Emulsion Stabilization with Lignosulfonates

Abstract

Lignosulfonates are biobased surfactants and specialty chemicals. Due to their amphiphilic nature, they can be utilized in many technical applications, such as plasticizers, dispersants, stabilizers, and agrochemical formulations. Here, their ability to stabilize emulsions plays an important role. This chapter hence explains the fundamentals of emulsion stabilization with lignosulfonates. First, basic concepts are introduced along with the production and chemical make-up of lignosulfonates. Second, the interfacial activity is discussed. Parameters that affect interfacial activity and emulsion stabilization efficiency of lignosulfonates are furthermore treated. Such parameters may include salinity, pH, the presence of cosolvents and cosurfactants. Third, the underlying mechanisms of emulsion stabilization are outlined. The goal of this chapter is to introduce the reader to important fundamentals, and to draw the link between basic research and industrial practice.

1. Introduction

Lignosulfonates are biobased specialty chemicals, which are generated as a by-product during sulfite pulping of wood [1]. They are water-soluble polyelectrolytes, as they contain ionizable moieties such as sulfonate and carboxylic acid groups. Having preserved the polyaromatic backbone of lignin, they are functional amphiphiles with surfactant-like behavior. Due to their rich chemistry, lignosulfonates are found in many industrial applications [2]. This book chapter discusses the fundamentals of one of those applications, that is, emulsion stabilization. By explaining the working principles of interfacial adsorption and emulsion stabilization, the goal is to provide a reference for applied and fundamental research, as well as industry and potential end-users of lignosulfonates.

Next to cellulose, lignin is considered the second most abundant and important polymeric compound in plants [1]. Here, it fulfills functions such as water-proofing the cell-walls and adding to the mechanical cohesion and strength. While wood may contain around 20–30% lignin, its abundance in aquatic and herbaceous angiosperms is generally less. In contrast to α-cellulose, lignin is an amorphous material with a randomly branched structure. This non-uniformity can make technical utilization challenging, as reactivity and availability of functional groups may vary [3]. Still, some applications may benefit from polydispersity. For specialty chemicals such as surfactants, it is the function or the effect that counts. Salinity changes may induce precipitation of an ionic surfactant. If the surfactant is polydisperse, precipitation may be limited to only a fraction of it. The remaining surfactant can then still perform the desired function. Moreover, mixtures of dissimilar surfactants can form synergies, where the performance may be superior over its individual components [4]. The application of lignin as specialty chemicals can thus benefit from its inherent characteristics, which includes its polydispersity. It is therefore not surprising that dispersants and surfactants are currently the dominant value-added applications of technical lignin.

It is frequently stated 50–70 million tons of technical lignin are produced per year, of which only 1–2% of are utilized for value-added products [5]. The majority is burned for heat and chemicals recovery in Kraft mills [6]. More than 1 million tons of lignosulfonates are produced annually [7], hence dominating the market of technical lignin. In contrast to many other lignin products, lignosulfonates are readily water-soluble. Their most common application is dispersants [8], for example as concrete plasticizers, drilling mud thinners, coal-slurry or dye dispersants [9]. Roughly 50% of the annual production of lignosulfonates were used for concrete admixtures in 1999 [10]. Other applications include dust binders, chelating and complexing agents, soil conditioning agents, floatation agents, and water-in-oil emulsion stabilizers [8, 9, 11, 12]. As the effect on interfacial tension is limited, high shear is usually required during emulsification [13]. Still, lignosulfonates can yield emulsions that are stable over months and longer, finding application for example in agrochemical formulations [8]. Other potential areas with less commercial importance include corrosion and scale inhibitors, CO₂ flooding and enhanced oil recovery, as well as polymer precursors and additives [2, 14, 15, 16, 17, 18, 19, 20].

Great importance is attributed to biopolymers these days, as they are inherently renewable and largely biodegradable. By substituting fossil-based polymers with biopolymers, the transition to more sustainable technologies is furthered. Expanding the use of lignosulfonates is therefore an important piece in the puzzle. Compared to the global production of surfactants or pulp and paper products, lignosulfonates are currently a niche application. There is hence a potential for growth, in particular for applications that involve liquid–liquid interfaces.

2. Fundamentals

2.1 Definitions and distinctions

The term lignin is used for the natural lignin as found in lignocellulosic biomass. It is also referred to as pristine lignin, as no treatment or modification has been done. Chemically speaking, lignin is a biopolymer consisting of the monolignol units sinapyl alcohol (S), coniferyl alcohol (G), and p-coumaryl alcohol (H) [21], which are connected by various oxygen- and carbon–carbon linkages. Lignin has been described as a polyaromatic and randomly branched biopolymer.

Technical lignin refers to the lignin-rich product obtained from biomass separation processes. While pristine lignin is a virtually “infinite” network [22], technical lignin is a fragmentated version thereof. The molecular weight, composition, and ratio of functional groups of technical lignin are hence different. Their abundance can be affected by parameters such as biomass feedstock, separation process, and purification steps. Technical lignin usually has a purity of at least 70% (Klason lignin + acid soluble lignin), with commercial products being closer to 85–100% of lignin per ash-free dry matter.

Pulping of lignocellulose biomass is conducted to obtain a fibrous material. Approximately 90% of the global pulp products are made from wood, whereas 10% originate from annual plants [1]. Common end-products include paper, cardboard, molded pulp, and specialty cellulose. Mechanical pulping applies force in a refiner to defibrate the feedstock into fibers and fibrils. Chemical pulping dissolves the lignin and other substances to liberate the cellulose fibers. Industrial processes can also be based on a combination of mechanical and chemical treatments. Today, technical lignin most commonly originates from chemical pulping.

A biorefinery is defined as “the sustainable processing of biomass into a spectrum of marketable products (food, feed, materials, chemicals) and energy (fuels, power, heat)” [23]. In other words, a biorefinery separates biomass into useful materials, chemicals, and energy. First generation biorefineries differ from second generation biorefineries by the choice of input material, i.e., food crops (first generation) instead of non-food crops (second generation) [24]. Pulping of wood is thus defined as a lignocellulosic crop, second generation biorefinery.

