Fractal Dimension of Cellulose Powders Analyzed by Multilayer BET Adsorption of Water and Nitrogen

Sorption: A Statistical Thermodynamic Fluctuation Theory

Can the sorption mechanism be proven by fitting an isotherm model to an experiment? Such a question arises because (i) multiple isotherm models, with different assumptions on sorption mechanisms, often fit an experimental isotherm equally well, (ii) some isotherm models [such as Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de Boer (GAB)] fit experimental isotherms that do not satisfy the underlying assumptions of the model, and (iii) some isotherms (such as Oswin and Peleg) are empirical equations that do not have a well-defined basis on sorption mechanisms. To overcome these difficulties, we propose a universal route of elucidating the sorption mechanism directly from an experimental isotherm, without an isotherm model, based on the statistical thermodynamic fluctuation theory. We have shown that how sorbate-sorbate interaction depends on activity is the key to understanding the sorption mechanism. Without assuming adsorption sites and planar layers, an isotherm can be derived, which contains the Langmuir, BET, and GAB models as its special cases. We have constructed a universal approach applicable to adsorption and absorption, solid and liquid sorbents, and vapor and liquid sorbates and demonstrated its efficacy using the humidity sorption isotherm of sucrose from both the solid and liquid sides.

Fractal Dimensions of Cell Wall in Growing Cotton Fibers

In this research, fractal properties of a cell wall in growing cotton fibers were studied. It was found that dependences of specific pore volume (P) and apparent density (ρ) on the scale factor, F = H/h, can be expressed by power-law equations: P = Po F(Dv−E) and ρ = ρo F(E−Dρ), where h is minimum thickness of the microfibrilar network in the primary cell wall, H is total thickness of cell wall in growing cotton, Dv = 2.556 and Dρ = 2.988 are fractal dimensions. From the obtained results it follows that microfibrilar network of the primary cell wall in immature fibers is loose and disordered, and therefore it has an increased pore volume (Po = 0.037 cm3/g) and low density (ρo = 1.47 g/cm3). With enhance days post anthesis of growing cotton fibers, the wall thickness and density increase, while the pore volume decreases, until dense structure of completely mature fibers is formed with maximum density (1.54 g/cm3) and minimum pore volume (0.006 cm3/g). The fractal dimension for specific pore volume, Dv = 2.556, evidences the mixed surface-volume sorption mechanism of sorbate vapor in the pores. On the other hand, the fractal dimension for apparent density, Dρ = 2.988, is very close to Euclidean volume dimension, E = 3, for the three-dimensional space.

Cladophora Cellulose: Unique Biopolymer Nanofibrils for Emerging Energy, Environmental, and Life Science Applications

Because of its natural abundance, hierarchical fibrous structure, mechanical flexibility, potential for chemical modification, biocompatibility, renewability, and abundance, cellulose is one of the most promising green materials for a bio-based future and sustainable economy. Cellulose derived from wood or bacteria has dominated the industrial cellulose market and has been developed to produce a number of advanced materials for applications in energy storage, environmental, and biotechnology areas. However, Cladophora cellulose (CC) extracted from green algae has unprecedented advantages over those celluloses because of its high crystallinity (>95%), low moisture adsorption capacity, excellent solution processability, high porosity in the mesoporous range, and associated high specific surface area. The unique physical and chemical properties of CC can add new features to and enhance the performance of nanocellulose-based materials, and these attributes have attracted a great deal of research interest over the past decade. This Account summarizes our recent research on the preparation, characterization, functionalization, and versatile applications of CC. Our aim is to provide a comprehensive overview of the uniqueness of CC with respect to material structure, properties, and emerging applications. We discuss the potential of CC in energy storage, environmental science, and life science, with emphasis on applications in which its properties are superior to those of other nanocellulose forms. Specifically, we discuss the production of the first-ever paper battery based on CC. This battery has initiated a rising interest in the development of sustainable paper-based energy storage devices, where cellulose is used as a combined building block and binder for paper electrodes of various types in combination with carbon, conducting polymers, and other electroactive materials. High-active-mass and high-mass-loading paper electrodes can be made in which the CC acts as a high-surface-area and porous substrate while a thin layer of electroactive material is coated on individual nanofibrils. We have shown that CC membranes can be used directly as battery separators because of their low moisture content, high mesoporosity, high thermal stability, and good electrolyte wettability. The safety, stability, and capacity of lithium-ion batteries can be enhanced simply by using CC-based separators. Moreover, the high chemical modifiability and adjustable porosity of dried CC papers allow them to be used as advanced membranes for environmental science (water and air purification, pollutant adsorption) and life science (virus isolation, protein recovery, hemodialysis, DNA extraction, bioactive substrates). Finally, we outline some concluding perspectives on the challenges and future directions of CC research with the aim to open up yet unexplored fields of use for this interesting material.

