Influence of the Particle Concentration and Marangoni Flow on the Formation of Cellulose Nanocrystal Films

Cellulose nanocrystals (CNCs), ribbonlike crystalline nanoparticles, are a biobased material that can be a great alternative to obtaining films with tunable optical properties. Iridescent and light-diffracting films can be readily obtained via the drying of a suspension of these cellulose nanocrystals. The characteristics of the particle deposition process together with the self-assembly in the precluding suspension has a direct effect on the optical properties of the obtained films. Particle deposition onto a substrate is affected by the flow dynamics inside sessile droplets and usually yields a ring-shaped deposition pattern commonly referred to as the coffee-ring effect. We set out to measure and describe the drying kinetics under different conditions. We found that the Marangoni flow inside the droplet was too small to counteract the capillary flow that deposits CNCs at the edges, resulting in the coffee-ring effect, irrespective of the atmospheric humidity. By varying the amount of ethanol in the atmosphere, we were able to find a balance between (1) colloidal stability in the droplet, which is reduced by ethanol diffusion into the droplet, and (2) increasing Marangoni flow relative to capillary flow inside the droplet by changing the droplet surface tension. We could thus make iridescent films with a uniform thickness

Cellulose Nanocrystals Grafted with Polystyrene Chains through Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP)

This paper reports the synthesis of cellulose nanocrystals grafted by polystyrene chains via surface-initiated ATRP. Naturally occurring cellulose was first hydrolyzed to obtain cellulose nanocrystals. Their surface was then chemically modified using 2-bromoisobutyryl bromide to introduce initiating sites for ATRP. A varying extent of surface modification was achieved by changing reaction conditions. Further initiation of styrene polymerization from these modified nanocrystals with a CuBr/PMDETA (N,N,N′,N′,N′′-pentamethyldiethylenetriamine) catalytic system and in the presence of a sacrificial initiator produced polysaccharide nanocrystals grafted by polystyrene chains. A range of nanocrystals-g-polystyrene with different graft lengths (theoretical polymerization degree = 27−171) was synthesized through this method and characterized by elemental analysis, XPS, FT-IR, TEM, and contact angle measurements. We are thus able to produce cellulose nanoparticles with varying grafting densities (by altering extent of initiator surface modification) and varying polymer brush length (through polymerization control). The nanocrystals-g-polystyrene (NC-g-PS) particles were tested for their capacity to absorb 1,2,4-trichlorobenzene from water. The results obtained show that they can absorb the equivalent of 50% of their weight in pollutant compared to 30 wt % adsorption for nonmodified nanocrystals, while also displaying faster absorption kinetics

Unravelling the Mechanism of Chitosan-Driven Flocculation of Microalgae in Seawater as a Function of pH

Chitosan is a nontoxic biobased polymer, attractive for the flocculation-based harvesting of microalgae. While it is generally effective to harvest algae in freshwater medium, its performance in seawater has been unpredictable. This study determined the optimal conditions for flocculation of the marine microalgae <i>Nannochloropsis oculata</i> using chitosan. Whereas in freshwater a low pH (<7.5) is required to protonate the amine groups and to activate the chitosan flocculation activity toward charge-neutralization and bridging, flocculation of <i>Nannochloropsis</i> in seawater only occurred at a high pH (>7.5). The dosage of chitosan required for flocculation of <i>Nannochloropsis</i> in seawater (75 mg/L) was higher than the reported dose to flocculate the freshwater microalgae (±10 mg/L) reported in the literature. Experiments carried out in synthetic seawater with modified magnesium concentration indicated that flocculation induced by chitosan at varying pH was not related to precipitation of magnesium hydroxides (so-called “autoflocculation”). Chitosan flocculation at high pH in seawater medium was found to be caused by precipitation of chitosan due to (partial) deprotonation of the amine groups, resulting in a sudden network formation that induces flocculation by a sweeping mechanism. Visual observations and viscosity measurements indeed confirmed the occurrence of precipitation of chitosan at pH > 7.5