An overview of existing biomass separation processes with the resulting lignin product is given in Figure 1. Technical lignin has traditionally been obtained by chemical pulping, i.e., Kraft, sulfite, or soda pulping. Organosolv pulping is a more recent invention, which utilizes organic solvents and often acid catalysis for delignification [26]. The original goal of organosolv pulping was to obtain a fiber material. Still, this process has also attracted interest for biorefinery applications, which subsequently process the cellulose for chemical utilization [27]. The production of technical lignin can hence be conducted as part of a pulping processes, but it is not limited to that. An example for a non-pulping type lignin would be hydrolysis lignin, which is produced in biorefinery sugar platforms. Here, the cellulose and hemicellulose are hydrolyzed to extract sugar monomers and oligomers. The sugar can further be converted to higher value products, for example ethanol or furfural. The residual solids are rich in lignin and therefore termed hydrolysis lignin. Residual cellulose and other impurities can impart poorer performance and hence lower value on this lignin type [3, 28]. Still, recent developments have yielded increased purity and reactivity, e.g., the Cellunolix^® lignin by St1 (Finnland) [29] or the Lignova™ lignin by Fibenol (Estonia) [30]. Because of these developments, it can be expected that hydrolysis lignin may play a larger role in the future. Steam explosion lignin is viewed as a separate category to hydrolysis lignin by some authors [3, 31]. Yet, both lignin types are similar in composition and the treatments are sometimes even cascaded. Steam explosion treatment subjects the substrate to steam at elevated temperature (ca. 160–280°C) and pressure (ca. 7–48 bar) [32]. This induces biomass disintegration and partial hydrolysis. Due to this, it will be considered as part of hydrolysis lignin, as shown in Figure 1. Hydrolysis lignins can further be subdivided into acid hydrolysis lignin and enzymatic hydrolysis lignin, with the latter being more reactive due to a lower degree of condensation [3]. Recent advances have also yielded novel processes, which are still in an early stage of their development. Ionic liquids have been demonstrated to function as lignin solvents [33]. Combined with other treatments, ionic liquids can yield new products and conversion routes with promising features; however, some economic and technical challenges still need to be addressed [34]. Another recent invention is the use of supercritical solvents, e.g., supercritical water yielding products such as Aquasolv lignin [35].

Figure 1.

Lignin extraction processes and their dominant products. Modified from Laurichesse and Avérous [25].

Lignosulfonates are traditionally extracted from sulfite spent liquors. Sulfite pulping is usually operated at low pH, but neutral or alkaline sulfite pulping have also been developed [1]. Sulfite liquors typically contain about 50–80% lignosulfonates, 30% hemicellulose and 10% inorganics per dry matter content [5]. Purification is often conducted by membrane filtration, removing low molecular weight components such as sugars. Commercial grade lignosulfonates are available in both purified and unpurified qualities. As a result of the pulping conditions, they also tend to exhibit a higher degree of condensation than, e.g., soda or organosolv lignin [3].

Sulfonated lignin is produced by chemical modification of lignin separated by a process other than sulfite pulping [2]. For example, MeadWestvaco produces Kraft lignin sulfonated with sulfite salts and an aldehyde, e.g., formaldehyde [5]. A variety of modification processes exist and the differences between lignosulfonates and sulfonated lignin can be marginal.

The counterion is the ion accompanying a second ionic species to maintain charge neutrality. Lignosulfonates contain covalently bond anionic moieties, which most importantly include sulfonate and carboxyl groups. A counter ion with positive charge is hence necessary for charge neutrality. This counterion is frequently a remnant of the pulping process, i.e., pulping with sodium, calcium or magnesium bisulfite will yield sodium, calcium, or magnesium lignosulfonates, respectively. Ion-exchange may also be conducted to replace the counterion. Protonation is furthermore possible, i.e., hydrogen as the counter ion. However, lignosulfonate dispersants are usually the salt of lignosulfonic acid, as this yields a more moderate pH and improves water-solubility.

A surfactant, i.e., surface-active agent, is a compound that can lower the surface or interfacial tension of a liquid in contact with another phase [36]. This effect is usually accompanied by adsorption at the surface or interface, i.e., enrichment at the phase boundary [37]. Surfactants usually contain hydrophilic and lipophilic moieties, which facilitate interfacial adsorption. Lignosulfonates have been shown to reduce the surface tension of water [38] and can hence be classified as surfactants.

Surface tension can be observed as the tendency of a liquid surface to assume the smallest possible surface area. It is linked to the intermolecular attraction forces within the liquid and is commonly denoted as force per unit length or energy per unit area. Dispersing a liquid with high surface tension hence requires more energy than dispersing a liquid with low surface tension. In this chapter, the surface tension will be used when discussing liquid–gas phase boundaries.

The interfacial tension is the equivalent to the surface tension at liquid–liquid interfaces. A system with low interfacial tension requires less work for emulsification than a system with high interfacial tension.

2.2 Production

Lignosulfonates are produced as a by-product during sulfite pulping of wood [1]. Sulfite pulping is a long-established method for producing cellulosic fibers [39]. The process usually operates at low pH, utilizing sulfite or bisulfite salts to soften and remove the lignin. The lignin undergoes a number of reactions, which include [1]:

• Hydrolysis

• Sulfonation

• Sulfitolysis

• Dissolution

• Degradation

• Condensation

During hydrolysis, the lignin-carbohydrate and, to a smaller extent, lignin-lignin linkages are broken down [1]. Sulfonation introduces sulfonate groups onto the lignin. When present in sufficient amounts, the sulfonate groups render the lignin water-soluble, which facilitates dissolution in the cooking liquor. Sulfitolysis and degradation may further reduce the molecular weight, whereas condensation reactions increase it. Condensation counteracts delignification by forming new carbon–carbon bonds [1].

Natural lignin is synthesized from the three monolignols, i.e., sinapyl alcohol (S), coniferyl alcohol (G), and p-coumaryl alcohol (H) units [21]. Coniferous lignin (softwood lignin) is composed mainly of G-units, whereas lignin from broad-leaved trees (hardwood lignin) contains a mixture of G- and S-units [40]. Lignin from annual plants also contains H-units in addition to G- and S-units [41]. The feedstock therefore affects the composition of the resulting lignosulfonates, as the three monolignols vary in their methoxy content and potential branching. The feedstock can furthermore affect the pulping process, since sulfonation of hardwood lignin is slower than of softwood lignin [1]. Moreover, softwood lignin is more prone to condensation reactions, resulting in higher molecular weight as compared to hardwood lignosulfonates [42]. An overview of the monolignols and a schematic of softwood lignin is given in Figure 2.

Figure 2.

Primary lignin monomers and their corresponding units (left) [25]. Schematic structure of softwood lignin (right) [43].

While alkali lignin can be separated by acid precipitation, this approach is not feasible for lignosulfonates. Instead, membrane filtration (ultrafiltration) is often conducted to purify the sulfite liquor [5]. Challenges can arise due to overlapping molecular weight of the lignosulfonates and dissolved hemicellulose. One approach is to cascade a series of membranes with different cut-off molecular weights [44]. An alternative is given by the Howard process [5], which uses lime (calcium oxide) to precipitate the lignosulfonate above pH 12 [45]. The precipitated solids can be separated mechanically and washed to improve the purity. Other potential approaches include amine extraction, electrolysis, ion-exchange resins, the Pekilo process (fermentation and ultrafiltration), and reverse osmosis [5].

