structural colors were clearly observed. Conclusively, we demonstrate a fast protocol to harvest different populations of CNCs, that enable tailored refinement of structural colors in CNC films.
Phase transition and gelation in cellulose nanocrystal-based aqueous suspensions studied by SANS
2024, Journal of Colloid and Interface ScienceAqueous suspensions of cellulose nanocrystals (CNC) form a re-entrant liquid crystal (LC) phase with increasing salinity. Phase separation occurs in this LC state leading to a biphasic gel with a flow programmable structure that can be used to form anisotropic soft materials. We term this state a Liquid Crystal Hydroglass (LCH). Defining the mechanisms by which the LCH forms requires detailed structural analysis at the mesoscopic length scale.
By utilising Small Angle Neutron Scattering (SANS), we investigated the microstructure transitions in CNC suspensions, with a particular focus on the unique LC re-entrancy and gelation into the biphasic LCH.
Scattering from LCH gels comprises contributions from a dispersed liquid state and static heterogeneity, characterised using a Lorentzian-Gaussian model of inhomogeneity. This conceptually supports a gelation mechanism (spinodal decomposition) in CNC suspensions towards a biphasic structure of the LCH. It also demonstrates that, with increasing salinity, the non-monotonic variation in effective volume fraction of CNC rods fundamentally causes the LC re-entrancy. This work provides the first experimental characterisation of the LC-re-entrancy and formation of an anisotropic LCH gel. The proposed mechanism can be extended to understanding the general behaviour of anisotropic colloids.