Attractive forces, emulsions stabilizing

In the interfacial tension theory, the adsorption of a surfactant lowers the interfacial tension between two liquids. A reduction in attractive forces of dispersed liquid for its own molecules lowers the interfacial free energy of the system and prevents the coalescence of the droplets or phase separation. Therefore the surfactant facilitates the stable emulsion system of the large interfacial area by breaking up the liquid into smaller droplets. However, the emulsions prepared with sodium dodecyl (lauryl) sulfate separate into two liquids upon standing even though the interfacial tension is reduced. The lowering of the interfacial tension in the stabilization of emulsions is not the only factor we should consider. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

Fig. 4 The total potential energy of interaction Vt as a function of distance of surface separation H for two similar oh droplets in an oil-in-water emulsion. (A) Electrostatic stabilization by a monolayer of ionic surfactant. (B) Steric stabilization by a monolayer of non-ionic surfactant. V van der Waals attractive force Vr electrostatic repulsive force Vs steric repulsive force.

Short Range Interactions and Emulsion Stability. The stability of macroemulsions in terms of short range (e.g. inter-droplet) interactions will be discussed in this section. The dispersion (London) forces arise from charge fluctuations within a molecule associated with the electronic motion (21). Therefore, these forces can operate even between nonpolar molecules. London (21) reported an equation for mutual attractive energy between two molecules in vacuum in the form... 

The formation and stabilization of 0/W emulsions prepared with mixed emulsifier systems has been extensively investigated. However, the mechanisms proposed differ greatly. One of the primary hypotheses attributes the enhanced stability to the formation of a molecular "complex" or layer at the oil/water interface (8-11). The mixture of emulsifier types increases the packing density of the adsorbed interfacial film. Several investigators have shown that more closely packed complexes produce more stable emulsions (9,12-14). Friberg, et al. (15-17) have attributed the enhanced stability of mixed emulsifier emulsions to the formation of liquid crystals at the oil/water interface, which reduce the van der Waals attractive forces. 

As an example, I would like to mention the effect of stirring speed on emulsion stability. In Figure 16, it can be seen that with increasing stirring speed in a homogenizer, the emulsion can be broken with less efficiency. This correlates with decreasing "Sauter diameter" Dp. The droplet diameter distribution of the emulsion becomes more and more uniform. The smaller the droplets, the smaller are the mutual attractive forces and the smaller is the probability of a collision of two particles. 

Figure 2 Aqueous droplets dispersed in crade oil and sub-jected to an electric field (a) no field (b) 5 s, 1 kV/cm -droplet orientation in chains along the direction of the field. The droplets become small net dipoles in the dielectric oil continuum and are attracted to each other, forming chains in the direction of the field. High field strengths will cause interdroplet membrane rupture and coalescence. The principle has been utilized for measuring emulsion stability (i.e., resistance to electrically forced breakdown) in the high voltage-time domain spectroscopy (HiV-TDS) (71,72) and conductivity techniques (73).

An emulsion has been defined above as a thermodynamically unstable heterogeneous system of two immiscible liquids where one is dispersed in the other. There are two principal possibilities for preparing emulsions the destruction of a larger volume into smaller sub-units (comminution method) or the construction of emulsion droplets from smaller units (condensation method). Both methods are of technical importance for the preparation of emulsions for polymerization processes and will be discussed in more detail below. To impart a certain degree of kinetic stability to emulsions, different additives are employed which have to fulfil special demands in the particular applications. The most important class of such additives, which are also called emulsifying agents, are surface-active and hence influence the interfacial properties. In particular, they have to counteract the rapid coalescence of the droplets caused by the van der Waals attraction forces. In the polymerization sense, these additives can be roughly subdivided into surfactants for emulsion polymerization, polymers for suspension and dispersion polymerization, finely dispersed insoluble particles (also for suspension polymerization), and combinations thereof (cf. below). 

Encounters between particles in dispersion can occur frequently due to any of the Brownian motion, sedimentation, or stirring. The stability of the dispersion depends upon how the particles interact when this happens. Table 4.2 lists some factors involved in determining the stability of emulsions. The main causes of repulsive or barrier forces may be electrostatic, steric, or mechanical. The main attractive forces are the van der Waals forces between objects. 

Interactions among atoms and molecules, as we have seen, are a result of various forces stemming from their atomic or molecular structure, including electrostatic or charge interactions, steric or entropic phenomena, and the ever-present van der Waals forces. Of these, electrostatic and steric interactions may be repulsive in that they act to force the interacting units apart or at least reduce the net attraction between units. The van der Waals forces, on the other hand, are usually (but not always) attractive. When one discusses the use of a surfactant as an emulsion stabilizer, as in the above sections, the concept of the function of the surfactant is that it have a strong tendency to... 

The presence of a sharply defined interface between a polymer solution and a solid wall leads to important modifications in the local polymer concentration with respect to the bulk concentration. These variations can be positive or negative depending on the sign of the interaction between the solid wall and the macromolecular chains immersed in the solvent. Attractive forces lead to adsorbed layers while repulsive forces lead to depletion layers. Due to their connection with important technological applications such as adhesives, protective coatings, microlithography, emulsion stabilizers, adsorbed polymer layers have been the subject of extensive theoretical studies. ... 

The stability of an emulsion system towards flocculation and coalescence may be better understood by considering the forces between emulsion droplets. These forces arise from a range of phenomena and vary from system to system. The most ubiquitous of these forces is the van der Waals force of attraction, which arises from momentary fluctuations in the charge distribution across molecules, giving them a flickering dipolar nature. The induction of complementary dipoles in adjacent molecules leads to a weak attractive force between them. A similar attraction occurs between colloidal particles, and the resulting potential decays with the inverse square of the separation between the droplets, as shown schematically in Figure 4.2. 

---