Study of Fractal Dimensions of Microcrystalline Cellulose Obtained by the Spray-Drying Method

In this research, the fractal structure of beads of different sizes obtained by the spray-drying of aqueous dispersions of microcrystalline cellulose (MCC) was studied. These beads were formed as a result of the aggregation of rod-shaped cellulose nanocrystalline particles (CNP). It was found that increasing the average radius (R) of the formed MCC beads resulted in increased specific pore volume (P) and reduced apparent density (ρ). The dependences of P and ρ on the scale factor (R/r) can be expressed by power-law equations: P = Po (R/r)E−Dp and ρ = d (R/r)Dd−E, where the fractal dimensions Dp = 2.887 and Dd = 2.986 are close to the Euclidean dimension E = 3 for three-dimensional space; r = 3 nm is the radius of the cellulose nanocrystalline particles, Po = 0.03 cm3/g is the specific pore volume, and d = 1.585 g/cm3 is the true density (specific gravity) of the CNP, respectively. With the increase in the size of the formed MCC beads, the order in the packing of the beads was distorted, conforming to theory of the diffusion-limited aggregation process.

Impact of Two-Dimensional Particle Size Distribution on Estimation of Water Vapor Diffusivity in Micrometric Size Cellulose Particles

This work aims at assessing the impact of two-dimensional particle size distribution (2D-PSD) on the identification of water vapor diffusivity in micrometric size cellulose particles displaying a size aspect ratio lower than 2 and a cylindrical shape. First, different methodologies to obtain the two-dimensional (2D) particle size distribution (diameter versus length) were compared, based on image analysis. Then, experimental sorption kinetics were obtained by using a quartz crystal microbalance (QCM) coupled with a water vapor adsorption system. Diffusivity values were estimated when considering either the 2D-PSD or global descriptors, such as the mean or median diameter and length of particles. Results revealed that the use of an analytical approach when considering the 2D mean-PSD or the median-PSD was the most accurate way to get diffusivity values at the scale of particles in a polydisperse sample of cellulose particles. Following this approach, a water vapor apparent diffusivity of 3.1 × 10⁻¹² ± 2.3 × 10⁻¹² m²·s⁻¹ was found for the considered cellulose sample. Neglecting PSD in diffusivity estimation led to an underestimation of a factor of 2. This procedure could be extended for all the polydisperse samples in order to have an accurate estimation of water vapor diffusivity at the scale of single particles.

Real-time adsorption and action of expansin on cellulose

BACKGROUND

Biological pretreatment is an environmentally safe method for disrupting recalcitrant structures of lignocellulose and thereby improving their hydrolysis efficiency. Expansin and expansin-like proteins act synergistically with cellulases during hydrolysis. A systematic analysis of the adsorption behavior and mechanism of action of expansin family proteins can provide a basis for the development of highly efficient pretreatment methods for cellulosic substrates using expansins.

RESULTS

Adsorption of Bacillus subtilis expansin (BsEXLX1) onto cellulose film under different conditions was monitored in real time using a quartz crystal microbalance with dissipation. A model was established to describe the adsorption of BsEXLX1 onto the film. High temperatures increased the initial adsorption rate while reducing the maximum amount of BsEXLX1 adsorbed onto the cellulose. Non-ionic surfactants (polyethylene glycol 4000 and Tween 80) at low concentrations enhanced BsEXLX1 adsorption; whereas, high concentrations had the opposite effect. However, sodium dodecyl sulfate inhibited adsorption at both low and high concentrations. We also investigated the structural changes of cellulose upon BsEXLX1 adsorption and found that BsEXLX1 adsorption decreased the crystallinity index, disrupted hydrogen bonding, and increased the surface area of cellulose, indicating greater accessibility of the substrate to the protein.

CONCLUSIONS

These results increase our understanding of the interaction between expansin and cellulose, and provide evidence for expansin treatment as a promising strategy to enhance enzymatic hydrolysis of lignocellulose.

