Nanocelluloses are anisotropic nanoparticles of semicrystalline assemblies of glucan polymers. They have great potential as renewable building blocks in the materials platform of a more sustainable society. As a result, the research on nanocellulose has grown exponentially over the last decades. To fully utilize the properties of nanocelluloses, a fundamental understanding of their colloidal behavior is necessary. As elongated particles with dimensions in a critical nanosize range, their colloidal properties are complex, with several behaviors not covered by classical theories. In this comprehensive Review, we describe the most prominent colloidal behaviors of nanocellulose by combining experimental data and theoretical descriptions. We discuss the preparation and characterization of nanocellulose dispersions, how they form networks at low concentrations, how classical theories cannot describe their behavior, and how they interact with other colloids. We then show examples of how scientists can use this fundamental knowledge to control the assembly of nanocellulose into new materials with exceptional properties. We hope aspiring and established researchers will use this Review as a guide.

The structure and dynamics of networks formed by rod-shaped particles can be indirectly investigated by measuring the diffusion of spherical tracer particles. This method was used to characterize cellulose nanofibril (CNF) networks in both dispersed and arrested states, the results of which were compared with coarse-grained Brownian dynamics simulations. At a CNF concentration of 0.2 wt% a transition was observed where, below this concentration tracer diffusion is governed by the increasing macroscopic viscosity of the dispersion. Above 0.2 wt%, the diffusion of small particles (20-40 nm) remains viscosity controlled, while particles (100-500 nm) become trapped in the CNF network. Sedimentation of silica microparticles (1-5 mu m) in CNF dispersions was also determined, showing that sedimentation of larger particles is significantly affected by the presence of CNF. At concentrations of 0.2 wt%, the sedimentation velocity of 5 mu m particles was reduced by 99 % compared to pure water.

The potential for large-scale applications of cellulose nanofibrils (CNFs) is limited by the high water content of the starting material, which leads to high transportation costs and undesirable environmental impact. However, drying of CNFs results in loss of their nanoscopic dimensions leading to deterioration of their unique inherent mechanical properties. Herein, thorough redispersion studies of both fundamental and applied nature have been conducted in order to evaluate the effect of charge, redispersing agent, and drying method. Freeze-dried CNF dispersions were successfully redispersed by either increasing the charge density or adding redispersing agents. The greatest effect on redispersibility was achieved with fractionated LignoBoost lignin as redispersing agent, and this is attributed to steric repulsion during water removal and reduced CNF adhesion. Furthermore, the results unexpectedly show that redispersion is easier when the CNFs are dried in the form of nanopapers. By using this approach, excellent redispersibility was achieved even without a redispersing agent. Nanopapers formed from the redispersed CNFs was found to have essentially the same mechanical properties as those made from never-dried CNFs. Hence, this work suggests solutions for making CNFs viable for large-scale application while maintaining their nanoscale dimensions and their ability to create nanopapers with excellent mechanical properties.

Over the past decades, concerns for the environment have increased and efforts to achieve a sustainable society have intensified. One particular challenge is to replace fossil-based plastics with biodegradable materials produced from renewable resources. Cellulose nanofibril (CNF)-based materials are strong candidates due to their excellent mechanical properties, nano-dimensions and molecular structure, which is suitable for modification. CNFs can be obtained from wood and are elongated, often charged, particles which are usually handled in aqueous dispersions. The colloidal stability is sensitive, and instability results in aggregation or transition to an arrested state. Since the properties of CNF-based materials rely on dispersion of the CNFs, an understanding of the colloidal behaviour is crucial.

This work has focused on the interactions and dynamics of CNFs in different colloidal states. Arrested states of CNFs were studied in detail and it was found that two types of arrested state exist, with different colloidal interactions and mechanisms governing their formation. The dynamics in arrested and dispersed states were studied by tracer diffusion measurements, and it was found that small amounts of CNFs can constitute an excellent stabiliser for other particle dispersions according to a so far unexplored mechanism.

The effects of altering the colloidal interactions using different strategies were also evaluated. The counterions of CNFs were exchanged and the impact on the swelling behaviour was measured. Based on the results, different contributions to the counterion-dependent interactions are discussed. Two strategies for using polymers to alter the interactions were furthermore studied. Polyethylene glycol (PEG) was grafted to CNFs in order to increase the arrested state threshold concentration. PEG, carboxymethyl cellulose and lignin, were also used as additives which improved the redispersion of dried CNF, especially in the case of samples containing lignin.

Under de senaste decennierna har oron för miljön ökat och strävan mot ett hållbart samhälle har intensifierats. En del i utmaningen är att ersätta fossilbaserad plast med bionedbrytbara material producerade från förnyelsebara råvaror. Cellulosa nanofibrill (CNF)-baserade material är starka kandidater för detta tack vare deras nanostorlek, utmärkta mekaniska egenskaper och gynnsamma molekylära struktur för kemisk modifiering. CNF kan utvinnas från träd och är avlånga, ofta laddade, partiklar som oftast hanteras i vattendispersioner. Den kolloidala stabiliteten styrs av subtila växelverkningar, och instabilitet leder till aggregering eller övergång till ett volymsomfattande låst tillstånd. Egenskaperna hos CNF-baserade material påverkas av hur väl dispergerade fibrillerna är, därmed är det viktigt att förstå de faktorer som styr det kolloidala beteendet.

