Emulsions attractive forces

Kizling and coworker [21] suggested that salts in the aqueous phase stabilised w/o HIPEs by two means. First, the Ostwald ripening process is inhibited due to the decreased solubility of the aqueous solution in the continuous oil phase. Secondly, the attractive forces between adjacent aqueous droplets are lowered, as a result of the increase in refractive index of the aqueous phase towards that of the oil phase. When the refractive indices of the two phases are matched, the attractive forces are at a minimum and highly stable, transparent emulsions are formed. The attractive force, A, is given by ... 

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. 

An important characteristics of this type of films (in contrast to the symmetric films foam and emulsion) is their ability to form thermodynamically stable films in the absence of a surfactant on the account of the molecular attraction forces [e.g. 522,523]. In the presence of a surfactant, however, it is possible to obtain stable asymmetric films stabilised with... 

Flocculation may occur in emulsions through a variety of different processes, described below, that either increase the attractive forces or decrease the repulsive forces between the droplets. 

Increased depletion attraction. The presence of nonadsorbing colloidal particles, such as biopolymers or surfactant micelles, in the continuous phase of an emulsion causes an increase in the attractive force between the droplets due to an osmotic effect associated with the exclusion of colloidal particles from a narrow region surrounding each droplet. This attractive force increases as the concentration of colloidal particles increases, until eventually, it may become large enough to overcome the repulsive interactions between the droplets and cause them to flocculate (68-72). This type of droplet aggregation is usually referred to as depletion flocculation (17, 18). 

The development of a suitable strategy to prevent droplet flocculation in an emulsion, therefore, depends on identification of the physicochemical origin of flocculation in this particular system. In the most general terms, flocculation can be prevented by ensuring that the repulsive forces dominate the attractive forces. 

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.

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. 

Sherman concluded that in the 0/W systems a small fraction of the continuous phase was immobilized by the dispersed phase either by attractive forces or by flocculation. Therefore, the apparent volume fraction was greater than the actual volume fraction of that component. The equations he used to describe the viscosities of these emulsions further extended the approach of Mooney and took account of the particle diameter. 

Halide-free anodic oxidation of alcohols mediated by an anionic WS-TEMPO (Figure 12.3) was also accomplished. It was suggested that the anionic WS-TEMPO might form an anionic oil-in-water emulsion including alcohol, and the attractive force between the sulfonate ion and the positive charge of anode would facilitate direct electron transfer from the N-oxyl moiety to the anode. 

Fig. 1.14. Mechanism of emulsion stabilisation a) electric repulsion, b) steric hindrance According to Hamaker (1937) the Free Energy of the attraction forces is approximately...

When the drops are close together, whatever the driving force that makes them approach, the film that is located between neighboring drops exhibits a complex drainage process that involves several different mcchanisnu. and this controls the second step of the emulsion decay. Some of them depend on the drop volume tike the van der Waals attractive forces, or the Archimedes pull, white others depend on the interdrop film physical properties such as viscosity, or on the interfacial phenomena chat occur whenever two interfaces approach at sub micrometer range. The first class of inlerfacial phenomena deals with static attractive or repulsive forces, like electrical, steric, or eniropic repulsions, while the second one has to do with dynamic processes like the steaming potential and interfaciat viscosity effects, as well as the more classical hydrodynamic considerations (19-23). 

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). 

Aggregation (route 3 in Figure 8.2 and Figure 8.3(a)) processes (flocculation, coagulation, coalescence) can take place when the average distance between the emulsion droplets is so close that attractive forces become dominating. It is useful to discriminate between flocculation on the one hand, and coagulation and coalescence, on the other hand, in the way that this is... 

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. 

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