Destabilization emulsions, electrostatic

Laser diffraction is most suitable for analyzing dilute emulsions that are fluid, and therefore competes directly with electrical pulse counting methods, which are applicable to similar systems (see Alternate Protocol). Most laser diffraction instruments can cover a wider range of particle sizes (i.e., 0.01 to 1000 pm) than electrical pulse counting instruments (i.e., 0.4 to 1000 pm using a number of different aperture sizes), and do not require the presence of electrolyte in the aqueous phase, which could destabilize some electrostatically stabilized emulsions. Nevertheless, electrical pulse counting techniques are considered to have greater resolution. 

Figure 13. Electrophoretic mobility (Fen Kem 3000) of the emulsion from Figure 12 after cationic polymer addition (A). The cationic polymer has neutralized the oil droplet surface charge and electrostatically destabilized the emulsion. The photomicrograph (B) shows this destabilized emulsion that has begun to flocculate or a lomerate but that is not coalescing. This electrostatic destabilization is not the only factor affecting emulsion stability. Factors such as interfacial tension and film strength can prevent coalescence of the emulsion droplets, even though they can now closely approach each other and ag omer-...

Steric stabilization differs from electrostatic stabilization in not being a function of a net force, but of the thickness of an adsorbed layer. When < >, equals 5-10%, stabilizing and destabilizing forces extend beyond the length of the electrostatic, interparticle barrier (Cabane et al., 1989). At this distance, attraction and repulsion are inconsequential, and electrolytes therefore have little effect. Bergenstahl (1988) proposed that the steric stabilization of emulsions by gums in the presence of a surfactant involves adsorption of the gum on the surfactant to form a combined structure constituted by a primary surfactant layer covered by an adsorbed polymer layer. 

In spite of the droplets being destabilized electrostatically, no evidence of droplet coalescence is seen. By the same token, an electrostatically stabilized emulsion might still coalesce and separate, sediment, or cream if other destabilizing forces overbalance the electrostatic component. Creaming refers to concentration of the dispersed phase without completely separating the oil and water phases. 

When the electrostatic stabilization of the emulsion is considered, the electrolytes (monovalent and divalent) added to the mixture are the major destabilizing species. The zeta potential of the emulsion particles is a function of the concentration and type of electrolytes present. Two types of emulsion particle-electrolyte (ions) interaction are proposed non-specific and specific adsorption.f H non-specific adsorption the ions are bound to the emulsion particle only by electrical double-layer interactions with the charged surface. As the electrolyte concentration is increased, the zeta potential asymptotes to zero. As the electrostatic repulsion decreases, a point can be found where the attractive van der Waals force is equal to the repulsive electrostatic force and flocculation of the emulsion occurs (Fig. 9A). This point is called the critical flocculation concentration (CFC). 

The interfacial tension, o, affects the rate ratio directly only through the capillary pressure, P = 2o/P. The electrolyte primarily affects the electrostatic disjoining pressure, n, which decreases as the salt content increases, thus destabilizing the OAV emulsion. It can also influence the stability by changing the surfactant adsorption (including the case of nonionic surfactants). 

Fig. 35. Schematic representation of formation of cohesive precipitate . Shown (left) is the emulsion with electrostatically stabilized polymer microparticles with extended surface bound sodium alginate chains. Shown (right) is the cohesive precipitate containing destabilized microparticles with collapsed surfactant and with some (assumed) ionotropic bridging chains...

The interfacial tension, o, affects directly the rate ratio in Eq. (89) through the capillary pressure, 2da. The addition of electrolyte would affect mostly the electrostatic component of the disjoining pressure (see Fig. 8a), which is suppressed by the electrolyte the latter has a destabilizing effect on OAV emulsions. In the case of ionic surfactant solutions the addition of electrolyte rises the surfactant adsorption and the Gibbs elasticity (see Fig. 5), which favors the stability of emulsion I. 

Emulsions can be destabilized by agitation, which leads to droplet coalescence, centrifuging which leads to creaming or by adding salt to electrostatically stabilized systems. Temperature changes (that cause freezing, for example) or filtration can also be used to break up emulsions. 

Figures 12.1-12.3 show the rheological investigations performed on emulsions stabilized by very hydrophobic, mildly hydrophilic and very hydrophilic silica particles, respectively. From Figure 12.1 it is clear that emulsions stabilized by means of very hydrophobic particles are very stable when no voltage is apphed. However, the stability of these emulsions towards electrostatic destabilization drops...

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