Surface potential, charged emulsion droplet

Figure 13. Simplified illustration of the surface and zeta potentials for a charged emulsion droplet dispersed in high and low electrolyte concentration aqueous solutions. (Courtesy of L. A. Ravina, Zeta-Meter, Inc., Long Island...

A reduction in the electrical charge is known to increase the flocculation and coalescence rates. Sufficient high zeta potential (> — 30 mV) ensures a stable emulsion by causing repulsion of adjacent droplets. The selection of suitable surfactants can help to optimize droplet surface charges and thus enhance emulsion stability. Lipid particles with either positive or negative surface charges are more stable and are cleared from the bloodstream more rapidly than those with neutral charge [192, 193]. 

Figure 11. Schematic representation of the electrophoretic mobility (A) measurement showing the major components. In an applied electric field, emulsion droplets move according to their surface charge. These charges can electrostatically stabilize an emulsion system by preventing the droplets from coming into contact and coalescing. The motion of the droplets is visually observed, and the electrophoretic mobilities of a number of particles are measured to determine zeta potential. The sedimentation potential (B) is also illustrated.

When charged coUoidal particles in a dispersion approach each other such that the double layer begins to overlap (when particle separation becomes less than twice the double layer extension), then repulsion will occur. The individual double layers can no longer develop unrestrictedly, as the limited space does not allow complete potential decay [3]. This is illustrated in Figure 10.8 for two flat plates, and shows clearly shows that when the separation distance h between the emulsion droplets become less than twice the doubly layer extension, the potential at the mid plane between the surfaces is not equal to zero (which would be the case if h were more than twice the double layer extension) plates. 

Thus, in the relatively simple case of oil in water emulsions, where a surface active agent such as a soap is used as the emulsifying agent, it is known that the soap adsorbed on the surface of the oil particles decreases the interfacial tension, thus stabilizing the emulsion. The adsorbed soap ions also give a net electrostatic charge to the dispersed oil droplets, serving to repel other oil droplets, with the net effect that flocculation is hindered (and stability is increased). It is even possible to measure the amount of adsorbed soap ions and to calculate the values of the surface potential. 

A consequence of the small size and large surface area in colloids is that quite stable dispersions of these species can be made. That is, suspended particles may not settle out rapidly and droplets in an emulsion or bubbles in a foam may not coalesce quickly. Charged species, when sedimenting, present a challenge to Stokes law because the smaller counterions sediment at a slower rate than the larger colloidal particles. This creates an electrical potential that tends to speed up the counterions and slow down the particles. At high enough electrolyte concentrations the electric potentials are quickly dissipated and this effect vanishes. 

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