Several factors affect the composition of lignosulfonates, which include but are not limited to:

• The choice of raw material, e.g., softwood or hardwood, affecting the monolignol composition

• The choice of pulping chemicals

• The pulping conditions

• Recovery and purification

• Post-treatment and chemical modification

The composition of lignosulfonates is inherently linked to its characteristics and behavior, and hence to the performance in technical applications [2].

2.3 Structure and composition

As discussed in the previous chapter, the structure and composition of lignosulfonates is strongly dependent on their origin and production. Still, there are certain characteristics that are worth discussing.

Due to their structure and monomeric configuration, cellulose and hemicellulose contain a higher percentage of oxygen than lignin. Lignin furthermore has the highest carbon content of these three biopolymers. A unique feature of lignosulfonates is their elevated sulfur content, which arises from the pulping process. Table 1 lists a range of elementary composition, as published in current literature. It should be mentioned that these values should only be taken as indicators, since individual samples may be different. In particular, the ash content may exhibit values up to 40 wt.%. Still, an elevated oxygen content and lower carbon content can in theory indicate lower purity, as the contributions from cellulose and hemicellulose would be greater.

Parameter	Approximate content in wt.%	Reference
Dry matter	94–96	[46]
Ash	4–10	[3, 47]
Carbon	29–47	[48, 49, 50]
Hydrogen	5	[48, 50]
Oxygen	37–54	[48, 49, 50]
Sulfur	2–10	[46]
Nitrogen	0.02	[3]

Table 1.

Approximate elementary composition of lignosulfonates.

The chemical structure of lignosulfonates mirrors that of both native lignin and the modifications done during sulfite pulping. As such, the skeletal configuration of lignin is preserved to some extent, while new carbon–carbon linkages have been formed. In addition, sulfonate and carboxylic acid groups were added, which are not found in native lignin to this extent. Generic structure models of lignosulfonates are shown in Figure 3. It is important to note that these are only approximate models, as lignosulfonates are a polydisperse mixture of many different macromolecules. The structure hence varies not only between different lignosulfonates, but also within a given sample. In other words, lignosulfonates should be considered statistical entities rather than classical chemical compounds [53].

Figure 3.

Generic (simplified) structure of lignosulfonates according to Kun and Pukanszky [51] (left), and Fiorani et al. [52] (right).

The chemical moieties of lignosulfonates can be categorized into ionizable, polar and non-polar groups [2]. Ionizable groups include sulfonate, carboxylic acid, and phenolic hydroxyl groups. At a neutral pH of 7, mainly the first two can be considered dissociated (ionized) and hence hydrophilic. Phenolic hydroxyl groups are usually ionized at around pH 9–10 [54, 55], but values extending to pH 5–12 have been reported [56]. Polar groups include various oxygen containing moieties due to the higher dipole moment of oxygen–carbon and oxygen-hydrogen linkages. These include ketones, aldehydes, and methoxy groups. Despite being polar, these groups are not considered operative hydrophilic groups [57]. Aliphatic hydroxyl and ether groups are also present to a great extent in lignosulfonates. These can be intrinsically hydrophilic; however, their functionality is determined by the surrounding molecular structure [57, 58, 59]. Because of this, the water-solubility of lignosulfonates arises from the presence of ionizable groups, i.e., sulfonate and carboxyl groups at neutral pH. Nonpolar groups include aromatic and aliphatic units, as found in the skeletal configuration of lignin. An overview of the common functional groups and linkages is given in Figure 4. This overview does not include additional functionalities, which may be grafted onto the lignosulfonates, e.g., by phosphorylation, alkylation, sulfobutylation or silylation [60].

Figure 4.

Commonly encountered chemical bounds and functional groups in technical lignin. Image taken from [2].

The abundance of functional groups can be of interest for two reasons. Firstly, this is an important parameter for chemical modification, as specific functional groups may be targeted. Secondly, the physicochemical properties of lignosulfonates are highly dependent on their composition. Hydrophobic interaction chromatography (HIC) has played an important role in characterizing lignosulfonates recently [61, 62, 63]. Based on this technique, the charge-to-size ratio was reportedly lower for more hydrophobic lignosulfonates [64]. Better performance as suspension or emulsion stabilizer was furthermore seen for more hydrophobic lignosulfonates [13, 63]. This example illustrates, how the abundance of functional groups may impact the performance in technical applications.

Compared to other technical lignins, lignosulfonates tend to exhibit a lower amount of phenolic hydroxyl groups [47]. While the sulfonate group is a distinct feature of lignosulfonates, the abundance of carboxylic acid groups is comparable to that of other technical lignins. A recent study was performed aqueous carbon black dispersions stabilized by sodium lignosulfonate, which also listed the composition of six commercial samples [46]. The values are summarized in Table 2. As all samples were of softwood origin, a higher methoxy content can be expected for hardwood lignosulfonates due to the monolignol composition.

Parameter	Approximate range
Mw in g/mol	1800–4000
Mn in g/mol	6000–330,000
Polydispersity index	3–83
Organic sulfur (wt.%)	2.1–9.4
Carboxyl groups (wt.%)	8.3–14.9
Phenolic hydroxyl groups (wt.%)	1.3–2.0
Methoxy groups (wt.%)	7.3–15.4

Table 2.

Abundance of functional groups of softwood sodium lignosulfonates according to Subramanian et al. [46].

Table 2 also lists the number-average (M_n) and mass-average molecular weight (M_w). A polydispersity index of up to 83 accounts for a great variety in molecular mass, which is characteristic for lignin from sulfite pulping. The molecular weight can have several implications on the properties of lignosulfonates. It is naturally linked to the diffusion coefficient [65], which will further affect interfacial adsorption and related phenomena [13, 66]. Research has shown that the degree of sulfonation decreases with increasing molecular weight [67]. Stearic screening can occur in lignosulfonates with high molecular weight [3]. This effect was used to explain, e.g., the lower effect on zeta potential by shielding ionizable groups [68] or the lower reactivity by screening of phenolic hydroxyl groups [3]. Overall, the molecular weight of lignosulfonates may span from less than 1000 g/mol to more than 400,000 g/mol [42, 67]. The molecular weight ranges are thus higher than for other technical lignins [2]. In comparison, values of (1000–15,000 g/mol) have been reported for soda lignin, 1500–25,000 g/mol for Kraft lignin, and 500–5000 g/mol for organosolv lignin [3]. Acid hydrolysis lignin is closer to lignosulfonates in terms of molecular weight, where values of 1500–50,000 g/mol have been reported [69]. The molecular weight distribution of various lignosulfonates is illustrated in Figure 5. As can be seen, the molecular weight of the hardwood samples was consistently lower than that of softwood lignosulfonates. This difference is likely the result of the monolignol configuration, which can further affect the sulfite pulping process.

