IN
DEGREE PROJECT CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS
,
STOCKHOLM SWEDEN 2020
Synthetic Functionalization of
Colloidal Lignin Particles for Wood
Adhesive Applications
Aalto-yliopisto, PL 11000, 00076 AALTO www.aalto.fi
Diplomityön
tiivistelmä
Author Fusi, Alexander Deen
Title of thesis Synthetic functionalization of Colloidal Lignin Particles for Wood Adhesive Applications
Master programme Polymer Technology Code CHEM3044
Thesis supervisor Prof. Mauri Kostiainen; Prof. Eva Malmström Jonsson Thesis advisor(s) Doctor of Science Kalle Lintinen
Abstract
Functionalizable spherical colloidal lignin particles (CLPs) represent a valuable asset for the valorization of lignin side-streams from the pulp industry. The spherical structure allows for the circumvention of the heterogeneous and poorly dispersible structure of the biopolymer. However, organic solvents and alkaline media degrade the particle structure and dissolve the polymers due to their chemical nature and solubility. The solvents will alter the aggregated polymers into irregular shapes that would correspond to inconsistent physicochemical properties. Then, the material will become unusable for advanced material applications, namely wood adhesives.
In this study, a replicable process to yield pH ca. 12 stable CLPs for wood adhesives or further functionalization for other advanced material applications was developed and optimized. Lignin was functionalized with cross-linkers, glyoxal or formaldehyde, and self-assembled into spherical structures in the micro emulsification of the organic solution. The formed colloids were partially rotary evaporated to retain organic solvents within the colloidal structures, and then be cured at 73-76 °C until pH stable and further functionalized for advanced material applications.
The functionalization with glyoxal was pursued further for its possibly increased reactivity and the health concerns associated with formaldehyde. The process requires the addition of glyoxal to lignin in an acidic organic media at ambient temperature, and the solution to react at 64 °C. Glyoxal is likely added to the polymer structure in its hydrated and dimerized form, and its attachment to lignin should be analyzed through the behavior of glyoxal in different media. The formed colloids were rotary evaporated to an organic solvent content of 60 wt. % of the spheres to allow the occurrence of the curing reaction. These materials were finally cured by thermosetting them at 73-76 °C until pH stable. The particles can be cured with base-catalysis through the controlled addition of the base NaOH(aq). However, the mode and rate of addition of the catalyst are critically important
for a non-degradative infusion of a base into solvent present ot removed particles without morphological changes. Further procedural improvement and larger batches are necessary to conduct CLP adhesive experiments.
Acknowledgements
Dear all,
This opportunity has been an enlightening experience. While the process may have been
intense, I am extremely happy about working on a project with an environmental focus
and would honestly repeat everything once more. The environment, supervision from both
Aalto and KTH Royal Institute of Technology, and the Nordic 5 Technology program have
been fantastic. You all have provided me with tools to tackle future endeavors, and the
project has been a rewarding experience that I cherish deeply. While hair may have been
lost during the coronavirus and thesis writing period, this task has taught me both
scientific and life lessons that I will never forget, carry on, and lead me onwards and
upwards.
The thesis work described hereafter was conducted in the Biohybrid Materials group at
Aalto University. In practice, I am grateful for the opportunity to have participated in the
wonderful, motivated, and community that is this Research group. The people, the
opportunity, the supervision, patience, and help from Prof. Mauri Kostiainen have been
excellent, and I could not have asked for more. Here, I would also like to particularly
thank my advisor, Kalle Lintinen, for his constant aid, support, and collective
brainstorming, which has always been insightful and pleasant. Moreover, I would like to
also acknowledge his unwavering guidance and patience, especially during the
manuscript development period. His excitement for scientific inquiries has positively
influenced my outlook. Lastly, I would also like to thank for the supervision of Prof. Eva
Malmström Jonsson, who has graciously helped as a supervisor during the thesis process.
Additionally, I want to thank:
• The people in C309 who have kept me company and have supported me throughout
the entirety of my work with colloidal lignin particles;
• Family and friends who have maintained my motivation sky-high during the
emotionally taxing thesis-writing, COVID-19 period;
• The coffee breaks and culture in Finland;
• All the people who have contributed to my project and visualization, including
Jani Seitsonen for TEM imaging, Aalto University RawMatters Facilities, and the
OtaNano Nanomicroscopy Center.
Thank you all.
Espoo 31.7.2020
Alexander Deen Fusi
Table of Contents
1. INTRODUCTION
1
1.1 Issue of Sustainability, Lignin Valorization, and Research Objectives 1
2. SCOPE AND STRUCTURE OF STUDY
2
2.1 Purpose of Study 2
2.2 Structure of the Manuscript and Disclosure 4
3. LIGNIN
4
3.1 Structure, Chemistry, and Properties 4 3.2 Commercial Production of Lignin 8 3.3 Current Uses, Applications, and Prospects 12 3.4 Colloidal Lignin Particles 15
3.4.1 Lignin Solubility in Media 15
3.4.2 Lignin Particle Formation Approaches and Considerations 18 3.4.3 Colloidal Lignin Particle Structure and Morphology 26
3.4.4 Colloidal Stability 27
3.5 Wood Adhesive Products 31
4. EXPERIMENTAL APPROACH
37
4.1 Overview and Theoretical Framework of the Functionalization of Dissolved Lignin, Colloid Formation, and Curing Process of Colloidal Lignin Particles 37
4.2 Materials 40
4.3 Lignin Particle Preparation 41
4.3.1 Methylolation of Lignin and Colloid Formation Preparation for Lignin-Formaldehyde
Particles (LF-CLPs) 41
4.3.2 Preparation of Spherical Lignin Colloids 41
4.4 Curing Process of Colloidal Lignin Particles 42
4.4.1 Curing Process Preparation and Set-up 42 4.4.2 Curing Process and Stability Analyses 42
4.5 Analysis of Colloidal Lignin Particles 43 4.6 CLP Final Curing and Adhesive Testing of colloidal Lignin Particles 45
5.1 LF-CLPs, Curing in the Bulk State, and the Issue of Lignin Dissolution by NaOH Catalyst Addition and High Temperature 46 5.2 Precuring Considerations, the Presence of Ionic Strength, and NaOH(aq) infusion into
LF-CLPs 48
5.3 The Use of Ethanol to Improve the Curing of LF-CLPs 58 5.4 The Transition to Automated Gradual NaOH(aq) Additions for the Curing of LF-CLPs 63 5.5 The Transition from Formaldehyde to Glyoxal Cross-linkers 67 5.6 The Adhesive Bond Test of Lignin-Glyoxal Colloidal Particles 73 5.7 The Optimization of the Glyoxalization Reaction and the Immediate Curing
Basification 75
5.8 The Development Process of pH ca. 12 Stable Lignin-Glyoxal Colloidal Particles 85
6. CONCLUSION
89
Abbreviations
AFxx Alexander Fusi experiment CLP colloidal lignin particle DLS dynamic light scattering
DLVO Derjaguin Landau Verwey Overbeek theory
HB heat block
HSQC Heteronuclear single quantum coherence KLxxxx Kalle Lintinen experiment
LF lignin-formaldehyde LG lignin-glyoxal
LF-CLP lignin-formaldehyde particle LG-CLP lignin-glyoxal particle LP lignin particle
L-vdW Lifshitz-van der Waals
MMF Melamine-Formaldehyde
NMR nuclear magnetic resonance
PF Phenol-Formaldehyde
RCF Relative Centrifugal Force
Symbols
M [mol/L] Molarity
Mn [g·mol-1] Number average molecular weight
Mw [g·mol-1] Weight average molecular weight
N [eq/L] Normality
T [°C] Temperature