Effect of Water Vapor Adsorption on Electrical Properties of Carbon Nanotube/Nanocrystalline Cellulose Composites

It has been long known that the electrical properties of cellulose are greatly influenced by adsorption of water vapor. Incorporating conductive nanofillers in a cellulose matrix is an example of an approach to tailor their characteristics for use in electronics and sensing devices. In this work, we introduce two new nanocomposites comprising carbon nanotubes (CNTs) and conventional or electrosterically stabilized nanocrystalline celluloses matrices. While conventional nanocrystalline cellulose (NCC) consists of a rigid crystalline backbone, electrosterically stabilized cellulose (ENCC) is composed of a rigid crystalline backbone with carboxylated polymers protruding from both ends. By tuning CNT loading, we can tailor a CNT/NCC composite with minimal electrical sensitivity to the ambient relative humidity, despite the fact that the composite has a high moisture uptake. The expected decrease in CNT conductivity upon water vapor adsorption, due to electron donation, is counterbalanced by an increase in the conductivity of NCC due to proton hopping at an optimum CNT loading (1-2%). Contrary to the CNT/NCC composite, a CNT/ENCC composite at 1% CNT loading shows insulating behavior for relative humidities up to 75%, after which the composite becomes conductive. This interesting behavior can be ascribed to the low moisture uptake of ENCC at low and moderate relative humidities due to the limited number of hydroxyl groups and hydrogen bond formation between carboxyl groups on ENCC, which endow ENCC with limited water molecule adsorption sites.

Transition from Bioinert to Bioactive Material by Tailoring the Biological Cell Response to Carboxylated Nanocellulose

This work presents an insight into the relationship between cell response and physicochemical properties of Cladophora cellulose (CC) by investigating the effect of CC functional group density on the response of model cell lines. CC was carboxylated by electrochemical TEMPO-mediated oxidation. By varying the amount of charge passed through the electrolysis setup, CC materials with different degrees of oxidation were obtained. The effect of carboxyl group density on the material's physicochemical properties was investigated together with the response of human dermal fibroblasts (hDF) and human osteoblastic cells (Saos-2) to the carboxylated CC films. The introduction of carboxyl groups resulted in CC films with decreased specific surface area and smaller total pore volume compared with the unmodified CC (u-CC). While u-CC films presented a porous network of randomly oriented fibers, a compact and aligned fiber pattern was depicted for the carboxylated-CC films. The decrease in surface area and total pore volume, and the orientation and aggregation of the fibers tended to augment parallel to the increase in the carboxyl group density. hDF and Saos-2 cells presented poor cell adhesion and spreading on u-CC, which gradually increased for the carboxylated CC as the degree of oxidation increased. It was found that a threshold value in carboxyl group density needs be reached to obtain a carboxylated-CC film with cytocompatibility comparable to commercial tissue culture material. Hence, this study demonstrates that a normally bioinert nanomaterial can be rendered bioactive by carefully tuning the density of charged groups on the material surface, a finding that not only may contribute to the fundamental understanding of biointerface phenomena, but also to the development of bioinert/bioactive materials.

Characterization of dielectric properties of nanocellulose from wood and algae for electrical insulator applications

Cellulose is one of the oldest electrically insulating materials used in oil-filled high-power transformers and cables. However, reports on the dielectric properties of nanocellulose for electrical insulator applications are scarce. The aim of this study was to characterize the dielectric properties of two nanocellulose types from wood, viz., nanofibrillated cellulose (NFC), and algae, viz., Cladophora cellulose, for electrical insulator applications. The cellulose materials were characterized with X-ray diffraction, nitrogen gas and moisture sorption isotherms, helium pycnometry, mechanical testing, and dielectric spectroscopy at various relative humidities. The algae nanocellulose sample was more crystalline and had a lower moisture sorption capacity at low and moderate relative humidities, compared to NFC. On the other hand, it was much more porous, which resulted in lower strength and higher dielectric loss than for NFC. It is concluded that the solid-state properties of nanocellulose may have a substantial impact on the dielectric properties of electrical insulator applications.

The hygroscopic power of amorphous cellulose: a modeling study

The relationship between cellulose and water was studied by building dense amorphous cellulose models and subjecting them to increasing moisture contents. When starting from completely dry cellulose, the first diffused water molecules were essentially individualized and hydrogen bonded exclusively to the O6 and O2 hydroxyl groups of cellulose. Upon continued hydration increase, the hydroxyl at O3 and then the acetal oxygens of cellulose also started to attract the upcoming water molecules, which were no longer isolated. They progressively became aggregated, first into clusters and then at high hydration content, into continuous capillary channels. A benefit of this study was to allow predicting a number of physical parameters of amorphous cellulose and their variation under hydration. With some parameters, the calculated values matched rather well the experimental literature determinations. This was the case for the hydration dependence of Tg, the stereoselectivity of the cellulose oxygen atoms for water molecules, together with the diffusion coefficients of water into cellulose. An estimate of the hygro-expansion of amorphous cellulose was provided.

