**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:


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 slopedecrease or plateau would be visible is in **Figure 7**. While this may be plausible, it has yet to be demonstrated experimentally.

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 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].*

**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.

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].

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].

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

**Figure 10.** *Surface tension of water and 1 g/l lignosulfonate in water. Image taken from [93].*

**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].*

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 waterin-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**.

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

#### **Figure 12.**

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

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 surfactantlike 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) [99–101]. By blending petroleum sulfonates, sodium chloride, 2-propanol, and lignosulfonates, interfacial tension values as low as 1 <sup>10</sup><sup>3</sup> mN*=*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 layerby-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.