cps [counts/s] Counts per second cP [mPa·s] Centipoise d.nm [nm] Colloid diameter g [9.8 m/s2] Gravity Å [1·10-10 m] Angstrom Đ Dispersity Index ζ [mV] Zeta potential
1. Introduction
1.1 Issue of Sustainability, Lignin Valorization, and Research Objectives
Lignin functionalization for advanced material applications is a strategy that can help purpose the quantity of by-product generated from established modern wood pulping processes, with a particular focus on side-streams from paper production1. Lignin, as one
of the most abundant side-products of paper-making available worldwide, is predominantly used for energy recovery through incineration due to its high calorific content resulting from the aromatic non-polar chemical structure. In addition, the inconsistent physicochemical properties of the heterogeneously composed material obtained through standard pulping processes render the material unusable for later applications. Through the formation of uniformly sized, spherical nano-scale colloidal lignin particles (CLPs), it is possible to develop a material able to circumvent the anisotropy associated with its heterogeneous composition, which can be then functionalized through common surface modifications based on secondary interactions, adsorption strategies, and enzymatic treatments2. However, the modification pathways
and valorization options are limited, and the reaction conditions of any functionalization for covalent bonds in organic and alkaline aqueous media affect the shape, size, and distribution of lignin particles (LPs), influencing the final properties as well.
To retain the spherical structure of CLPs during their functionalization for advanced material applications, it is necessary for the particles to be resistant to solubilizing mediums. As a result, we aim to cure the CLPs by cross-linking and determine the parameters that influence the quality of the particles of interest, optimize the curing step to produce a chemically stable material, and determine the applicability of CLPs as a bio-based adhesive. The ortho-/para- directing reactive groups of the phenolic moieties allow for the functionalization of the aromatics with cross-linking agents, which can then react with other lignin units to cure the different polymer chains into a chemically, thermally, and mechanically resistant spherically shaped strong network structure that can ensure isotropic properties throughout the entirety of the material. The setting of these particles
can widen the range of possible applications for lignin, thereby increasing the usage of lignin as an environmentally friendly substitute to petroleum polymers, with distinct consideration towards phenolic adhesives for wood.3
2. Scope and Structure of Study
2.1 Purpose of Study
The study is set to develop to produce a sustainable and resistant material to offer valuable sustainable alternatives to lignin incineration. Consequently, a scalable process for the development of functionalizable lignin dispersions has been investigated to convert lignin particles into chemically and mechanically resistant spheres.
This research focuses on the development of a universal method to cure colloidal lignin particles, and the approach proceeds as follows in figure 1.
Figure 1. Development process of high pH stable colloidal lignin particles; a) functionalization of
dissolved lignin polymers with cross-linkers; b) self-assembly of lignin polymers in colloidal spheres; c) curing reaction for the formation of high-pH stable colloidal lignin particles; d) modification of cured lignin particles for wood adhesive application.
Dissolved lignin can be reacted with cross-linkers and then converted into spherical colloids. The spherical nature of lignin colloids reduces the influence of the heterogeneity of the major section of the lignin structure and ensures isotropic properties throughout the material. The functionalized lignin in the spherical colloids can be cured to bind the different polymer units together to a degree where particles can sustain any environment and be modified further without compromising their macrostructure. Post-cure, the surface functional moieties can be modified for widespread applicability, thereby
circumventing the issue of inconsistent microstructure and predominantly focusing on the presence of functional groups on the surface of the particles.
The resulting chemical stability, availability, and low cost of CLPs allows further functionalization and use in large scale applications, such as adhesives and coatings, and minorly, wound-healing properties, anti-fouling function. Moreover, the particles are also applicable as pickering emulsifiers and for encapsulation of valuable compounds. However, there can be limitations and concerns for the toxicity of the functionalized CLP formulation.
The work done in this study is based on a preliminary process for the development of cured colloidal lignin particles developed previously in the Biohybrid Research group (figure 2). However, while the premise is theoretically sound, the inconsistent addition of base catalysts and uncontrolled temperature of curing yields degraded particles of high dispersity (Đ; measure of heterogeneity of the size of particles within mixtures), size, and lower quality unfit for further functionalization. As a result, we aimed to develop the current set-up further to produce “environmentally” stable CLPs via base-catalyzed curing reactions. Furthermore, we will determine the influencing curing parameters, develop and optimize the production process, and test the applicability of dense lignin particles for advanced material applications by purposing them as a bio-based adhesive.
Figure 2. Provided initial colloidal lignin formaldehyde particle development process flow and
2.2 Structure of the Manuscript and Disclosure
The manuscript is divided into three major sections, namely a literature review, an overview of the studies and experimental approach, and the results and discussion of the findings. The following chapter provides a literature review of lignin’s properties, processes, applications, and insights of adhesion science. The second section of the chapter describes an overview of the preparation and characteristics of colloidal lignin particles, and the theoretical framework for its application as an adhesive. The manuscript will then delve into a description of the materials and methods of the preparation of the functionalization of lignin, formation of colloidal lignin dispersions, and the curing of CLPs to be then applied as an adhesive, the findings of which are discussed in the fourth chapter.
Within chapter 2.4.3 “Colloidal Lignin Particle Structure and Morphology”, information regarding the surface topography and morphology of particles was empirically obtained and derived by research efforts of Kalle Lintinen of the Biohybrid Materials group. It is based on unpublished confidential data that is present in disclosure IPID2415 of Aalto University.
3. Lignin
3.1 Structure, Chemistry, and Properties
Lignin is a large, heterogeneous, energy-rich, organic macromolecule, which forms complex three-dimensional structures. It is one of the most abundant biological polymers available and is one of the critical structural components of vascular plants, accounting for up to 40 % of the dry biomass of wood. The network behaves as the glue connecting the polysaccharides in the cell wall of plants through covalent binding (figure 3)4–6.
The biopolymer is composed of aromatic molecules, known as monolignols. The three main monomer precursors consist of a substituted aromatic ring with a propenol side moiety bound to the aromatic carbon C1; these are named p-coumaryl, coniferyl, and sinapyl alcohol or designated with the letters H, G, S, based on whether the ring is unmethoxylated, mono- or di-substituted with methoxy groups, respectively.