Figure 5.

Molecular weight distribution of lignosulfonates originating from spruce [LS1 (♦), LS2 (▀) and LS3 (+)], a spruce-birch blend [LS5 (▲)], aspen [LS6 (*)], and eucalyptus [LS7 (●) and LS8 (×)]. Image taken from [42].

3. Adsorption on surfaces and interfaces

3.1 Adsorption

It is generally agreed that adsorption of lignosulfonates follows the Langmuir isotherm [70, 71, 72, 73]. The according framework was first published by Irving Langmuir in 1916, which described the adsorption of gasses on solids [74]. A common expression of the Langmuir equation is given in Eq. (1), were $\mathrm{\Gamma }$ corresponds to the amount of adsorbed species, ${\mathrm{\Gamma }}_{\max }$ is the maximum amount adsorbed, $c$ is the concentration of adsorbing species in bulk, and $K$ is the Langmuir equilibrium constant.

$$\mathrm{\Gamma }=\frac{{\mathrm{\Gamma }}_{\max }\mathit{Kc}}{1+\mathit{Kc}}$$E1

The parameters ${\mathrm{\Gamma }}_{\max }$ and $K$ can be determined via mass-balancing, i.e., measuring the adsorbed amount in dependence of concentration. This approach is simple for adsorption on solid surfaces, as the amount of non-adsorbing surfactant is equivalent to the remaining bulk concentration. An example for the adsorption of lignosulfonates on carbon black is given in Figure 6. The characteristic behavior accompanying Langmuir adsorption isotherms is a sharp increase of adsorbed amount at low concentrations, whereas higher concentrations yield a plateau.

Figure 6.

Adsorption isotherm of lignosulfonate on 1 wt% carbon black along with Langmuir isotherm fitting. Image taken from [46].

Determination of the Langmuir equation parameters to liquid–liquid systems is less straight forward than for liquid–solid dispersions. Emulsification is accompanied by an increase in specific surface area, which can be difficult to quantify. The surface or interfacial tension can be used instead of mass-balancing. Langmuir type adsorption implies the applicability of the Szyszkowski equation, as given in Eq. (2). Here, the surface pressure $\mathrm{\Pi }$ is calculated as the difference of surface or interfacial tension without surfactant (${\gamma }_0$) and at concentration $c$ ($\gamma \left(c\right)$). The equation further involves the temperature $T$, the ideal gas constant $R$, and the constant $n$.

$$\mathrm{\Pi }={\gamma }_0-\gamma \left(c\right)=n\mathrm{RT}{\mathrm{\Gamma }}_{m}ln\left(1+\mathit{Kc}\right)$$E2

The surface excess ${\mathrm{\Gamma }}_m$ can further be determined using the Gibbs adsorption isotherm. As stated in Eq. (3), this framework relies on the linear-logarithmic progression of surface tension $\gamma$ with concentration $c$. It is important to note that the linear-logarithmic regime does not span over the entire concentration range. At low surfactant concentrations, the surface or interfacial tension approaches the surfactant-free case ${\gamma }_0$ asymptotically. At high lignosulfonate concentrations, agglomeration and other effects will decrease the observed slope [38, 75]. A more detailed discussion of the surface and interfacial tension of lignosulfonates is given in Section 3.2.

$${\mathrm{\Gamma }}_m=-\frac{1}{n\mathrm{RT}}\left(\frac{\partial \gamma }{\partial lnc}\right)$$E3

Assumptions can furthermore be made to determine the constant $n$. In case of strong surfactant electrolytes, such as $R-O-S{O}_3^{-}{\mathit{Na}}^{+}$, both the surfactant ion and the counterion must absorb, yielding a value of $n=2$ [76]. A value of $n=1$ corresponds to non-ionic surfactants or ionic surfactants in presence of high concentrations of an indifferent electrolyte, such as NaCl. Both assumptions have been applied to lignosulfonates and were found to yield conclusive results [13, 66].

By applying the above framework, other information can also be extracted from surface and interfacial tension measurements. For example, the surface excess ${\mathrm{\Gamma }}_m$ can be used to calculate the area per molecule, as stated in Eq. (4) [76]. Here, $N_{\mathrm{av}}$ denotes the Avogadro’s constant $6.022\times {10}^{23}\mathrm{mo}{\mathrm{l}}^{-1}$.

$${\mathrm{A}}_{\mathrm{m}}=\frac{1}{{\mathrm{\Gamma }}_m{N}_{\mathrm{av}}}$$E4

Several authors have studied the adsorption kinetics of lignosulfonates on solid surfaces [71, 72, 77, 78, 79, 80]. The reports generally agree that pseudo second-order kinetics provided the best fit. Such kinetic can be expressed as written in Eq. (5), where $q_t$ is the adsorbed amount at time $t$, $q_e$ is the amount adsorbed at equilibrium, and $k$ is the rate constant.

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{k{q}_e^2}+\frac{t}{q_e}$$E5

Eq. (5) can be fitted to experimental data by plotting $\frac{t}{q_t}$ against $t$, yielding $\frac{1}{q_e}$ as the slope and $\frac{1}{k{q}_e^2}$ as the intersection. This model is, however, limited to adsorption from liquid onto solid phase. Kinetic modeling of lignosulfonate adsorption at liquid–liquid interfaces has also been attempted; yet, the results indicated that water–oil adsorption was not diffusion-controlled [66]. It usually takes hours or days to attain an equilibrium state at the liquid-air or liquid–liquid interface [66, 81, 82]. This time span is greater than for simple monodisperse surfactants. Different explanations have been given for this behavior. The lignosulfonate can undergo diffusion exchange at the interface, replacing molecules with a lower diffusion coefficient but higher effect on interfacial tension. In addition, the individual macromolecules may be subject to rearrangement, both with respect to the interface and to each other [2]. Such realignment of conformation has been described, e.g., for petroleum asphaltenes [83, 84]. Both lignosulfonates and asphaltenes share common characteristics, e.g., both are polybranched, exhibit a tendency for self-association, and require overnight storage, as the emulsions would be less stable if processed immediately after emulsification [13, 85]. Analogies have therefore been drawn between these two species in terms of their interfacial behavior [2, 66].

