Hydrodynamics of kraft lignins

Mass Transport of Lignin in Confined Pores

A crucial step in the chemical delignification of wood is the transport of lignin fragments into free liquor; this step is believed to be the rate-limiting step. This study has investigated the diffusion of kraft lignin molecules through model cellulose membranes of various pore sizes (1–200 nm) by diffusion cells, where the lignin molecules diffuse from donor to acceptor cells through a membrane, where diffusion rate increases by pore size. UV–vis spectra of the donor solutions showed greater absorbance at higher wavelengths (~450 nm), which was probably induced by scattering due to presence of large molecules/clusters, while acceptor samples passed through small pore membranes did not. The UV–vis spectra of acceptor solutions show a characteristic peak at around 350 nm, which corresponds to ionized conjugated molecules: indicating that a chemical fractionation has occurred. Size exclusion chromatography (SEC) showed a difference in the molecular weight (Mw) distribution between lignin from the donor and acceptor chambers. The results show that small pore sizes enable the diffusion of small individual molecules and hinder the transport of large lignin molecules or possible lignin clusters. This study provides more detail in understanding the mass transfer events of pulping processes.

Revisiting the Molar Mass and Conformation of Derivatized Fractionated Softwood Kraft Lignin

The limited utilization of reliable tools and standards for determination of the softwood kraft lignin molar mass and the corresponding molecular conformation hampers elucidation of the structure-property relationships of lignin. At issue, conventional size exclusion chromatography (SEC) is unable to robustly measure the molar mass because of a lack of calibration standards with a similar structure to lignin. In the present work, the determination of the absolute molar mass of acetylated technical lignin was revisited utilizing SEC combined with multi-angle light scattering with a band pass filter to suppress the fluorescence. Fractionated lignin isolated using sequential techniques of solvent and membrane methods was used to enhance the clarity of light-scattering profiles by narrowing the molar mass distribution of lignin fractions. Further information on the molecular conformation of derivatized samples was studied utilizing a differential viscometer, and chemical structures were identified by NMR spectroscopy analysis. Through the help of fractionation, intrinsic viscosity values were determined for the different fractions as a function of molecular weight cut-off membranes. The derivatized acetone-soluble lignin was found to possess a lower molecular weight and an extremely compact structure relative to the derivatized acetone-insoluble fraction based on a significantly lower "α" value in the Mark-Houwink-Sakurada plot (0.15 acetone-soluble vs 0.33 acetone-insoluble). The differences in geometry were supported by the linkage analysis from NMR showing the acetone-soluble part containing fewer native linkages. In both of these examples, kraft lignin behaved like a solid sphere, limiting the ability to provide entanglements between molecular chains. From this standpoint, macroscopic properties of lignin are justified with this knowledge of a dense and extremely compact structure.

Nano-Structured Lignin as Green Antioxidant and UV Shielding Ingredient for Sunscreen Applications

Green, biocompatible, and biodegradable antioxidants represent a milestone in cosmetic and cosmeceutical applications. Lignin is the most abundant polyphenol in nature, recovered as a low-cost waste from the pulp and paper industry and biorefinery. This polymer is characterized by beneficial physical and chemical properties which are improved at the nanoscale level due to the emergence of antioxidant and UV shielding activities. Here we review the use of lignin nanoparticles in cosmetic and cosmeceutical applications, focusing on sunscreen and antiaging formulations. Advances in the technology for the preparation of lignin nanoparticles are described highlighting structure activity relationships.

Polarity of Cationic Lignin Polymers: Physicochemical Behavior in Aqueous Solutions and Suspensions

The structure of cationic monomers can significantly impact the polarity of lignin after polymerization. Cationic hydrolysis lignin (CHL) polymers were produced by polymerizing hydrolysis lignin (HL) with [3-(methacryloylamino)propyl] trimethylammonium chloride (MAPTAC) or [2-(methacryloyloxy)ethyl] trimethyl ammonium chloride (METAC). The METAC monomer has an oxygen atom, with larger electronegativity, in its molecular structure, whereas the MAPTAC monomer contains a nitrogen atom, as well as an extra nonpolar CH₂ group, facilitating investigation into the effects of the polarity of CHLs on their physicochemical performance in an aqueous system. CHL polymers are analyzed and their interactions with clay particles are determined in colloidal systems. CHLs are designed to have similar charge densities (2.1-2.2 mmol g^-1) and molecular weights (55000-60000 g mol^-1 ). The hydrodynamic radius (H_y) and radius of gyration, (R_g) of HL-METAC are larger than those of HL-MAPTAC, implying a more 3-dimensional structure of HL-METAC in aqueous solution. The stability ratio of kaolin particles affirms the better performance of HL-METAC in comparison to HL-MAPTAC, which reflects the better flocculation efficiency of HL-METAC. The results also reveal that salt and urea aqueous solutions affect the H_y, R_g, and configuration of CHL polymers, which alters the flocculation efficiency of HL-METAC and HL-MAPTAC polymers in kaolin suspensions.