Lignin is biosynthesized, and the formation and monomer ratios are cell and location specific7,8. The main monomers are formed through a gene-controlled pathway that
converts the amino acids, phenylalanine and tyrosine, into the molecules of interest through several enzymatic steps within the cytosol9,10. Then, the precursors enter the
polymerization step through an enzyme-induced oxidative coupling of monomers and the addition of phenoxy radicals to the growing polymer via lactase and peroxidase11,12. Once
the unpaired electrons form, the reaction proceeds randomly with functional groups connecting with other sites based on their reactivity and be attached to each via various reactions, and bridged differently (figure 4)13.
The oxidative process creates a delocalized radical highly resonance stabilized on the phenolic moiety at the C-1, C-3, C-5, or O-4 position, or the propanolic double bond, thus allowing for a variety of resonance structures that can connect to other molecules. Covalent bonds are formed, and the chains can propagate further, or react with the radical of another molecule, terminating the polymer growth.11 The molecules formed are bound
majorly through aryl ether linkages (β-O-4′), comprising approximately 50–60 % of intermolecular linkages in hardwoods and softwoods14. Other ether linkages are found
between 4-O-5′, 1-O-4′ and C-C bonds, known as condensed bonds, are observed at the 5–5′, β-5′, β-β′, β-1′ positions12. The monomers are also thought to polymerize with
minorly present molecules, such as variants of monolignols, hydroxycinnamic acid, and aldehydes. The inclusion of these variations and other minor lignin constituents elevates the complexity of the structure and to the heterogeneity of the network system. The
Figure 4. Main monolignol units, nomenclature for the carbons, and linkages present in the lignin
polymer (adapted from Barros et al.)8.
4 3 2 1 6 5 OH ⍺ β 𝝲 H O 4 5 6 1 2 3 OH ⍺ β 𝝲 HO OCH3 4 3 2 1 6 5 OH ⍺ β 𝝲 HO OCH3 OCH3 S G H
p-Coumaryl alcohol Coniferyl alcohol Sinapyl alcohol
A β-O-4, β-ether B 4-O-5 biphenyl ether C β-β resinol D β-5 phenylcoumaran HO HO H3CO H3CO OH O H3CO H3CO OH OH O OCH3 HO HO O OCH3 OH HO O S S OCH3 H3CO HO OH O OCH3 HO OH O OH O OH OCH3 OH HO O OCH3 HO OH O O OCH3 OH OH O O O O H3CO OH HO OCH3 OCH3 O OH OH OH OCH3 OCH3 OCH3 OCH3 O HO OCH3 HO O HO OCH3 H3CO HO HO O O OH OCH3 OCH3 S S S S S S G G G G G G G G H CA A B C D
heterogeneity and lack of comprehensive formulas invalidates all-encompassing structures that can summarize the material’s chemical organization in space across all plant species. Moreover, there is a lack of processes that maintain the integrity of the native lignin structure for characterization. This consideration brings forth the use of milled wood lignin, extracted through mild conditions, for the characterization and general representation of the native structure.15
The spatial disposition of the network is irregular, thereby amorphous. The structure can be more or less rigid or branched contingent upon the ratio of monomers found in the vascular tissue. For example, hardwood lignin is predominantly composed of G and S rings, which are substituted by methoxy groups that may restrict the mobility of the fibers and allow for a more rigid structure. Alternately, softwood lignin is characterized mainly by G units and, minorly, by H units. These differences can be determined by several analytical techniques.16,17
The functional groups and the hydrophobic backbone of the polymer predominantly induce four types of attractive interactions, namely intermolecular hydrogen bonding, hydrophobic and electrostatic interactions, and 𝜋-stacking. These cause the aggregation of lignin and influence its micro and macroscale properties18. The rigid aromatics and the
cross-links restrict the movement of the different backbones, which analytical techniques suggest form ellipsoid basal subunit shapes with different degrees of association based on the extraction process used19. The restricted movement confers rigidity to the compact
structure, allowing the organism to retain its structure. Furthermore, the rigid material is compact enough to limit the penetration of degrading enzymes into the polysaccharide region to a minimum, acting as a basic form of protection against parasitic organisms and external agents20,21.
The polymer is also known to be biocompatible, UV-absorbent and oxidatively resistant due to the absorbing ability of its chromophoric conjugated aromatic structure, which confers thermal stability and the dissipation of energy22–27. Furthermore, lignin is chemical
stable, heat resistant, and has a radical scavenging ability that is dependent on the hydroxyl group content, molecular weight, and various other factors28.
The combination of these types of groups and the consequent aggregations contribute to lignin insolubility in aqueous media. However, the separation processes employed in the pulp and paper industry introduce new functionalities through harsh reactions. The extractions consequently modify the solubility properties of the biomass and increase the polar group content to a point that permits dissolution in water, as observed in lignosulfonates18.
Different structural information can be obtained through common techniques. Gel permeation chromatography (GPC) is used for the determination of the isolated chain fractions’ molecular weights. One and two-dimensional nuclear magnetic resonance (NMR) provide structural elucidation of the units and linkages present, and Fourier transform spectroscopic analysis for identification of functional components of lignin and their ratios29. Additional information can be obtained by small-angle neutron and X-Ray
scattering (SANS; SAXS), which show the size and shape of individual lignin molecules on the order of 10–1’000 Å.
3.2 Commercial Production of Lignin
Lignin, essentially, is the by-product created from the separation of lignocellulosic biomass in biorefineries and the pulp and papermaking industry. Paper-focused cellulose extractions are based on breaking the macrostructure of lignin into smaller molecules through the cleavage of inter-unit linkages present between monolignols. However, the chemically heterogeneous pristine wood structure is strong, and the extraction may require different harsh treatment strategies to separate the components. The process introduces different types of structural variations, such as polar groups and wide distributions of macromolecule molecular weights.29,30 The alterations ultimately affect the
properties and applications of the biopolymer. The different delignification alterations will vary the heterogeneous macromolecular and chemical structure of lignin further, which then augment or decrease the propensity of specific properties. These variations will cause inconsistent behaviors across materials from varying feedstocks.30
A variety of strategies have been developed and form a classification based on the inorganic sulfur content introduced during the separation operations. The different types can be subdivided in sulfur-containing lignin, which encompasses the leading measures of cellulose extraction nowadays, or sulfur-free lignin, which includes soda lignin and materials from second-generation bio-refinery processes, namely organosolv polyphenols (figure 5).31
Within the first-mentioned group, the types of by-products are Kraft, sulfite, and hydrolyzed lignin, of which processes have reported lignin yields of 65–90 wt.%32. The
Kraft procedure is the primary type of chemical pulping and comprises approximately 80 % of the total chemical processes available. Wood chips are boiled with sodium hydroxide (NaOH) and sodium sulfite (Na2S), known as "white liquor," at high pressure and
temperatures within the 150–180 °C range.31 Here, the Na
2S prevents condensation
reactions and improves the dissolution of lignin while reducing that of cellulose33. The
base deprotonates the phenolic groups, increases the hydrophilicity of lignin, and facilitates its dissolution and separation from other wood components. The reagents break the macromolecule into oligomers specifically by cleaving the alkyl-aryl bonds and separates the cellulose fibers from each other. The spent liquor and degraded contents form a weak black liquor that is removed from the pulp, evaporated, and concentrated via evaporator systems to a solids content of 65 % (figure 6)31,34.