3.2 Effect on surface and interfacial tension

Measuring surface tension is a useful tool, e.g., for assessing the behavior of lignosulfonates in solution or their interactions with other components [38, 86]. In addition, the interfacial tension provides a measure for the ease of emulsification [13]. The effect on surface or interfacial tension is related to the amphiphilic property of lignosulfonates, containing both hydrophilic and hydrophobic moieties. From a molecular point of view, the lignosulfonate macromolecule can attain a lower state of energy by extending its hydrophilic moieties into the water, while facing the hydrophobic parts away from it. The decrease in surface tension at increasing surfactant concentration can be explained by different mechanisms. Firstly, adsorption and desorption are occurring simultaneously, where an equilibrium is attained if the adsorption rate is equal and opposite to the desorption rate. By increasing the surfactant concentration, it is statistically more likely for a surfactant molecule to be in the right position and conformation to undergo adsorption. This would hence drive the adsorption–desorption equilibrium towards a higher surface coverage. Secondly, lignosulfonates are polyelectrolytes. By increasing the lignosulfonate concentration, the electrolyte concentration is also increased. As has been shown by small-angle X-ray scattering (SAXS), the effective surface charge of lignosulfonates decreases at increasing concentration [87]. Higher concentrations of a common ion indeed enhance effects such as counterion condensation, charge screening, and the dissociation equilibrium [88]. Water-solubility is ensured by the ionic groups of lignosulfonates, so a lower effective charge would have destabilizing effect. At a lower water-solubility, the equilibrium would hence be shifted to increased surface or interface adsorption. This effect concurs with the observations that lignosulfonates can be precipitated by salt addition (salting-out) [89] and that emulsion stabilization is highest, if the stabilizers are on the verge of precipitation [90].

Lignosulfonate adsorption at aqueous surfaces is evidenced, among others, by a decrease in surface tension [13, 38, 91]. Within a certain range, this decrease follows a linear-logarithmic progression with concentration [13, 38]. Below this range, the surface tension of the pure liquid is approached asymptotically. At high lignosulfonate concentrations, the slope of the surface tension decreases. Some authors have explained this behavior with the onset of lignosulfonate aggregation [75, 92], but other effects cannot be ruled out. Measurements of the surface tension with respect to concentration are shown in Figure 7.

Figure 7.

Equilibrium surface tension in dependence of surfactant concentration. Comparison of lignosulfonates (LS) with polyelectrolyte polymers (left) [38] or with the surfactants dodecyl benzenesulfonate (DBS), nonylphenyl polyoxyethylene glycol (PONP) and polysorbate 20 (PS20) (right) [92].

In comparison with commercial surfactants, the effect on surface or interfacial tension is often less at the same mass concentration [75, 82, 92]. However, in comparison with other polyelectrolyte polymers, the effect on surface tension can be considerably higher [38]. This circumstance is likely related to the chemical composition and structure, which define macroscopic properties such as solubility, hydrophilic–lipophilic balance (HLB), or surface coverage.

3.3 Parameters affecting the surface activity of lignosulfonates

There are several external factors, which govern the surface activity of lignosulfonates, in addition to the intrinsic properties such as composition and abundance of functional groups. These factors include, but are not limited to:

• Salinity

• pH

• Temperature

• Co-solvents in the aqueous phase

• Oil phase composition

It has been shown that increasing salinity with simple monovalent electrolytes, such as NaCl, can have the same effect as increasing lignosulfonate concentration [38, 66]. The effect of NaCl concentration on surface or interfacial tension is illustrated in Figure 8, which is marked by same linear-logarithmic progression as in Figure 7. As discussed earlier, increasing the concentration of a common ion facilitates a lower effective surface charge, which further enhances surface or interface adsorption. The similarities in observed surface tension hence corroborate that the surface adsorption of lignosulfonates is also driven by salinity. It could further be argued that outside the tested concentration range in Figure 8, the same slope-decrease or plateau would be visible is in Figure 7. While this may be plausible, it has yet to be demonstrated experimentally.

Figure 8.

Effect of NaCl concentration on the surface tension of 0.01 wt.% lignosulfonate in water (left) [38]. Effect of NaCl or lignosulfonate concentration on the tension of the water-xylene interface (right) [66].

However, the data exhibited in Figures 7 and 8 represents a simplified case. In both instances, the counterion was solely the monovalent sodium ion, as only sodium lignosulfonate and NaCl were used. In presence of multivalent cations, the surface phenomena of lignosulfonates are more complex. Di- or trivalent cations, for example, were shown to induce interface gelling [66]. As illustrated in Figure 9, such interfaces behave inelastically, exhibiting wrinkles and cracks upon deformation. Pendant drop tensiometry relies on the applicability of the Young-Laplace equation. Since this is not the case for inelastic interfaces, the technique fails to predict accurate interfacial tension. Similar challenges are evident for other techniques, such as the Du Noüy ring method or spinning drop tensiometry.

Figure 9.

Droplet retraction in pendant drop video-tensiometry. In incompressible interface layer is visible as wrinkling of the droplet surface, which was formed by sodium lignosulfonate in presence of CaCl₂. Image taken from [66].

Lowering the pH can further enhance the effect of lignosulfonates on surface tension [91]. This circumstance is in analogy to the salting-out effect of simple electrolytes. Phenolic moieties are said to ionize at around pH 9–10, while the carboxylic acid groups ionize at pH 3–4 [54, 55]. A lower pH can hence reduce the total charge of the lignosulfonate macromolecules. This would further reduce the water-solubility and thereby drive the equilibrium towards higher adsorption. However, the discussed effect is valid only if the lignosulfonates remain water-soluble. If precipitation is evident, then the bulk concentration would decrease, which would also reduce surface and interface adsorption.

As depicted in Figure 10, increasing the temperature can also increase the effect of lignosulfonates on surface tension. At higher temperature, the hydrodynamic radius of lignosulfonates decreases [93]. Thermodynamically speaking, the entropy is higher at elevated temperatures. This can enable a larger number of possible conformations, thus reducing the average molecular dimensions [88]. As a result, lignosulfonate aggregation and a reduction of zeta potential have been reported at elevated temperature [94]. These two effects indicate solution-destabilization, which could further promote surface adsorption.

Figure 10.

Surface tension of water and 1 g/l lignosulfonate in water. Image taken from [93].

Co-solvents in the aqueous phase can alter characteristics such as the solution parameter and the dielectric constant. This will inevitably also affect the surface behavior of the lignosulfonates. The addition of low molecular weight alcohols yielded a decrease of surface charge and slight interparticle association [87]. The effect was thereby similar as to increasing the ionic strength, implying solution destabilization of lignosulfonates by alcohol addition. On the other hand, hydrophobic interaction chromatography uses water/ethanol and water/2-propanol mixtures to eluate the more hydrophobic fractions [64], which would suggest that water/alcohol mixtures are a better solvent for these. This is corroborated by the fact that the surface pressure decreased due to ethanol addition [81], i.e. fewer lignosulfonate molecules would enter the interface. Corresponding measurements of interfacial tension are shown in Figure 10. Overall, the effects of adding alcohols are hence counteracting, i.e., reduction of effective charge (destabilizing) and solubility improvement (stabilizing). It is thus not surprising that the emulsion stability could be both improved and reduced after alcohol addition (Figure 11) [81].