Effect of Gelation on the Colloidal Deposition of Cellulose Nanocrystal Films

One of the most important aspects in controlling colloidal deposition is manipulating the homogeneity of the deposit by avoiding the coffee-ring effect caused by capillary flow inside the droplet during drying. After our previous work where we achieved homogeneous deposition of cellulose nanocrystals (CNCs) from a colloidal suspension by reinforcing Marangoni flow over the internal capillary flow (Gençer et al. <i>Langmuir</i> <b>2017</b>, <i>33</i> (1), 228–234), we now set out to reduce the importance of capillary flow inside a drying droplet by inducing gelation. In this paper, we discuss the effect of gelation on the deposition pattern and on the self-assembly of CNCs during droplet drying. CNC films were obtained by drop casting CNC suspensions containing NaCl and CaCl₂ salts. A mixed methodology using rheological and depolarized dynamic light scattering was applied to understand the colloidal behavior of the CNCs. In addition, analysis of the mixture’s surface tension, viscosity, and yield stress of the suspensions were used to gain deeper insights into the deposition process. Finally, the understanding of the gelation behavior in the drying droplet was used to exert control over the deposit where the coffee-ring deposit can be converted to a dome-shaped deposit

Effect of Gelation on the Colloidal Deposition of Cellulose Nanocrystal Films

One of the most important aspects in controlling colloidal deposition is manipulating the homogeneity of the deposit by avoiding the coffee-ring effect caused by capillary flow inside the droplet during drying. After our previous work where we achieved homogeneous deposition of cellulose nanocrystals (CNCs) from a colloidal suspension by reinforcing Marangoni flow over the internal capillary flow (Gençer et al. <i>Langmuir</i> <b>2017</b>, <i>33</i> (1), 228–234), we now set out to reduce the importance of capillary flow inside a drying droplet by inducing gelation. In this paper, we discuss the effect of gelation on the deposition pattern and on the self-assembly of CNCs during droplet drying. CNC films were obtained by drop casting CNC suspensions containing NaCl and CaCl₂ salts. A mixed methodology using rheological and depolarized dynamic light scattering was applied to understand the colloidal behavior of the CNCs. In addition, analysis of the mixture’s surface tension, viscosity, and yield stress of the suspensions were used to gain deeper insights into the deposition process. Finally, the understanding of the gelation behavior in the drying droplet was used to exert control over the deposit where the coffee-ring deposit can be converted to a dome-shaped deposit

Green One-Step Synthesis of Catalytically Active Palladium Nanoparticles Supported on Cellulose Nanocrystals

Palladium nanoparticles (PdNPs) supported on cellulose nanocrystals (CNXL) were synthesized in a single step from Pd­(hexafluoroacetylacetonate)₂ (Pd­(hfac)₂) in subcritical and supercritical carbon dioxide. CNXLs acted as both the reducing agent and support material for the obtained nanoparticles. Dry Pd nanoparticles supported on the cellulose nanocrystals (PdNP@CNXL) were obtained by simply venting the CO₂ and were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). The results show that the Pd nanoparticle diameters varied between 6 and 13 nm with varying pressure (240–2200 psi), reaction time (2–17 h), and weight ratio of the precursor Pd­(hfac)₂ to CNXL (1–4% w/w). Particles with diameters above 13 nm appeared not to remain attached to the CNXL surface. Reaction conditions also affected the Pd loading in the final PdNP@CNXL composite. Finally, the PdNP@CNXL composites were shown to be effective catalysts for carbon–carbon bond formation in the Mizoroki–Heck cross-coupling reaction, in line with other reports