The resulting lignin are low purity oligomers (Mn=1’400–6’000 g·mol-1) with highly
condensed structures, -SH groups (1.5–3 wt.% S). The persisting predominant bonds, determined through the quantitative NMR and GPC analysis of Kraft softwood, are the arylglycerol-β-aryl ether (β-O-4’), stilbene inter-units, Cβ-H of secoisolariciresinols,
pinoresinol β-β’ bonds, and minor abundances of other linkages. The major functional groups noted were the aromatic C-H, methoxy, quinone groups, aliphatic and phenolic hydroxyls, and minorly carboxyl -OH moieties. Moreover, there is a significant abundance of aliphaticities originally associated with wood extractives. The groups are speculated to be derived from radical-induced reductions of lignin sidechains. The harsh conditions of Kraft pulping cause a variety of side reactions, including redox and ionic, that produce “single-linked” phenol branching up to approximately 84 % of the aromatics present17.
Post-extraction, the lignin by-product can be isolated from black liquor through the LignoBoostTM process (Innventia). The separations leads to the formation of lignin as
small particles, which feature higher heating values and unique physicochemical properties. The resulting moist lignin has lower softening points, and its dry version (moisture content<10 wt.%) possesses great reactivity due to the increased surface areas of the small particles formed35.
An analogous operation to Kraft pulping is the acid sulfite process, based on high pressure "cooking" (T=140–170 °C) and large scale batches or continuous flow. The cooking chemicals in use are sulfuric acid, to break and dissolve lignin, alongside sodium, magnesium, calcium, or ammonium bisulfate to buffer the acidic caustic action of the
reagent36. Sulfur dioxide is forms from the acid and is converted into hydrogen sulfide,
which allows the removal of lignin their attachment to it via sulfonication and hydrolysis reactions in the β-aryl ether bonds. The material formed is a low-purity, structurally dense, lignosulfonate oligomer (Mn=15’000 to 50’000 g·mol-1) with high ash content and -SO3
groups (4–8 wt.% S)37,38. The process introduces polar sulfone groups in lignin, which
increases its solubility in aqueous media, association with other polar impurities, and reduces the formation of lignin aggregates in organic media39.
The last wide-spread sulfur-containing material is hydrolyzed lignin, produced through a dilute acid or alkali pretreatment and the subsequent hydrolysis of carbohydrates by cellulolytic enzymes while lignin persists as an insoluble residue. When the polysaccharides are removed, a mixture of 65–80 % lignin, 0–1.0 wt.% sulfur, ash, protein impurities, and approximately 7–8 % carbohydrates that cannot be removed with prolonged treatments is formed40.
Alternately, some options are not based on the addition of inorganic sulfur and yield higher purity lignin. Purer lignin side-streams are produced by biorefinery processes, and predominantly from solvent, and soda pulping in the pulp and paper making industry. Soda pulping capitalizes on the separation of lignin from cellulose through the cooking of wood chips at 160–170 °C with NaOH, for cleaving reactions. Optionally, anthraquinone can be used as a pulping additive to reduce carbohydrate degradation41. The obtained
product is of low purity, does not contain any sulfur, and has greater ash content compared to other described processes42.
Differently, solvent pulping, the organosolv process, is dependent on the solubilization of lignin and hemicellulose with organic solvents. In this case, wood chips are boiled at 100– 200 °C with alcohol or alcohol/water mixtures with formic acid, acetic acid, and, optionally, sulfuric acid. The resulting lignin has lower ash content, which is precipitated out by solvent exchange, that is recovered as high purity spherical aggregates43. The relevant
mechanisms for the dissolution are the acid-mediated hydrolysis of the α-aryl ether C-O-C bonds44. Lignin derived from this process has been reported to have an M
n of 500–
3.3 Current Uses, Applications, and Prospects
Lignin represents a promising material due to its properties and low-cost, abundant volume, sustainable production, and biodegradability. Alongside the inherent variation given by the plant source, the combination of properties will create unique behaviors in the material of interest and provide a finely tuned match between applications and species-specific lignin.45
Traditional approaches in lignin side-stream management employ the simple, economic incineration tactic due to its high production volume and calorific value. Through the aid of salts and metal oxides in gasification, the material can be converted into synthesis gas, and further alterations through petroleum technology produce other types of fuel.46–48 The
energy produced is applied to power industrial-scale pulping machines and create self-sustaining processes for pulp mills.29,36,49–51 Other biomass prospects include the
conversion of the biomass to specialty chemicals and carbonization for low-cost carbon fibers reinforcements in advanced material applications. Lignin is fractionated and purified into small, low molecular weight reactive intermediates. In most cases, the conversion occurs post-controlled fracture and macromolecule depolymerization to yield chemically modifiable fragments into well-defined products. Alternately, other processes directly produce valuable molecules with catalyst aids, as occurs in gasification.29
The purified macromolecule is converted into specialty chemicals by means of pyrolysis, acid or base hydrolysis associated with depolymerization, oxidative or reductive conversion. Alternatives require many extensive steps for the removal of various functional groups, on account of oxidative or reductive conversion, for the development of fine chemicals such as alkyl or pure benzenes, toluene, xylene, hydrocarbons, ring-opening monomers, and several aromatic and cyclic compounds29,49,52. Moreover, lignin
oligomers can be selectively converted into chemicals such as phenols, aldehydes, and aromatic alcohols (figure 7)52. However, the molecules produced can be polymerized
Other purified lignin applications are dependent on the physicochemical properties of lignin from specific extraction processes. The wide array of polymer allows for widespread applicability in major industries ranging from platform chemical production, composite and construction specialties, electrochemical processes, biomedical applications, and agriculture. Actual use is majorly found in lignosulfonates, as starting materials for vanillin and in construction applications, as dispersants, water reducers, viscosity controllers, and plasticizers in cement mixtures.53,54 Here, lignin can reinforce the strength of the product
and, with its hydrophobic and chemically resistant structure, protect the external areas from moisture and acid rain. The hydrocarbon structure derived from the lignin fragments confer applicability as a dispersant and hydrophobic additive material29,45,55–57.
Furthermore, the specific inclusion of sulfonate groups to lignin and the resulting associated highly negative zeta-potential (ζ-potential) lead to the electrostatic repulsion of cement particles, thereby improving the composites’ compressive strength58.