Figure 11.

Tension of the mineral oil–water interface in dependence of ethanol concentration. The samples contained 1 g/l lignosulfonate (LS) and 20 mM NaCl as background electrolyte. Figure taken from [81].

The addition of non-solvents to water will eventually lead to lignosulfonate precipitation [87]. In this case, the interfacial activity would also decrease, as the bulk concentration is lower [66]. Solvent shifting can be useful for the production of functional micro- and nanoparticles from lignin [95]. However, the technology bears limited importance for this chapter, as the resulting Pickering-emulsions tend to be less stable [66].

At last, the oil phase composition can also affect the interfacial activity of lignin. A simple explanation for this can be based on the concept of the hydrophilic–lipophilic balance (HLB). Surfactants can generally be classified according to their HLB, where lower HLB values account for more lipophilic surfactants and high values for more hydrophilic ones. It is generally agreed that an HLB of 3–6 is characteristic for water-in-oil (W/O) emulsifiers, whereas 8–18 are suited for oil-in-water (O/W) emulsifiers [76]. According to the HLB model, optimum emulsion stability is given, when a surfactant-blend matches the HLB value of the emulsified liquids. In analogy to that, the interfacial tension is also said to exhibit a minimum at the optimal HLB of the surfactant mixture. The concept is illustrated in Figure 12.

Figure 12.

Variation of emulsion stability, droplet size, and interfacial tension with percentage of surfactant with high HLB. Figure reproduced from [76].

Oils with low polarity, such as paraffins and mineral oil, tend to have a required HLF for O/W emulsions of 9–11, whereas more polar oils are in the range of 12–17, e.g., oleic acid, chlorinated paraffins, and aromatics such as toluene [96]. The HLB scale can hence also be an indicator for the compatibility or affinity of surfactants towards a certain oil phase. Lignosulfonates were reported to have an HLB of 11.6 based on their composition [97], but the effective HLB is likely higher than this, as the cited value is a rough estimation based on the elementary composition. Among others, it does not consider stearic effects. In addition, the contribution of the sulfonate group to the effective HLB is generally high. There are few studies comparing the stability of different oils emulsified with lignosulfonates. Experience shows that aromatic solvents, e.g., toluene or xylene (HLB = 14–15), tend yield more stable than emulsions with mineral oil (HLB = 10) [13]. Accordingly, it was demonstrated that the effect on interfacial tension of xylene was greater than on mineral oil, as plotted in Figure 12. It would only make sense that lignosulfonates, comprising a polyaromatic structure, also have a higher affinity to aromatic oils than to paraffinic ones. Based on their behavior, lignosulfonates can be classified as O/W emulsion stabilizers with an effective HLB of 10–18 (Figure 13).

Figure 13.

Effect of molecular weight and oil phase on interfacial tension. Figure taken from [13].

3.4 Interactions with other surfactants

The presence of other surfactants can affect the surface activity of lignosulfonate. Surfactants can be distinguished based on their charge, i.e., non-ionic, anionic, cationic, and zwitterionic surfactants. Another classification would be based on the structure, that is, polymeric and non-polymeric surfactants. Electrostatic interactions with lignosulfonates are eminent, if the co-surfactant carries a charge. Surfactant interactions will hence be discussed for each individual type in this sub-chapter.

Interactions with non-ionic surfactants can be due to effects such as hydrogen bonding or hydrophobic interactions. Straight-chain alcohols can exhibit surfactant-like properties, if the chain length is at least four carbon atoms or more [98]. The effect of these was studied by Qiu et al., who concluded that alcohols can improve the surface activity of lignosulfonate [86]. The authors attributed the largest effect to alcohols with a chain-length of at least 10 carbon atoms, which was evidence by an increase in zeta potential of TiO₂ particles. Such behavior would suggest cooperative adsorption. Still, the author conducted experiments at a constant alcohol/lignosulfonate ratio, which makes delineating individual contributions difficult, as the lignosulfonate concentration would increase at increasing alcohol levels. Low molecular weight alcohols can indeed increase counterion condensation on lignosulfonates [87], which could facilitate a higher surface coverage. Simon et al. further studied the interfacial tension of lignosulfonate solutions in presence of asphaltenes [82]. The authors concluded that interfacial adsorption was competitive, which is a potential detriment for emulsion stability. At last, Askvik et al. concluded that lignosulfonates and non-ionic surfactants did not associate, due to the small contribution of hydrophobic interaction [38]. Current literate hence disagrees, whether blending lignosulfonates with non-ionic surfactants has a positive effect. Yet, some cases may indeed benefit from such mixtures.

Anionic surfactants can interact with lignosulfonates by electrostatic repulsion, which would suggest competitive adsorption. Still, combining lignosulfonates with other anionic surfactants can be beneficial, as both species increase the ionic strength. As has been discussed previously, increasing salinity will also facilitate more interfacial adsorption [66]. This perception is supported by a study on blending lignosulfonates and sodium dodecyl sulfate (SDS) [75], which showed that the presence of lignosulfonates decreased the critical micelle concentration (CMC) of SDS. A beneficial effect was also found in the context of enhanced oil recovery (EOR) [99, 100, 101]. By blending petroleum sulfonates, sodium chloride, 2-propanol, and lignosulfonates, interfacial tension values as low as $1\times {10}^{-3}\mathrm{mN}/\mathrm{m}$ were obtained [100]. Another implementation would be the use of lignosulfonates as sacrificial adsorbents [16, 102]. Here, the rock formations are initially saturated with lignosulfonates, after which a second flood with a different surfactant mixture would be injected. While this application does not directly relate to emulsion stabilization, it shows that adsorption was also competitive.

Cationic surfactants interact with lignosulfonates electrostatically, which can yield the formation of lignosulfonate-cationic surfactant complexes [38]. Improved solubility of such complexes in oil media has been reported [103]. Still, a challenge with such systems is the formation of a water-insoluble complexes. In such cases, the surfactants precipitate and are no longer available for interfacial adsorption, hence yielding mixed effects on emulsion stabilization [38].

It has long been established that lignosulfonates can associate with cationic polyelectrolytes, forming insoluble complexes, colloids, and macroscopic precipitates [104]. Association of lignosulfonates with chitosan reportedly forms such complexes at a sulfonate/amine ratio close to 1.0, suggesting that all sulfonate groups are accessible for interactions [105]. No complex formation was reported at pH 8, which entails that it was indeed electrostatic interactions governing the association of these two compounds. Another interesting application is the formation of multilayers via layer-by-layer association of lignosulfonates and cationic polyelectrolytes [106, 107]. This self-assembly was reportedly governed by electrostatic interactions, hydrogen bonding, and cation-π interactions.

