**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]:


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 watersoluble, 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**.

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:

#### **Figure 2.**

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


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.

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**.

*Emulsion Stabilization with Lignosulfonates DOI: http://dx.doi.org/10.5772/intechopen.107336*


#### **Table 1.**

*Approximate elementary composition of lignosulfonates.*

**Figure 3.**

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

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].

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 [57–59]. 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].

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 [61–63]. 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


#### **Figure 4.**

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


#### **Table 2.**

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

**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].*

were of softwood origin, a higher methoxy content can be expected for hardwood lignosulfonates due to the monolignol composition.

**Table 2** also lists the number-average (Mn) and mass-average molecular weight (Mw). 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.