Potentially, the different lignin types can be applied for nanomaterials, energy storage, and various specialized applications that exploit its passive functionality. These properties are related to adsorption, i.e. for environmental remediation as pickering
emulsions, entrapment and triggered release of valuable active compounds, as seen in pharmaceutical applications.59,60
In polymeric materials, Kraft, Organosolv, and lignosulfonates have been studied as drug carriers, scaffolds, and hydrogels for biomedical applications. Alternately, through the copolymerization of lignin with commercial polymers through grafting, the lignin-based polymers have been considered for advanced materials applications as surfactants or additives to improve 3D-printed resin properties, polymer blends, and composites. The rigid, functional aromatic rings confer chemical, oxidative, UV, and thermal stability of the polymer.61 The electron-delocalizing conjugated systems of lignin can form stable radical
bearing species, functioning as an organic hydroperoxide decomposer, UV absorbent, and energy dissipating reinforcement24,61
For energy storage, Kraft lignin and lignosulfonates have been included as an expander in lead-acid batteries, to increase their efficiency and durability, or as green carbon precursors for electrode binders in lithium-ion batteries62. More specifically, the
water-soluble salt form of lignosulfonates has been thoroughly studied as a low-cost binder in silicon anodes for lithium-ion energy storages63.
Alternately, Soda and Organosolv lignin applications can be used as precursors for carbon materials, biofuels, building materials as well. Specialized applications for LignoBoost-purified Kraft lignin have been specifically studied for the replacement of phenolic resins components, and as dispersants or adsorbents for purification purposes. Lastly, literature reports the use of lignin for agricultural, textile, and fire suppression functions as soil conditioners, controlled release agents for fertilizers and pesticides, and as fire retardants.55,64–68 Interests in lignin adhesive or multifunctional material, or building
blocks for bio-based products, composites, and biomedical end-use, have been reported as well37,60,69.
Recently, interest in lignin from Organosolv and Soda processes has increased due to their purity, low dispersity, and lack of sulfur within the macromolecules, which allow for applications to the field of medicine and nanoscience, a rising field of lignocellulosic science38. The increased interactivity and self-aggregative properties of micro and nano
scale colloidal lignin in aqueous media, pronounced in nanoscale particles, allows for easier and greater ranges of valorization options and directs the biopolymer’s functionalizable surface toward high values applications.
3.4 Colloidal Lignin Particles
Colloidal lignin dispersions can be produced in aqueous continuous phases by microemulsification of organic solvents, in which lignin is solubilized, and the adsorption of lignin polymers to these emulsions. Various subsets of colloids can be differentiated based on their diameter size as the particle dimensions can range from 1–1’000 nm in diameter (d.nm). All colloids of interest are formed with particular attention toward the solubilities of the species of lignin extracted, and special considerations are given to narrow distributions and consistent characteristics of these dispersions due to the wide interest in industrial applications70. The homogeneity in the surface of colloidal lignin
structures will allow for the fine-tuning of the associated properties through synthetic functionalization.
3.4.1 Lignin Solubility in Media
To be able to exploit the aggregation function of lignin particles, it is crucial to understand the polymer’s behavior in different liquid environments. The system is bound to behave according to the principles of mixing thermodynamics. The basis of association is dependent on the forces at play within the macromolecules themselves and with the liquid media, aqueous or organic. Lignin dissolves spontaneously in solution when the Gibbs free energy of mixing is less than 0. This event occurs especially when the polymer-solvent attractive interactions are more significant than the ones found between chains. The interaction between components is denoted by the model-specific Flory-Huggins interaction parameter X12, which characterizes the interaction energy per solvent
molecule and, summarized, the enthalpy and entropy of mixing in the aforementioned theory. The X12 factor can be determined through the solvent activity in vapor sorption
studies, the interaction with a polymer packed column in inverse chromatography, with osmotic pressure data derived from a simple membrane osmometer alongside the second viral coefficient, and a few other methods71.
Lignin solubility will be dependent on the different types and propensity of interactions between the different types of macromolecules and solvent, which can vary based on the origin and extraction process used. The analysis of the chemical structure enables us to categorize the inter and intra-chain existing forces to hydrophobic and electrostatic interactions, 𝜋 − 𝜋 stacking, hydrogen bonding, and stereoregular association, the combination of all, alongside high molecular weights, branching, and the consequential entanglements allow for associative properties72,73. These solubilities will vary depending
on the pulping process the biomass undergoes, and, generally, different molecular weight fractions can be solubilized with increasing the non-polar content in the solvent system74.
With these bonds, lignin, in short segments comprised of four to ten monolignols approximately, assemble into rod-shaped or cylindrical building blocks. These can assemble further into highly branched network structures with low cross-linking densities, analogous to native lignin disposition in space. The branching present in the oligomers are thought to widely influence the association, where highly branched structures will lead to the formation of considerably sized particles, and low-branching will allow for better packing of the molecules.75,76 The lack of aggregates in solution from the disruption of
non-bonded orbital interaction between the phenolic rings through the presence of iodine in THF attributes the major contributing factor for lignin molecular aggregation to 𝜋 − 𝜋 interaction (bond energy=4–30 kJ·mol-1)66,67. Minor contributions were found from
H-bonding and other types of interactions. Studies report tightly aggregated network core regions in lignin aggregates still stable after the addition of electrolytes to lyophobic lignin colloids. At the same time, loose non-covalent bonding given by the combined action of attractive H-bonding, intermolecular van der Waals forces, and repulsive electrostatics, was destabilized79–81. Argyropoulos et al. noted the same through the absence of
H-bonds of both hardwood and softwood lignin in THF, which still allowed the association of lignin molecules73. Although minor with respect to aromatic interaction, the association
in solution is additionally dependent on the ionization of polar groups, predominantly phenolic -OHs, which dictate the overall charge of the molecules and behavior in solution. The deprotonation of these groups can increase the interactivity between water and lignin, forcing the macromolecule into solution in the form of random coils at pH=9.5 and fully solvated chains at greater pH82,83. Within the pH range 3.7–8.6, the decreasing
ionization of carboxyl groups and oxygen in hydroxides and ethers in lignin increased the number of H-bonds present81.
The manipulation of polymer-polymer and polymer-solvent interactions through pH, mediums, ionic strength, co-solvents and temperature, can control lignin aggregation and create a system in which lignin aggregates with particular shapes and spatial geometry dependent properties are present.