It can be concluded that strong interactions exist between lignosulfonates and cationic surfactants or polymers. Still, these interactions frequently yield precipitates, which would shift the emulsion stabilization mechanism from interfacial adsorption to that of a Pickering emulsion. Beneficial interactions can occur due to mixing lignosulfonates with anionic or non-ionic surfactants, but these may depend on the actual system and the application mode.

4. Emulsion stabilization mechanisms

4.1 Basic concepts

Emulsion stabilization entails the prevention or delay of the coalescence event. Coalescence is the fusing of two or more droplets to form one larger droplet. The limiting case for coalescence is complete phase separation, i.e., the oil and the water being separated into two distinct phases. Coalescence involves film drainage, during which the continuous phase is displaced between the coalescing droplets. Flocculation usually precedes coalescence, in which the dispersed droplets collect to form larger aggregates. Creaming or sedimentation occurs, if the flocculated droplets accumulate at the top (creaming) or the bottom (sedimentation) of the continuous phase. Centrifugation of emulsions stabilized with lignosulfonates can yield the formation of a dense packed layer (DPL) of droplets, which exhibit thixotropy and viscoelastic behavior [13].

Fundamentally speaking, the use of a stabilizer (surfactant and/or polymer) introduces an energy barrier between the droplets [76]. The lowest state of energy would be a system, which is completely phase separated. Yet, the transition from emulsion to complete phase separation may become noncontinuous in presence of a stabilizer. Emulsions stabilized with lignosulfonates are hence only kinetically stable. This entails that with time, the emulsions are expected to return to original state, i.e., a fully coalesced and phase separated system. Still, this transition is hindered to the extent that emulsion can remain in their emulsified state over a period of months or even years.

The interfacial tension is an important parameter, as low interfacial tension reduces the energy required for emulsification. In addition, this parameter can be used to study interfacial phenomena and interactions in the aqueous phase. A known stabilizer tends to be more effective at parameters, which yield a higher reduction of interfacial tension. Still, the reduction of interfacial tension in general is no guarantee for forming stable emulsions. There are examples of surfactants, which substantially decrease the interfacial tension, but do not produce stable emulsions or even destabilize existing systems. The latter are referred to as demulsifiers or emulsion breakers. The important point is that interfacial tension measurements can yield complementary information, but it should not be taken as a sole measure to probe emulsion stability. Interfacial adsorption is a prerequisite for an efficient stabilizer; however, emulsion stabilization involves several other mechanisms, which will be discussed in detail further on.

4.2 Stearic hindrance

Stearic hindrance or stearic repulsion relies on the presence of the surfactant or polymer at the interface. By imposing spatial obstacles, the oil within the droplets is prevented from coalescing. Stearic repulsion is an important mechanism for non-ionic surfactants and polymers, as these lack the contribution from electrostatic repulsion.

Stearic repulsion is aided by a positive osmotic free energy of interaction, which states that the affinity of the adsorbed species to the continuous phase (water) is greater than to each other [76]. As such, complete film drainage can be prevented, as the surfactant or polymer favors the retention of water between the oil droplets.

A second effect is of entropic nature, also referred to as volume restriction or elastic interaction [76]. A significant overlap of the polymer chains can be favored by hydrophobic or van der Waals interactions. Separating these chains would require energy, which can act as a barrier to prevent coalescence. This phenomenon will be discussed in further detail during Section 4.5.

4.3 Marangoni effect

The Marangoni effect, also referred to as Marangoni-Gibbs effect, is a common effect that facilitates emulsion stabilization with simple surfactants. It is related to the mass transfer along the interface between two fluids due to a concentration gradient. In analogy to Fick’s law of diffusion, mass transfer of surfactant molecules is directed towards areas with lower concentration. As the oil droplets approach, the surfactant layer between each droplet and the continuous phase can be displaced. This will in turn yield a concentration gradient. Surfactant molecules are hence drawn back into the contact area between the oil droplets, yielding a stabilizing effect. An illustration of the Marangoni effect is given in Figure 14.

Figure 14.

Illustration of the Marangoni effect during film drainage. Image taken from [108].

Attribution of the Marangoni effect to emulsion stabilization with lignosulfonates is given only implicitly, as the contribution of individual effects is often difficult to delineate. Still, the described effect is likely of importance, as lignosulfonates are subject to interfacial diffusion in the same manner as other surfactants.

4.4 Electrostatic repulsion

In aqueous solution, lignosulfonates can attain a negative charge due to the dissociation of anionic groups. When adsorbed at the interface, these groups will contribute to an overall negative charge. Coulomb forces are then acting between the interfaces of different oil droplets, yielding electrostatic repulsion. This mechanism has been described for stabilization of both particles and emulsions with lignosulfonates [38, 46, 109]. Related phenomena are frequently studied by measuring the electrophoretic mobility of the dispersed particles or droplets, i.e., the zeta potential [38, 70].

Electrostatic repulsion is highly affected by the composition of the aqueous phase. Increasing electrolyte concentrations or the presence of less polar solvents will affect counterion condensation [87]. Charge screening and a lower degree of dissociation can then lessen the repulsion between anionic groups [88]. This could explain the destabilizing effect, which the presence of low molecular weight alcohols can have [81]. Still, increasing the concentration of simple electrolytes tends to improve stability, if the lignosulfonate is not precipitated from solution [66]. This effect is likely attributed to the fact that increasing salinity can enhance other effects, such as increased interfacial adsorption. The pH furthermore affects electrostatic repulsion. It has been reported that higher pH induces larger changes of the zeta potential, as the lignosulfonate macromolecules exhibit a higher degree of ionization [70, 110].

4.5 Viscoelastic interface films

It has long been established that droplet coalescence is affected by the stability of interfacial layers [111]. From a purely mechanical point of view, an emulsion would naturally be more stable, if the dispersed droplets were coated by a rigid interface layer. The term rigid hereby refers to the inelastic behavior, as can also be observed for lignosulfonates [66]. These systems are, however, not entirely rigid. Deformation may occur to some extent, bearing both elastic and viscose contributions, hence the terminology viscoelastic interface films.

