**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

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

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 ashfree 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

#### **Figure 1.**

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

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].

**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.
