**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.