The low polarity and aromatic content of Kraft lignin decreases its water-solubility in acidic and neutral conditions, and the higher ionic strength combined with high temperature will affect this property further by decreasing the degree of ionization and inducing lignin aggregation84. The ionization of the phenolics, seen in pulping processes, will result in
the dissociation of lignin molecules and form expanded aggregates, thus allowing for the solvent to enter in between and surround the structures. In lignin from this process, two major fractions, acetone soluble and insoluble, can be isolated and are denoted either by a heterogenous structure with high aliphatic hydroxyl content, high inter-unit bonding, and lower degrees of aromatic substitutions (compared to previously analyzed Kraft lignin), observed in the lower yield acetone insoluble fraction (reported 30 wt.% yield), or by a more homogenous, lower molecular weight structure (reported 70 wt.%) with smaller amounts of side chains, end group units and aliphatic -OHs, and greater branching and phenolic content17. The high -OH content, alongside the low concentration of hydrophobic
aromatics and high molecular weight (reported Mn=3’300 g·mol-1; Mw=12’000 g·mol-1),
increase the H-bonding between molecules and the difficulty in solvating the large macromolecules, and thereby less soluble in organic solvents, while the smaller (reported Mn=1’000–2’000 g·mol-1; Mw=1’000–3’000 g·mol-1) more hydrophobic aromatic fraction
allows for solubilization in acetone.
Lignosulfonates (20’000–50’000 g·mol-1), instead, are water-soluble soluble at any pH
possible, due to the introduced polar sulfonate groups, and a high anionic character will prevent the formation of colloidal aggregates by precipitation in aqueous media39. Other
types of lignin-derived from procedures that do not introduce additional polar functionalities, such as the organosolv process, will be more soluble in organic solvents and, consequently, can be precipitated as aggregates.
The analysis of the chemical structure and its mixing thermodynamics by several polymer-characterization methods available, namely multiple-angle light scattering techniques, enables us to determine the most optimal solubilization or aggregation tactic for lignin.
3.4.2 Lignin Particle Formation Approaches and Considerations
A variety of methods have been developed to form lignin nanoparticles. A general categorization of the particle formation approaches is found in the subdivision of the production of lignin particles in their dry form or an aqueous dispersion as colloids. Methods that lead to the formation of dry lignin particles include the use formation of aerosol flow, dissolution and controlled evaporation of atomized droplets, mechanical shearing, or solvent evaporation. Alternately, aqueous dispersions of lignin particles can be formed through solvent shifting, acidification, CO2 precipitation, and reverse micelle
formation (figure 8).85–88 The more notable recent studies focus on lignin particle
formation via mechanical shearing and sonication, aerosol flow, anti-solvent precipitation, interfacial polymerization, and self-assembly in organic microemulsions. These options will the primary approaches discussed.
Mechanical shearing and sonication represent the more crude and cost-effective approaches for the formation of irregularly shaped particles of reported 10–30 nm and 100–500 nm86,87. The subjugation of lignin to the intense mechanical shearing action of a
homogenizer results in the development of small size nanoparticles (diameter size - d.nm<100 nm), which were successfully included in blends to improve the properties of the matrix (figure 9a). The subsequent structural NMR analyses indicate small changes in chemical composition with no difference in molecular weight and dispersity.86 Similarly
was found for the sonication process. Structural, compositional, and morphological studies indicate that the ultrasonic irradiation can yield irregular nanoparticles of varying size as visualized through microscopy (figure 9b). However, much like mechanical shearing, reaction patterns, namely the depolymerization by side-chain cleavage, and repolymerization due to oxidative coupling were observed, causing a change in the structure and the degradation of phenyl coumaran, pinoresinol, and β-O-4 subunits. These unwanted reactions could be reduced by modifying the process parameters involved, and these resulting products can provide utility as additives in blends and composites for their thermal stability.87 However, contributions to mechanical properties
of materials are not significantly promising due to their irregular structure that causes the
material to behave anisotropically and heterogeneously throughout the composite product.87 Less damaging approaches can be found in the aerosol flow process, where
the atomized lignin droplets carried by N2 gas are injected in a laminar flow reactor that
evaporates the solvent through heat and form low-density dry particles of 30 nm to 2 μm applicable for pickering emulsions (figure 9c)88.
Figure 9. Lignin particles resulting from (a) mechanical shearing, (b) sonication, and (c) aerosol
flow86–88.
In the development of aqueous lignin dispersions through acid precipitation and anti-solvent precipitation, the particles of aggregated lignin in mediums would generally be stabilized electrostatically through the charges of the carboxy- and phenolate groups pointing into the aqueous solution. The acidification process of Gupta et al., in which ethylene glycol was used to dissolve Kraft lignin, and acid was added to induce supersaturation by overcoming the polymer-solvent interactions, driving the precipitation of lignin in the form of particles (figure 10a). These particles were reported to be of 30– 762 nm in size and were successfully infused with several compounds to be applied for anti-microbial functions as a more environmentally friendly alternative.89,90 Works of
literature report the use of different acids to achieve this result with more or less control over the size range; theoretically, multivalent ions could be employed to de-solvate the
a)
lignin91. Dioxane and alkali lignin (AL) spheres were produced through the same
theoretical principle by precipitating the macromolecules in an acetone/water mixture (9:1, v/v) into water, thus creating approximately 100 nm-sized colloids of broad-range distribution as visualized through field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) (figure 10b).92 Any broad distribution
observed was attributed to the competitive processes during which polymers may solidify before or after the solvent propagation reaches equilibrium. The formed products retained and exhibited a more prominent manifestation of the parent properties of lignin24. The
size-dependent properties, customarily attributed to the high surface area to volume ratio of nanomaterials, increases the value of the sustainable biomass-derived products for energy-based research, environmental, and medical purposes. The nano-scale size suggests further exploitation and application in various other fields93.
Figure 10. Kraft lignin particles formed through (a) the acidification process and (b) anti-solvent
precipitation with dioxane and water89,92.
Another known approach to colloid formation is to create aqueous emulsions of water-soluble lignin, such as lignosulfonate, cross-link the superficial molecules by means of interfacial polymerization, and remove the solvents to create hollow lignin spheres for encapsulation of valuable compounds22. On account of the selective polyaddition of
di-isocyanates in non-polar medium, the lignin units at the interface of the emulsions cross-linked with adjacent units, thus forming a solid shell that would remain post-solvent removal. Reported particle diameter sizes of 162–220 or 300–390 nm were found, depending on the medium the nanocarriers were held within, and temporal stability, i.e., few months, was observed in both aqueous and organic solvents (figure 11a). The particles were also shown to degrade through fungal enzymatic activity. The concept can be applied to Kraft lignin, as seen in Crestini et al.’s study. In this study, the dissolution of lignin in alkali media and its consequent irradiation with ultrasound formed aggregates of biomass and mixing oil droplets that would infuse into the particles formed through
aggregation23. The oil-infused hollow particles were then cross-linked at the surface to
produce stable lignin microcapsules of 0.3–1.1 μm (figure 11b). The introduced aliphatic -OHs confer greater amphiphilicity to lignin polymers, which then packed around the organic phase, much like a pickering emulsion, while the oxidative coupling of phenoxy radicals allowed for the polymerization to occur in this specified process. The increased cross-linked density from telechelic ethylene glycol-like groups improved the structural stability of the material, ensuring improved structural properties23.
a)
b)
Figure 11. (a) Lignosulfonate particles cross-linked with di-isocyanates and (b) Kraft lignin
Other minor methods were found, such as the formation of nanotubes through the polymerization of monolignols assisted by the catalytic activity of horseradish peroxide on the porous walls of alumina membranes and the latter’s subsequent dissolution in phosphoric acid to reveal final lignin macrostructures84.