Interaction of synthetic and lignin-based sulfonated polymers with hydrophilic, hydrophobic, and charged self-assembled monolayers

There is a need to understand the role of polymer structure on its interaction with surfaces to produce effective functional surfaces. In this work, we produced two anionic polymers of lignin-3-sulfopropyl methacrylate (L-S) and poly(vinyl alcohol-co-vinyl acetate)-3-sulfopropyl methacrylate (PVA-S) with similar charge densities and molecular weights. On the gold-coated surface, we deposited self-assembled monolayers (SAM) bearing different terminal moieties namely, hydroxyl, carboxyl, methyl, and amine groups of alkanethiols. This study highlighted the difference between the interaction of L-S and PVA-S and functionalized self-assembled surfaces. The information was generated using advanced tools, such as an X-ray photoelectron spectroscopy (XPS), and a quartz crystal microbalance with dissipation (QCM-D), which facilitated the correlation development between polymer properties and deposition performance on the functionalized surfaces. The higher deposition of PVA-S than L-S onto OH and COOH surfaces was observed due to its greater hydrogen bonding development and higher solubility. The solubility and structure of PVA-S were also beneficial for its higher adsorption than L-S onto CH₃ and NH₂ surfaces. However, the variation in pH, temperature, and salt significantly affected the adsorption of the macromolecules.

The interaction mechanism of synthetic and lignin based sulfonated materials with well-designed functional surfaces was investigated systematically.

Phenomenological Changes in Lignin Following Polymerization and Its Effects on Flocculating Clay Particles

Cationic kraft lignin (CKL) macromolecules were produced via polymerizing kraft lignin (KL) with [2-(acryloyloxy)ethyl]trimethylammonium chloride (ATAC) or [2-(methacryloyloxy)ethyl]trimethylammonium methyl sulfate (METAM). Despite slightly different charge densities (2.3-2.5 mmol/g) of CKL, lignin-METAM (KL-METAM) had a significantly larger molecular weight and radius of gyration. A correlation was observed between the structure of CKLs and their impacts on the surface hydrophilicity of kaolin particles. In interacting with kaolin particles, KL-METAM generated larger and stronger flocs with looser structures than did KL-ATAC. Compared to ATAC, METAM had one additional methyl substituent on its structure, which provided fundamental evidence on how a small group (i.e., a methyl group) on the structure of a cationic monomer can have a substantial influence on its polymerization with lignin and subsequently on the efficiency of the induced macromolecule as a flocculant in a kaolin suspension system.

Using Fractionation and Diffusion Ordered Spectroscopy to Study Lignin Molecular Weight

Recent reports demonstrate that applications of the biopolymer lignin can be helped by the use of a fraction of the lignin which has an optimal molecular weight range. Unfortunately, the current methods used to determine lignin's molecular weight are inconsistent or not widely accessible. Here, an approach that relies on 2D DOSY NMR analysis is described that provides a measure of lignin's molecular weight. Consistent results were obtained using this well-established NMR technique across a range of lignins.

The Self-Assembly of Lignin and Its Application in Nanoparticle Synthesis: A Short Review

Lignin serves as a significant contributor to the natural stock of non-fossilized carbon, second only to cellulose in the biosphere. In this review article, we focus on the self-assembly properties of lignin and their contribution to its effective utilization and valorization. Traditionally, investigations on self-assembly properties of lignin have aimed at understanding the lignification process of the cell wall and using it for efficient delignification for commercial purposes. In recent years (mainly the last three years), an increased number of attempts and reports of technical-lignin nanostructure synthesis with controlled particle size and morphology have been published. This has renewed the interests in the self-assembly properties of technical lignins and their possible applications. Based on the sources and processing methods of lignin, there are significant differences between its structure and properties, which is the primary obstacle in the generalized understanding of the lignin structure and the lignification process occurring within cell walls. The reported studies are also specific to source and processing methods. This work has been divided into two parts. In the first part, the aggregation propensity of lignin based on type, source and extraction method, temperature, and pH of solution is discussed. This is followed by a critical overview of non-covalent interactions and their contribution to the self-associative properties of lignin. The role of self-assembly towards the understanding of xylogenesis and nanoparticle synthesis is also discussed. A particular emphasis is placed on the interaction and forces involved that are used to explain the self-association of lignin.

