**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**.

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.

**Figure 14.** *Illustration of the Marangoni effect during film drainage. Image taken from [108].*

### **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*<sup>∗</sup> can be modeled as in Eq. (6), i.e., as the sum of the elastic contribution *E*<sup>0</sup> and the viscose contribution *E*00. 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 *ω*. This allows the complex modulus *E*<sup>∗</sup> to be determined in terms of the change of interfacial tension *dσ*ð Þ*t* divided by the change in interface area *d* ln *A t*ð Þ.

$$E^\*\left(\boldsymbol{\alpha}\right) = E'(\boldsymbol{\alpha}) + iE''(\boldsymbol{\alpha}) = \frac{d\sigma(t)}{d\ln A(t)}\tag{6}$$

Pendant drop tensiometry has the advantage of yielding information on both interfacial tension and interfacial rheology. The technique ceases to function,

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

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**.

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

#### **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].*

**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].*

particles, hence shifting the stabilization mechanism to that of a Pickering emulsion. This type of mechanism will be discussed in the next sub-chapter.

#### **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

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

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.