As an alternative, one of the significant methods for the development of lignin colloids is through the simple self-assembly via solvent-exchange. Qian et al. report the chemical modification and self-assembly of alkali lignin into colloidal spheres of defined size as a consequence of the addition of the non-solvent water. The alkaline biomass was acetylated to change the hydrophilic hydroxyl groups into the more hydrophobic acetyl ester groups to increase the degree of hydrophobic interaction present in the structure. The increased hydrophobic character then allowed for colloid formation at a water content of 67 v/v%. With the low presence of water (0–13 v/v%), light scattering data indicates a lignin hydrodynamic radius, <Rh>, of 1 nm, which starts to increase at the H2O content of
13 v/v%. The addition of the aqueous medium was found to induce the formation of clusters of increasing <Rh> with hydrophobic skeletons of lignin chains aggregating as the core and the charged groups pointing outwards to the point at which colloids formed at the critical water concentration of 44 v/v%. The increase of <Rh> stops once the water content is 67 %, and tetrahydrofuran (THF) swollen hydrophobic cores with hydrophilic outer layers are observed by TEM (figure 12). The hydrodynamic radius can be shrunk down with greater volumes of water, and, with the full recycling of the non-polar solvent via rotary evaporation, the lowest <Rh> values are observed and defined spherical particles are created. The study reports particle stability in aqueous media ranging from acidic solutions to pH=12, the point at which acetylated colloidal lignin disaggregates from the removal of the charge neutralizing H+, and the electrostatic repulsive double-layer
Figure 12. Self-assembly of acetylated lignin in THF with increasing volume of water. TEM
visualization of colloidal particle formation at A) 0–13 v/v% H2O, B) 13–44 v/v% H2O, C) 44–67
v/v% H2O, and D) >67 v/v% H2O94.
The colloidalization of AL was scrutinized further through the efforts of Lievonen et al., whose study led to the development of spherical particles through a dialysis-mediated nucleation growth mechanism method without the need for acetylation of softwood lignin (figure 13a). In the process, there is a reduced use of organic solvents, and the final product is an environmentally friendly aqueous dispersion95. The process was optimized
further by Lievonen’s collaborators Lintinen et al., who have presented a proof-of-concept, low-energy demanding (55 MJ·kg-1 CLP), scalable method to produce a high
concentration of CLPs (up to 2.8 wt.%) for advanced material applications, with no significant difference in particle quality between the dialysis-mediated and the rapid direct micro emulsification. The colloid formation does not vastly differ from the last three mentioned approaches and includes the use of ethanol (EtOH) as an organic co-solvent and surfactant of lignin. The study elucidates a five-step, mixed batch-flow reactor, closed-cycle process for the formation of dry particles. At first, softwood lignin is shown dissolved into a solvent system consisting of THF:EtOH:water, in which ash and low molecular weight sulfur impurities can be removed by decanting the solution. The dissolved material is then introduced into excess water in one go (t<1 sec) to form colloidal lignin, THF, EtOH emulsions to be rotary evaporated for the full recovery and reuse of the organic solvents for the first step until dense lignin particles of 200 d.nm remain in the dispersion. In this step, the ethanol and lignin molecules behave as
surfactants that stabilize organic microemulsions (droplet diameter size ca. 600-700 d.nm) formed in the aqueous medium. In the rotary evaporation process, ethanol solvates the polymer chains enough to allow their rearrangement into a denser structure1. The
CLP dispersion is then ultra-filtrated with a regenerated cellulose membrane (150’000 g·mol-1) to concentrate the solution further and spray dried with 180 °C air to yield
sonication-redispersible dry micro-clusters.3 With Qian et al.’s acetylation of lignin, there
is an increase in hydrophobic interaction that, alongside the aromatic orbital interaction of phenolic groups, produces a particle with an <Rh> of approximately 106 ± 4 nm and a dispersity index of 0.022. The study reports stability at different pHs with the disaggregation of colloids at pH=12 due to the deprotonation of negatively charged groups, which leads to ACL particle-particle repulsion and consequential stability in media (figure 13b).
In Lievonen and Lintinen’s proposed methods, it was found possible to form stable colloidal dispersions of 2.8 wt.% lignin particle of ca. 200 d.nm depending on the concentration of lignin in the THF and EtOH without the need for acetylation. The CLP diameter was noted to be the lowest possible at the minimum concentration of 1 mg of lignin in 1 ml of H2O with an increasing d.nm until the concentration reached 20 mg·ml-1.
The dispersities of the different CLP samples ranged from 0.15 to 0.56, with no discernible trend until the lignin concentration reached 10 mg·ml-1. Non-uniform
dispersities are associated with high concentrations of lignin that lead to an uncontrolled
1 Unpublished hypothesis of Kalle Lintinen
nucleation growth mechanism. The size distribution is further affected in the case of rapid solvent exchange. A slow addition will result in particles of different sizes due to the changing microemulsion sizes and concentrations of lignin and solvents in water.3,95
For this experimental study, colloidal lignin particles developed from Lintinen et al. will be used, and their properties will be discussed in the following section.
3.4.3 Colloidal Lignin Particle Structure and Morphology
The theory presented within this section is based on unpublished work of Kalle Lintinen. Considering the formed shape, distribution of functional groups, and quality of lignin materials, it is important to note characteristics associated with colloidal dispersions, namely zeta-potential, stability in different conditions, and size and distribution.