Thermodynamic Study of Ion-Driven Aggregation of Cellulose Nanocrystals

The thermodynamics of interactions between cations of the second group of the periodic table and differently negatively charged cellulose nanocrystals was investigated using isothermal titration calorimetry (ITC). The interaction of cations with the negatively charged CNCs was found to be endothermic and driven by an increase in entropy upon adsorption of the ions, due to an increase in degrees of freedom gained by the surface bound water upon ion adsorption. The effect was pH-dependent, showing an increase in enthalpy for cellulose suspensions at near-neutral pH (6.5) when compared to acidic pH (2). Sulfated cellulose nanoparticles were found to readily interact with divalent ions at both pH levels. The adsorption on carboxylate nanocrystals was found to be pH dependent, showing that the carboxylic group needs to be in the deprotonated form to interact with divalent ions. For the combined system (sulfate and carboxylate present at the same time), at neutral pH, the adsorption enthalpy was higher than the value obtained from cellulose nanocrystals containing a single functional group, while the association constant was higher due to an increased favorable entropic contribution. The higher entropic contribution indicates a more restricted surface-bound water layer when multiple functionalities are present. The stoichiometric number n was nearly constant for all systems, showing that the adsorption depends almost completely on the ion valency and on the amount of ionic groups on the CNC surface, independent of the type of functional group on the CNC surface as long as it is deprotonated. In addition, we showed that the reduction in Gibbs free energy drives the ionotropic gelation of nanocellulose suspensions, and we show that ITC is able to detect gel formation at the same time as determining the critical association concentration

Ferrocene-Decorated Nanocrystalline Cellulose with Charge Carrier Mobility

Ferrocene-decorated cellulose nanowhiskers were prepared by the grafting of ethynylferrocene onto azide functionalized cotton-derived cellulose nanowhiskers using azide–alkyne cycloaddition. Successful surface modification and retention of the crystalline morphology of the nanocrystals was confirmed by elemental analysis, inductively coupled plasma-atomic emission spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The coverage with ferrocenyl is high (approximately 1.14 × 10^–3 mol g^–1 or 4.6 × 10¹³ mol cm^–2 corresponding to a specific area of 61 Ų per ferrocene). Cyclic voltammetry measurements of films formed by deposition of ferrocene-decorated nanowhiskers showed that this small spacing of redox centers along the nanowhisker surface allowed conduction hopping of electrons. The apparent diffusion coefficient for electron (or hole) hopping via Fe­(III/II) surface sites is estimated as <i>D</i>_app = 10^–19 m²s^–1 via impedance methods, a value significantly less than nonsolvated ferrocene polymers, which would be expected as the 1,2,3-triazole ring forms a rigid linker tethering the ferrocene to the nanowhisker surface. In part, this is believed to be also due to “bottleneck” diffusion of charges across contact points where individual cellulose nanowhiskers contact each other. However, the charge-communication across the nanocrystal surface opens up the potential for use of cellulose nanocrystals as a charge percolation template for the preparation of conducting films via covalent surface modification (with applications similar to those using adsorbed conducting polymers), for use in bioelectrochemical devices to gently transfer and remove electrons without the need for a solution-soluble redox mediator, or for the fabrication of three-dimensional self-assembled conducting networks

Toward Improved Understanding of the Interactions between Poorly Soluble Drugs and Cellulose Nanofibers

Cellulose nanofibers (CNFs) have interesting physicochemical and colloidal properties that have been recently exploited in novel drug-delivery systems for tailored release of poorly soluble drugs. The morphology and release kinetics of such drug-delivery systems heavily relied on the drug–CNF interactions; however, in-depth understanding of the interactions was lacking. Herein, the interactions between a poorly soluble model drug molecule, furosemide, and cationic cellulose nanofibers with two different degrees of substitution are studied by sorption experiments, Fourier transform infrared spectroscopy, and molecular dynamics (MD) simulation. Both MD simulations and experimental results confirmed the spontaneous sorption of drug onto CNF. Simulations further showed that adsorption occurred by the flat aryl ring of furosemide. The spontaneous sorption was commensurate with large entropy gains as a result of release of surface-bound water. Association between furosemide molecules furthermore enabled surface precipitation as indicated by both simulations and experiments. Finally, sorption was also found not to be driven by charge neutralization, between positive CNF surface charges and the furosemide negative charge, so that surface area is the single most important parameter determining the amount of sorbed drug. An optimized CNF–furosemide drug-delivery vehicle thus needs to have a maximized specific surface area irrespective of the surface charge with which it is achieved. The findings also provide important insights into the design principles of CNF-based filters suitable for removal of poorly soluble drugs from wastewater