On the solution structure of kraft lignin in ethylene glycol and its implication for nanoparticle preparation

Ethylene glycol (EG) starts to attract attention as a robust solvent for lignin processing. However its solution structure has not been revealed. In this effort, small angle neutron scattering (SANS) and dynamic light scattering are used to understand the dissolution of kraft lignin in EG and the impact of the resultant solution structure on nanoparticle preparation. Lignin solutions with different concentrations (0.6 to 13.0 wt%) were explored by SANS, allowing evaluation of the solvent quality, the conformation of lignin subunits and their aggregation in EG. Molecular interactions between EG and lignin were discussed and compared to those between DMSO and lignin. The process of nanoparticle preparation from EG solutions upon addition of anti-solvents was also discussed.

Fractionation and DOSY NMR as Analytical Tools: From Model Polymers to a Technical Lignin

One key challenge hindering the valorization of lignin is its structural complexity. Artificial lignin-like materials provide a stepping stone between the simplicity of model compounds and the complexity of lignin. Here, we report an optimized synthesis of an all-G β-O-4 polymer 1 designed to model softwood lignin. After acetylation, the polymer Ac-1(V) was fractionated using a protocol that involved only volatile organic solvents, which left no insoluble residue. Using diffusion ordered spectroscopy NMR in combination with gel permeation chromatography, it was revealed that this fractionated material behaved like a flexible linear polymer in solution (average α > 0.5). Acetylated kraft lignin was subsequently processed using the same fractionation protocol. By comparison with the model polymer, we propose that the acetylated kraft lignin is composed of two classes of materials that exhibit contrasting physical properties. One is comparable to the acetylated all-G β-O-4 polymer Ac-1, and the second has a significantly different macromolecular structure.

Effect of ionic liquid treatment on the structures of lignins in solutions: molecular subunits released from lignin

The solution structures of three types of isolated lignin--organosolv (OS), Kraft (K), and low sulfonate (LS)--before and after treatment with 1-ethyl-3-methylimidazolium acetate were studied using small-angle neutron scattering (SANS) and dynamic light scattering (DLS) over a concentration range of 0.3-2.4 wt %. The results indicate that each of these lignins is comprised of aggregates of well-defined basal subunits, the shapes and sizes of which, in D(2)O and DMSO-d(6), are revealed using these techniques. LS lignin contains a substantial amount of nanometer-scale individual subunits. In aqueous solution these subunits have a well-defined elongated shape described well by ellipsoidal and cylindrical models. At low concentration the subunits are highly expanded in alkaline solution, and the effect is screened with increasing concentration. OS lignin dissolved in DMSO was found to consist of a narrow distribution of aggregates with average radius 200 ± 30 nm. K lignin in DMSO consists of aggregates with a very broad size distribution. After ionic liquid (IL) treatment, LS lignin subunits in alkaline solution maintained the elongated shape but were reduced in size. IL treatment of OS and K lignins led to the release of nanometer-scale subunits with well-defined size and shape.

Distribution of counterions around lignosulfonate macromolecules in different polar solvent mixtures

Lignosulfonate is a colloidal polyelectrolyte that is obtained as a side product in sulfite pulping. In this work we wanted to study the noncovalent association of the colloids in different solvents, as well as to find out how the charged sulfonate groups are organized on the colloid surface. We studied sodium and rubidium lignosulfonate in water-methanol mixtures and in dimethyl formamide. The number average molecular weights of the Na- and Rb-lignosulfonate fractions were 7600 g/mol and 9100 g/mol, respectively, and the polydispersity index for both was 2. Anomalous small-angle X-ray scattering (ASAXS) was used for determining the distribution of counterions around the Rb-lignosulfonate macromolecules. The scattering curves were fitted with a model constructed from ellipsoids of revolution of different sizes. Counterions were taken into account by deriving an approximative formula for the scattering intensity of the Poisson-Boltzmann diffuse double layer model. The interaction term between the spheroidal particles was estimated using the local monodisperse approximation and the improved Hayter-Penfold structure factor given by the rescaled mean spherical approximation. Effective charge of the polyelectrolyte and the local dielectric constant of the solvent close to the globular polyelectrolyte were followed as a function of the methanol content in the solvent and lignosulfonate concentration. The lignosulfonate macromolecules were found to aggregate noncovalently in water-methanol mixtures with increasing methanol or lignosulfonate content in a specific directional manner. The flat macromolecule aggregates had a nearly constant thickness of 1-1.4 nm, while their diameter grew when counterion association onto the polyelectrolyte increased. These results indicate that the charged groups in lignosulfonate are mostly at the flat surfaces of the colloid, allowing the associated lignosulfonate complexes to grow further at the edges of the complex.