The colloid of interest is structured by a hydrophobic core of lignin polymers, shielded from the aqueous medium by an anionically charged surface layer given by the ionizable groups present in lignin that are oriented outwards to stabilize the emulsion (figure 9). The size of the observed particles can vary and is dependent on the degree of interaction, concentration of the biomass in solution, and on the method of colloidalization involved. Concerning the proton concentration in the medium of the dispersion, these particles can persist at different pH levels even though different colloidal behaviors are observed. The significant point to be noted is that, at alkalinities proximal to pH ca.13, the particles start to dissolve, most likely due to dissociation of ionizable groups carboxylic and phenoxy- groups, leading to increased polar interaction with the aqueous medium and dissolution.95
Moreover and based on Lintinen’s unpublished data, the pH of CLP formation can affect the morphology of the spheres. At different pH levels for the colloidal lignin particle
formation, there are specific degrees of dissociation of the carboxylic groups into sodium carboxylates within the lignin polymers. If not neutralized, the ionizable groups will lead to the development of colloidal particles with rough and un-defined surfaces. In slightly acidic and in increasingly alkaline conditions (i.e. pH>4.64), the surface morphology is increasingly rougher and phase segregation within the colloid is visibly present with transmission electron microscopy. It is hypothesized that the high pH possibly breaks the particles into smaller colloidal spheres due to the high curvature of the emulsion droplet formed, causing the formation of rough-surfaced or smaller structures. Within the pH range 4.00–4.64, the phase segregation is less apparent as TEM analyses will display uniform non-segregated colloids. The still present carboxylates affect colloid behavior as seen with pronounced foaming during the removal of organic solvents during rotary evaporation. Analogously to higher pHs, the ionizable groups create nanoscopic hydrophilic regions that act as THF-(semi)impermeable barriers that induce violent foaming, akin to boiling over phenomena, when THF escapes the swollen cores. These particles may still be functional when rotary evaporated, but the solvent removal process then requires extensive time to be complete. Nonetheless, at pH<4.0, the concentrations of dissociated functionalities are low enough to prevent the influence ionized lignin units and guarantee a defined, smooth surface with negligible internal boundaries present in the colloid2.
Outside of these parameters, the sphere size appears to remain constant over time. However, the analysis of stability demonstrates changes depending on the presence of solvents, such as THF, and on the characteristics of the system the material is dispersed within. Factors such as salt concentration and pH have been documented to affect colloid quality and will be discussed in the following section.
3.4.4 Colloidal Stability
In accordance to the Derjaguin-Landau-Verwey-Overbeck (DLVO) theory of colloidal stability, the stability of the lignin dispersion in media is dependent on the net forces resulting from the balance of Lifshitz-van der Waals and electrostatic double layer
interactions, associated with the surfaces’ phenolic hydroxy and carboxyl groups. Highly negative potentials are associated with the repulsive forces associated with the surfaces’ phenolic hydroxy and carboxyl groups. These functional moieties confer the surface charge that is attracts counter-ions and creates a concentration gradient dictated by the distance from the charged surface. The ion cloud that forms around the CLPs is called the electrical double layer (EDL), composed of the Stern layer, the section with an excess of counter-ions that balances the surface potential, and the diffuse layer, which is comprised of a decreasing counter-ion and increasing anion concentration gradient the more distant from the particle surface. Within the diffuse section, the surrounding ions do not necessarily move alongside the Brownian motion of LPs. If distant enough from the core, the ions “slip off” the physically bound ions in the diffuse layer, and the boundary at which the phenomenon occurs is denoted by the ζ-potential (figure 10).96
The high negative charge of lignin creates an EDL that, when in contact with other particles, forms an overlap of concentrated counter-ions with little co-ions and increases the osmotic pressure present, which equilibrates the chemical potential in solution by diffusion. Moreover, these repulsions are balanced out by the attractive Lifshitz-van der
Figure 15. Electrical double layer of an anionically charged particle and difference in chemical
Waals (LW) forces, which represent the interactions between permanent dipoles (Keesom interactions), induced dipole moments (London interactions), and between each other (Debye interactions). The interactions in particles are distance dependent and can be described by the van der Waals force equation, which is balanced by the double layer forces (eq. 1-2):
𝐹!"#(𝐷) = − $!
%&'"
(#("
((#*(") (eq. 1)
𝐹,-,./01.2- "456-, -27,0(𝐷) ≈ 2𝜋𝜀8𝜀0𝑅𝜅𝜙&𝑒(9:') (eq. 2)
Where, in eq. 1, 𝐴; is the Hamaker constant, 𝐷 the distance between particles, and 𝑅% and 𝑅& are the radii of the particles. In eq. 2, 𝜀8 and 𝜀0 are the vacuum permittivity (or
dielectric constant) and the relative constant of a solvent, 𝜅 is the inverse of the Debye length, and 𝜙 is the surface potential.
The combination of interactions between EDL and LW forces is additive and describes the Derjaguin-Landau-Verwey-Overbeck (DLVO) theory of colloidal stability. However, there are many other possible surface interactions at play, specifically polymer-related forces, such as steric stabilizations and bridging flocculations, and minorly hydration forces, which are additional short range steric repulsions due to strongly bound water molecules present at high salt concentrations97. Given the dependence on the surface
charge, the ζ-potential, the electrical potential at the shear plane of a particle, is often a good indication of colloidal stability, and different ranges of stability are presented in the following table98:
Table 1. Stability of particles with varying zeta-potentials (adapted from Kumar et al.88)
Zeta-Potential Value (mV) Stability Behavior
0 to ±5 Flocculation or coagulation
±10 to ±30 Incipient instability
±30 to ±40 Moderate stability
±40 to ±60 Good stability
Both the colloidal dispersions from Lievonen et al. and Lintinen et al. are connotated by a mid-level to high negative ζ-potential when compared to other noted potentials of LPs, implying the good stability of the lignin dispersion3,95. Dialysis prepared particles are
denominated by ζ-potential of -60 mV, which allows for sufficient repulsion and stability in water solution over the course of 60 days95. Similar results were obtained by Lintinen
et al., who produced particles of similar size and characteristics, but with the ζ-potential of -40 mV, high lignin concentrations and the use of the emulsifier ethanol as co-surfactant for the formation of stable colloids. This negative value is comparatively high when considered alongside other LPs produced through different approaches. The majority of lignin dispersions developed fall within the range of -60 to -33 mV with stability over time up until 84 days, and exceptional values as high as -91.5 mV for potential have been documented at pH 1190,99–101. The stability for these other particles over time can
range depending on the pH sensitivity of the particles.
In Lievonen’s case, the absolute value of the CLPs’ potential decreases with the increase of ionic strength in solution as seen in accordance with the DLVO theory. With the increase of sodium ions in solution, the thickness of the EDL decreases to a point that LF interactions become dominant and cause attraction between particles, leading to aggregation, increased particle size and, ultimately, instability of the colloids. Stability up to 7 days has been noted for concentrations as high as 500 mM NaCl, but 1 M concentrations are enough to screen the EDL95. Other studies, up until 500 mM NaCl
concentrations excluded, observed potentials as low as -12 mV and no significant size change nor aggregation as a result of the aforementioned non-DLVO forces at play102.
The similar cases can be seen when observing the effects of pH outside the range 4–12. Within that range, ζ-potential did not noticeably change. In highly acidic solutions, instead, the potential decreased surmountedly due to the protonation of ionizable groups and the low repulsion from the thinning of the EDL, leading to visible aggregation. Alternately, high alkalinity resulted in the full dissociation of phenolic hydroxyl and carboxy-groups and the dissolution of the particles, as indicated by the reduced haziness and increase in coloring of solution. These phenomena are supported for most lignin particles (LPs) available including acetylated samples as well, where high pH favored the hydrolysis of acetyl ester groups and subsequent dissolution94.