Upon deformation, a purely elastic film will store the exerted work as potential energy. If the force is no longer acting, the system will return to the originate state or even oscillate, if the viscose contribution is small or negligible. Viscose forces can be viewed in analogy to friction, since energy is dissipated upon deformation, which cannot be retrieved during relaxation. Based on this, the complex modulus $E^{\ast }$ can be modeled as in Eq. (6), i.e., as the sum of the elastic contribution $E^{\prime }$ and the viscose contribution $E^{\prime \prime }$. These contributions are also referred to as the apparent elastic dilatational modulus and the apparent viscous dilatational modulus, respectively, as Eq. (6) is based on the framework of dilatational interfacial rheology. During this measurement technique, a droplet is suspended in a continuous phase in presence of surfactants (see Figure 9), while undergoing volume expansion and contraction at the frequency $\omega$. This allows the complex modulus $E^{\ast }$ to be determined in terms of the change of interfacial tension $\mathit{d\sigma }\left(t\right)$ divided by the change in interface area $dlnA\left(t\right)$.

$$E^{\ast }\left(\omega \right)=E^{\prime }\left(\omega \right)+i{E}^{\prime \prime }\left(\omega \right)=\frac{\mathit{d\sigma }\left(t\right)}{dlnA\left(t\right)}$$E6

Pendant drop tensiometry has the advantage of yielding information on both interfacial tension and interfacial rheology. The technique ceases to function, however, if droplet contraction and expansion is not entirely viscoelastic, as the Young Laplace equation is no longer valid. Other techniques to measure surface rheology include the oscillating barrier method, capillary waves, Langmuir trough, and surface rotational shear rheometry [112]. The latter two have been successfully applied for studying the interfacial behavior of lignosulfonates.

Results from interfacial shear rheology of lignosulfonates are depicted in Figure 15. Frequency sweeps are of interest, as these can be used to characterize the interfacial properties. As can be seen, the elastic modulus E’ and the viscose modulus E” are intersecting or approaching each other in cases, where only monovalent cations are present. The interfacial response is hence entirely viscoelastic. In presence of multivalent cations, i.e., Ca²⁺ and Al³⁺, the elastic and viscose modulus are approximately parallel to each other. This behavior is characteristic for that of gelled interfaces. It appears that multivalent cations can function as connectors between the lignosulfonate molecules, imposing a stronger cohesion to the interface film. This gelling is a reasonable explanation for the observed rigidity in Figure 9.

Figure 15.

Frequency sweep of 1.8 g/l sodium lignosulfonate and in presence of different electrolytes at 0.2% strain. The oil phase is made of xylene isomer blend. Figure taken from [66].

Increasing the lignosulfonate concentration has shown to decrease the interfacial tension (see Figure 8). While the same could be expected for the interfacial modulus, experimental evidence shows that this is not the case. As depicted in Figure 16, the interfacial modulus will increase to a maximum, after which a decrease is noted. It appears that electrolytic effects govern this behavior, as the same maximum was observed for adding simple electrolytes. The maximum has indeed been correlated with a maximum in emulsion stability and the onset of lignosulfonate precipitation [66]. The observed phenomena are hence in line with two long established principles: Emulsions can be stabilized by the formation of viscoelastic interface layers and the emulsion stability tends to be best, if the stabilizing agent is on the verge of precipitation [90, 111]. If the solubility limit is exceeded, precipitation yields the formation of particles, hence shifting the stabilization mechanism to that of a Pickering emulsion. This type of mechanism will be discussed in the next sub-chapter.

Figure 16.

Interfacial moduli in dependence of lignosulfonate or added salt concentration. Data was obtained using interfacial shear rheology and xylene isomer blend as the oil phase. Figures taken from [66].

4.6 Particle stabilization

Particle stabilization of emulsions involves the presence of a third (solid) phase. The solid phase is usually present as a colloidal dispersion, exhibiting particle sizes of approximately 1 nm to 1 μm. These particles can adsorb at the oil–water interface, forming bilayers and bridging monolayers [113]. The stabilizing mechanism is based on coherent particle layers around the dispersed liquid, preventing coalescence by acting as a steric (mechanical) barrier. A particle stabilized emulsion is also referred to as Pickering emulsion. Interest has currently shifted from using inorganic particles to adopting bio-based systems, which includes the use of cellulose, chitin, starch, proteins, and lignin [114].

Particle stabilization is usually not the primary mechanism, with which lignosulfonates stabilize emulsions. In aqueous solution at low salinity, lignosulfonates are usually well-dissolved and hence act by molecular adsorption at the interface. Certain conditions, however, can induce lignosulfonate precipitation, which will shift the stabilization mechanism to that of a Pickering emulsion [66]. In addition, lignosulfonate aggregation can occur at concentrations as low as 0.05 g/l [55]. Aggregation is reportedly facilitated by π–π stacking, hydrogen bonding, and hydrophobic interactions [93, 115, 116]. At the right conditions, aggregate dimensions can be in the range of colloidal dispersions. Based on the diffusion coefficient of lignosulfonates obtained from dilatational interfacial rheology, it was concluded that lignosulfonates undergo interfacial adsorption in the aggregated state [66]. Interfacially adsorbed aggregates could furthermore act by particle stabilization. It is important to note that lignosulfonate aggregation occurs gradually over a broad concentration range. The contribution of aggregates to the overall emulsion stability would hence build up in increments. This contrasts with precipitation, where an immediate shift to particle stabilization is observed at the precipitation onset.

5. Summary and conclusion

This chapter detailed the fundamentals of emulsion stabilization with lignosulfonates. First, basic concepts, the industrial production, as well as chemical make-up and structure of lignosulfonates were discussed. Second, the effect of lignosulfonates on surface and interfacial tension was described. Third, the fundamental mechanisms behind emulsion stabilization with lignosulfonates were explained.

Lignosulfonates are bio-based specialty chemicals and function as surfactants due to their amphiphilic property. Lignosulfonates are readily water-soluble as they comprise ionizable moieties, i.e., sulfonate and carboxylic acid groups at neutral pH. They can hence be utilized in a variety of technical applications, which includes the stabilization of oil-in-water emulsions. Surface and interface adsorption of lignosulfonates is evidenced by a reduction in surface and interfacial tension. Parameters that can enhance the effect on interfacial tension include increased salinity, a reduction in pH, and the presence of co-solvents or co-surfactants. A high reduction of interfacial tension is generally beneficial, as this suggests enhanced interfacial adsorption and reduces the energy required during emulsification. While interfacial adsorption is a prerequisite for an efficient emulsion stabilizer, the stabilization mechanism is related to other phenomena as well. For lignosulfonates, these mechanisms include stearic hindrance, electrostatic repulsion, the Marangoni-Gibbs effect, the formation of viscoelastic interface layers, and particle stabilization. The stabilization mechanisms are furthermore affected by the composition of the lignosulfonates as well as the aqueous and oil phase. Parameters that increase the interfacial activity and yield the formation of more cohesive interface layers are beneficial, as both effects tend to promote emulsion stability.

There has been an increased interest in biopolymers recently, as these are inherently more sustainable than petroleum-derived polymers. Lignosulfonates are no exception to that trend, which is also mirrored by the large variety of lignosulfonate products on the market today. Emulsion stabilization is one of the application areas, which still has the potential for growth. This chapter was hence dedicated to the fundamentals of emulsion stabilization with lignosulfonates, in the hope that this may aid their utilization in new areas and products.

Conflict of interest

The author declares no conflict of interest.