Small-angle X-ray scattering and rheological characterization of aqueous lignosulfonate solutions

Lignosulfonate is a colloidal polyelectrolyte widely used as a dispersant in various industrial applications and produced during chemical pulping of wood chips. Here we present a systematic small-angle X-ray scattering (SAXS) and rheological study of fractionated lignosulfonate (mass weighted molar mass M w 18 000 g/mol) dissolved in water and 0.2 M NaCl. The concentration range varied from semidilute to concentrated regime. SAXS intensity of all solutions followed the Porod law at all concentrations, which is a clear indication of a compact shape of the lignosulfonate particle. In water, below 10 mass % lignosulfonate, the average interparticle distance obtained from SAXS patterns relates to concentration via a power law with exponent -0.28. Deviation of the power law exponent from ideal -0.33 and a linear decrease in volume fraction normalized Porod constant as a function of concentration are taken as indications of self-association of lignosulfonate. In saline solutions at high lignosulfonate mass fractions the average distance between lignosulfonate particles was longer and the average particle size was larger than those in aqueous solutions. The intrinsic viscosity in saline solution also was larger than that in aqueous solution. Lignosulfonate solutions showed Newtonian viscosity, except at very high concentrations. The variation of the relative zero-shear viscosity eta(0),r) with concentration was interpreted with the Krieger-Dougherty equation. An oblate spheroid shape with an axial ratio of 3.5 describes the average shape of the lignosulfonate particles in saline solutions based on SAXS intensities, the size distribution obtained using gel permeation chromatography, and rheological characterization. The largest dimension of the particles was about 8 nm. SAXS and rheology studies as a function of temperature reveal indications of temperature-dependent self-association.

Smooth model surfaces from lignin derivatives. I. Preparation and characterization

Lignin model surfaces were prepared from aqueous alkaline solutions by spin-coating on silica wafers. Films of thicknesses between 20 and 140 nm were easily made by variations in the spinning rate or in the lignin concentration. The roughnesses of the lignin surfaces were relatively low, approximately 1.1 nm (rms) on an area of 25 microm2, as determined by atomic force microscopy imaging. The stability of the lignin films in aqueous solutions was found to be excellent. No changes in the thickness of model surfaces immersed in slightly alkaline solutions (pH 9.2) could be detected even after 5 h soaking. A 10 percent reduction in the thickness of the lignin film was observed after 5 h of exposure to a solution containing 0.1 M NaCl. This novel preparation method opens great possibilities for further fundamental studies, where interactions between lignin and other substances are of interest to investigate.

Morphology of dry lignins and size and shape of dissolved kraft lignin particles by X-ray scattering

Lignin is a highly branched polymer consisting of phenylpropane units, and it is one of the ingredients of the supporting matrix in plant cell walls. The morphology of several lignins extracted from plant cell walls using different methods was studied by small-angle and ultra-small-angle X-ray scattering. A power-law type intensity was observed for the dry lignins, but on the basis of the power-law exponent the fractal approach often applied to lignins is not fully justified. However, the intensity of kraft lignin did show a power law with surface fractal dimension D(s) = 2.7 +/- 0.1. The specific surface area of the lignins ranged from about 0.5 to 60 m(2)/g with 20% relative accuracy. The radius of gyration was determined from small-angle X-ray scattering data for aqueous solutions of kraft lignin. The shape of the particles in NaCl and NaOH solutions was found to be elongated. The particles were about 1-3 nm thick, while the length (5-9 nm) depended on the solvent and on the lignin concentration. The size of these primary particles was approximately the same as the size of the pores in the fractal aggregates of the dry kraft lignin. Their size was determined to be about 3.5 nm.

Localized interaction of the polyamine methylspermidine with double-helical DNA as monitored by 1H NMR self-diffusion measurements

The 1H NMR pulsed field gradient self-diffusion method has been used to measure the diffusion coefficient of the polyamine analogue methylspermidine (completely N-methylated spermidine) in DNA solution, as a function of the concentration ratio of methylspermidine to DNA phosphate. Three different DNA's have been investigated: d(GC)4 (8 base pairs), core length calf thymus DNA (approximately 120 base pairs), and sonicated high molecular weight calf thymus DNA (average 7500 base pairs). For a constant ratio of methylspermidine to DNA phosphate, the diffusion coefficient decreases with increasing DNA length. Moreover, at low concentration ratios the diffusion coefficient of methylspermidine approaches a limiting value that is close to that of the DNA molecule. The experimental data are well reproduced by a two-state diffusion model. In this model the diffusion coefficient of the polyamine is a population-weighted average of polyamine associated with DNA (with a diffusion coefficient given by that of the DNA molecule) and polyamine free in solution.