Detta arbete har fokuserat på interaktionerna och dynamiken hos CNF i olika kolloidala tillstånd. Låsta tillstånd av CNF undersöktes och resultaten visade att det finns två typer av låsta tillstånd, med olika interaktioner och mekanismer som ligger bakom hur de genereras. Dynamiken i olika kolloidala tillstånd undersöktes genom att mäta diffusionen av sfäriska partiklar i systemen, och det påvisades att små mängder CNF kan skapa en stabilitet hos andra partiklar via en ny, ännu inte helt utredd, mekanism.

De kolloidala interaktionerna modifierades på olika sätt för att klarlägga effekterna på CNFs associationstillstånd. I en serie försök undersöktes hur ett byte av motjoner till ytladdningarna påverkade växelverkan genom att mäta svällningen av nanopapper. Vidare undersöktes två strategier där polymerer användes för att öka repulsionen mellan CNFs. Polyetylenglykol (PEG) kopplades kovalent till CNF för att förhöja koncentrationen för övergången till ett låst tillstånd. PEG, karboximetylcellulosa och lignin användes även som additiv vilket förbättrade återdispergeringen av torkad CNF, speciellt när lignin användes.

Cellulose nanofibrils (CNFs) assemble into water-resilient materials in the presence of multivalent counter-ions. The essential mechanisms behind these assemblies are ion-ion correlation and specific ion effects. A network model shows that the interfibril attraction indirectly influences the wet modulus by a fourth power relationship to the solidity of the network (E-w proportional to phi(4)). Ions that induce both ion-ion correlation and specific ion effects significantly reduce the swelling of the films, and due to the nonlinear relationship dramatically increase the wet modulus. Herein, this network model is used to explain the elastoplastic behavior of wet films of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized, carboxymethylated, and phosphorylated CNFs in the presence of different counter-ions. The main findings are that the aspect ratio of the CNFs influences the ductility of the assemblies, that the bivalency of phosphorylate ligands probably limits the formation of interfibril complexes with divalent ions, and that a higher charge density increases the friction between fibrils by increasing the short-range attraction from ion-ion correlation and specific ion effects. These findings can be used to rationally design CNF materials for a variety of applications where wet strength, ductility, and transparency are important, such as biomaterials or substrates for bioelectronics.

Ion-induced assemblies of highly anisotropic nanoparticles can be explained by a model consisting of ion-ion correlation and specific ion effects: dispersion interactions, metal-ligand complexes, and local acidic environments. Films of cellulose nanofibrils and montmorillonite clay were treated with different ions, and their subsequent equilibrium swelling in water was related to important parameters of the model in order to investigate the relative importance of the mechanisms. Ion-ion correlation was shown to be the fundamental attraction, supplemented by dispersion interaction for polarizable ions such as Ca2+ and Ba2+, or metal-ligand complexes for ions such as Cu2+, Al3+ and Fe3+. Ions that form strong complexes induce local acidic environments that also contribute to the assembly. These findings are summarized in a comprehensive semi-quantitative model and are important for the design of nanomaterials and for understanding biological systems where specific ions are involved.

EDC-mediated coupling has frequently been utilized to poly(ethylene glycol) functionalize (PEGylate) cellulose-based materials, but no work has previously been reported on the direct N-(3-dimethylaminopropyl)-N-ethylcarbodiimide (EDC)-mediated PEGylation of cellulose nanofibrils (CNF). Herein, we report the first study where CNF has been directly sterically stabilized with amine-terminated PEG employing N-hydroxysuccinimide (NHS)-assisted EDC-coupling. This work has shown that this coupling reaction is highly sensitive to the reaction conditions and purification procedures, and hence an optimized coupling protocol was developed in order to achieve a reaction yield. Elemental analysis of the nitrogen content also showed the successful PEGylation. It was also shown that a surprisingly low PEGylation (1%) is sufficient to significantly improve the colloidal stability of the PEGylated samples, which reached dispersion-arrested-state-transitions at higher concentrations than neat CNF. The colloidal stability was preserved with increasing ionic strength, when comparably long polymer chains were grafted, targeting only 1% PEGylation.

Microcapsules with specific functional properties, related to the capsule wall and core, are highly desired in a number of applications. In this study, hybrid cellulose microcapsules (1.2 +/- 0.4 mu m in diameter) were prepared by nanoengineering the outer walls of precursor capsules. Depending on the preparation route, capsules with different surface roughness (raspberry or broccoli-like), and thereby different wetting properties, could be obtained. The tunable surface roughness was achieved as a result of the chemical and structural properties of the outer wall of a precursor capsule, which combined with a new processing route allowed in-situ formation of silica nanoparticles (30-40 nm or 70 nm in diameter). By coating glass slides with "broccoli-like" microcapsules (30-40 nm silica nanoparticles), static contact angles above 150 degrees and roll-off angles below 6 degrees were obtained for both water and low surface-tension oil (hexadecane), rendering the substrate superamphiphobic. As a comparison, coatings from raspberry-like capsules were only strongly oleophobic and hydrophobic. The liquid-core of the capsules opens great opportunities to incorporate different functionalities and here hydrophobic superparamagnetic nanoparticles (SPIONs) were encapsulated. As a result, magnetic broccoli-like microcapsules formed an excellent superamphiphobic coating-layer on a curved geometry by simply applying an external magnetic field.