A size-exclusion nanocellulose filter paper for virus removal

This is the first time a 100% natural, unmodified nanofibrous polymer-based membrane is demonstrated capable of removing viruses solely based on the size-exclusion principle, with a log10 reduction value (LRV) ≥ 6.3 as limited by the assay lower detection limit and the feed virus titre, thereby matching the performance of industrial synthetic polymer virus removal filters.

Study of enzymatic digestion of cellulose by small angle neutron scattering

Small angle neutron scattering (SANS) was used to study the structure of Avicel (FD100) microcrystalline cellulose during enzymatic digestion. Digestions were performed in either of two modes: a static, quiescent mode or a dynamic mode using a stirred suspension recycled through a flow cell. The scattering pattern for as-received Avicel in D(2)O buffer is comprised of a low Q power law region resulting from the surface fractal character of the microcrystalline fibers and a high Q roll-off due to scattering from water-filled nanopores with radii approximately 20 A. For digestions in the dynamic mode the high Q roll-off decreased in magnitude within approximately 1 h after addition of enzymes, whereas in the static digestions no change was observed in the high Q roll-off, even after 60 h. These results indicate that only with significant agitation does enzyme digestion affect the structure of the nanopores.

Stress-transfer in anisotropic and environmentally adaptive cellulose whisker nanocomposites

Quantitative insights into the stress-transfer mechanisms that determine the mechanical properties of tunicate cellulose whisker/poly(vinyl acetate) nanocomposites were gained by Raman spectroscopy. The extent of stress-transfer is influenced by local orientation (or anisotropy) of the whiskers, which in turn is governed by the processing conditions used to fabricate the nanocomposites. Solution-cast materials display no microscopic anisotropy, while samples that were cast and subsequently compression molded contain both isotropic regions as well as domains of locally oriented whiskers. Polarized optical microscopy showed these regions to have dimensions in the hundreds of mum. Polarized Raman spectroscopy of the 1095 cm(-1) Raman band, associated with C-O ring stretching of the cellulose backbone, was used to quantify the local orientation of the cellulose whiskers. Clear and discernible shifts of this Raman band upon uniaxial deformation of nanocomposite films were further used to determine the level of stress experienced by the cellulose whiskers, ultimately reflecting the levels of stress-transfer predominantly between the poly(vinyl acetate) matrix and the tunicate whiskers, but also between the whiskers within the network. In the isotropic regions, where whiskers form a percolating network, the observed Raman shift rate with respect to strain is smaller than in the regions where the whiskers are uniaxially orientated. The Raman shift is strongly affected by the presence of water, leading to a lack of stress-transfer when the samples are fully hydrated, which is clearly detected by the Raman technique. Heating of the nanocomposites above the glass transition temperature of the poly(vinyl acetate) matrix also reduces the stress experienced by the individual whiskers.

A novel high specific surface area conducting paper material composed of polypyrrole and Cladophora cellulose

We present a novel conducting polypyrrole-based composite material, obtained by polymerization of pyrrole in the presence of iron(III) chloride on a cellulose substrate derived from the environmentally polluting Cladophora sp. algae. The material, which was doped with chloride ions, was molded into paper sheets and characterized using scanning and transmission electron microscopy, N 2 gas adsorption analysis, cyclic voltammetry, chronoamperometry and conductivity measurements at varying relative humidities. The specific surface area of the composite was found to be 57 m (2)/g and the fibrous structure of the Cladophora cellulose remained intact even after a 50 nm thick layer of polypyrrole had been coated on the cellulose fibers. The composite could be repeatedly used for electrochemically controlled extraction and desorption of chloride and an ion exchanging capacity of 370 C per g of composite was obtained as a result of the high surface area of the cellulose substrate. The influence of the oxidation and reduction potentials on the chloride ion exchange capacity and the nucleation of delocalized positive charges, forming conductive paths in the polypyrrole film, was also investigated. The creation of conductive paths during oxidation followed an effective medium rather than a percolative behavior, indicating that some conduction paths survive the polymer reduction steps. The present high surface area material should be well-suited for use in, e.g., electrochemically controlled ion exchange or separation devices, as well as sensors based on the fact that the material is compact, light, mechanically stable, and moldable into paper sheets.

Mesopore Structure of Microcrystalline Cellulose Tablets Characterized by Nitrogen Adsorption and SEM: The Influence on Water-Induced Ionic Conduction

Tablets of microcrystalline cellulose were formed at different compaction pressures and physical properties, such as pore size distribution, surface area, and pore surface fractality, were extracted from N2 adsorption isotherms. These properties were compared to previously published data on the water-induced ionic conductivity of the tablets. The conduction process was shown to follow a percolation model with a percolation exponent of 2 and a porosity percolation threshold of approximately 0.1. The critical pore diameter for facilitated charge transport was shown to be in the 5-20 nm range. When the network of pores with a diameter in this interval is reduced to the point where it no longer forms a continuous passageway throughout the compact, the conduction process is dominated by charge transport on the surfaces of individual microfibrils mainly situated in the bulk of fibril aggregates. A fractal analysis of nitrogen adsorption isotherms showed that the dominant interface forces during adsorption is attributed to surface tensions between the gas and the adsorbed liquid phase. The extracted fractal dimension of the analyzed pore surfaces remained unaffected by the densification process at low compaction pressures (< approximately 200 MPa). At increased densification, however, pore-surface structures smaller than approximately 100 nm become smoother as the fractal dimension decreases from approximately 2.5 at high porosities to approximately 2.3 for the densest tablets under study.

Influence of cellulose powder structure on moisture-induced degradation of acetylsalicylic acid

The stability of crystalline acetylsalicylic acid (ASA) powder in binary mixtures with cellulose powders was investigated to reveal information about the influence of the cellulose structural properties on the moisture-induced ASA degradation. Different cellulose powder samples were manufactured and characterized by X-ray diffraction and N2 BET gas adsorption. The degradation patterns in ASA/cellulose mixtures were monitored as a function of salicylic acid increase versus time under various relative humidity conditions at 50 degrees C. The crystallinity index of cellulose samples varied between approximately 49 and 95%. The results indicated that cellulose powder with the lowest crystallinity index exhibited lower degradation rates than the samples with the higher crystallinity index. It should be noted that higher ASA degradation rates were observed in the samples with comparably lower moisture contents. This effect was most pronounced in the 1:3 (w/w), ASA/cellulose mixtures, whereas in 3:1 (w/w), ASA/cellulose mixtures the effect was less obvious. The findings emphasise the importance of cellulose structural organisation when governing the moisture's partition between cellulose and ASA during the hydrolytic degradation.

Viscoelastic characterization of compacted pharmaceutical excipient materials by analysis of frequency-dependent mechanical relaxation processes

A newly developed method for determining the frequency-dependent complex Young's modulus was employed to analyze the mechanical response of compacted microcrystalline cellulose, sorbitol, ethyl cellulose and starch for frequencies up to 20 kHz. A Debye-like relaxation was observed in all the studied pharmaceutical excipient materials and a comparison with corresponding dielectric spectroscopy data was made. The location in frequency of the relaxation peak was shown to correlate to the measured tensile strength of the tablets, and the relaxation was interpreted as the vibrational response of the interparticle hydrogen and van der Waals bindings in the tablets. Further, the measured relaxation strength, holding information about the energy loss involved in the relaxation processes, showed that the weakest material in terms of tensile strength, starch, is the material among the four tested ones that is able to absorb the most energy within its structure when exposed to external perturbations inducing vibrations in the studied frequency range. The results indicate that mechanical relaxation analysis performed over relatively broad frequency ranges should be useful for predicting material properties of importance for the functionality of a material in applications such as, e.g., drug delivery, drug storage and handling, and also for clarifying the origin of hitherto unexplained molecular processes.

Characterization of the Drug Release Process by Investigation of Its Temperature Dependence

Temperature-dependent drug release from disintegrating tablets made of NaCl-containing agglomerated micronized cellulose (AMC) granules has been studied to characterize the release process. Release measurements on tablets compacted at three different compaction pressures; 50, 100, and 200 MPa, were performed at seven different temperatures; 6, 23, 33, 43, 50, 55, and 63 degrees C using the recently developed alternating ionic current method. Tablets compacted at different compaction pressures showed similar release rates. The release process was found to be diffusion-controlled, and the activation energy of the diffusion coefficient was comparable to that obtained for diffusion in pure water. The results show that the AMC granules in contact with water swell to a size and shape that is only slightly affected by their compaction history and the ion diffusion operates mainly within liquid-filled pores within the AMC granules. By using the temperature dependence of the release process, it was possible to reach this conclusion without any assumptions concerning the number and radii of the granules into which the tablets disintegrated. Further, the magnitude of the effective diffusion coefficient was found to be approximately 7.5 x 10(-10) cm(2)/s, which is approximately four orders of magnitude lower than for unhindered diffusion of Na(+) and Cl(-) in water but similar to the diffusion coefficient for protons and OH(-) ions in microcrystalline cellulose.