Emulsion electrostatic stabilization

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. 

For ionic surfactants another effect often dominates and usually salt tends to stabilize emulsions. Reason without salt the distance between surfactants in the interface is large because the molecules electrostatically repel each other. This prevents a high surface excess. The addition of salt reduces this lateral repulsion and more surfactant molecules can adsorb at the interface. Then, according to the Gibbs adsorption isotherm Eq. (3.52), the surface tension is reduced and the emulsion is stabilized. 

The emulsion polymerization process involves the polymerization of liquid monomers that are dispersed in an aqueous surfactant micelle-containing solution. The monomers are solubilized in the surfactant micelles. A water-soluble initiator catalyst, such as sodium persulfate, is added to the aqueous phase. The free radicals generated cause the dispersed monomers to react to produce polymer molecules within the micellar environment. The surfactant plays an additional role in stabilizing dispersion of the produced polymer particles. Thus, the surfactants used both provide micelles to house the monomers and macroradicals, and also stabilize the produced polymer particles [193,790], Anionic surfactants, such as dodecylbenzene sulfonates, are commonly used to provide electrostatic stabilization [193], These tend to cause production of polymer particles having diameters of about 0.1-0.3 pm, whereas when steric stabilization is provided by, for example, graft copolymers, then diameters of about 0.1-10 pm tend to be produced [790,791]. 

Many food colloids are stabilized from proteins from milk or eggs [817]. Milk and cream, for example, are stabilized by milk proteins, such as casein micelles, which form a membrane around the oil (fat) droplets [817]. Mayonnaise, hollandaise, and bearnaise, for example, are O/W emulsions mainly stabilized by egg-yolk protein, which is a mixture of lipids (including lecithin), proteins, and lipoproteins [811,817]. The protein-covered oil (fat) droplets are stabilized by a combination of electrostatic and steric stabilization [817]. Alcohols may also be added, such as glycerol, propylene glycol, sorbitol, or sucrose sometimes these are modified by esterification or by... 

Part of the process to make cheese involves the flocculation of an electrostatically stabilized colloidal O/W emulsion of oil droplets coated with milk casein. The flocculation is caused by the addition of a salt, leading to the formation of networks which eventually gel. The other part of the process involves reaction with an enzyme (such as rennet), an acid (such as lactic acid), and possibly heat, pressure and microorganisms, to help with the ripening [811]. The final aggregates (curd) trap much of the fat and some of the water and lactose. The remaining liquid is the whey, much of which readily separates out from the curd. Adding heat to the curd (-38 °C) helps to further separate out the whey and convert the curd from a suspension to an elastic solid. There are about 20 different basic kinds of cheese, with nearly 1000 types and regional names. Potter provides some classification [811]. 

Non-dairy creams (cream alternatives) are O/W emulsions stabilized by milk proteins. A relatively thick adsorption layer provides stability, mostly by steric stabilization and partly by electrostatic stabilization [829]. Figure 13.3 shows an example of a soybean-oil and milk-protein emulsion stabilized by fat globules and protein membranes. Stabilizers, such as hydrocolloid polysaccharides, are added to increase the continuous phase viscosity and reduce the extent of creaming. They must be stable enough to have a useful shelf-life but de-stabilize in a specific way when they are... 

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. 

One topic which is not covered in this section is the preparation and stabilization of emulsion type flowables (oil in water flowables ) (7 ). This type of flcwable is just beginning to receive widespread attention and is still very much in the experimental stage. In addition to the other factors considered above, factors which influence oil droplet coalesence, such as Interfacial free energy, electrostatic stabilization and steric stabilization, must be taken into consideration for this special type flowable. 

Surfactants are employed in emulsion polymerizations to facilitate emulsification and impart electrostatic and steric stabilization to the polymer particles. Sicric stabilization was described earlier in connection with nonaqueous dispersion polymerization the same mechanism applies in aqueous emulsion systems. Electrostatic stabilizers are usually anionic surfactants, i.e., salts of organic acids, which provide colloidal stability by electrostatic repulsion of charges on the particle surfaces and their associated double layers. (Cationic surfactants are not commonly used in emulsion polymerizations.)... 

Reduced electrostatic repulsion. Electrostatically stabilized emulsions may flocculate when the electrostatic repulsive interactions between the droplets are reduced. This can be achieved by altering the pH so that the electrical charge on the droplets is reduced, adding multivalent counter ions that bind to the surface... 

Electrokinetics. Bottle tests and centrifugation may be somewhat crude, but they do offer a relative measure of emulsion stability that combines, to some extent, all of the factors that affect stability. Electrokinetic measurements are somewhat more elegant because they allow direct measurement of the degree of electrostatic stability in an emulsion system. The zeta potential, or relative magnitude of the electric charge on the surface, is... 

A dispersed emulsion droplet (electrostatically stabilized) can be thought of as a charged center with a pliable cover of compensating charge due to orientation of the water molecules (or counterions) around the droplet. When exposed to an electric field, the droplet will move according to the surface charge. 

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.

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. 

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.

Emulsions have been widely used as vehicles for oral, topical, and parenteral delivery of medications. Although the product attributes of an emulsion dosage form are dependent on the route of administration, a common concern is the physical stability of the system, in particular the coalescence of its dispersed phase and the consequent alteration in its particle-size distribution and phase separation. The stabilization mechanism(s) for an emulsion is mainly dependent on the chemical composition of the surfactant used. Electrostatic stabilization as described by DLVO theory plays an important role in emulsions (0/W) containing ionic surfactants. For 0/W emulsions with low electrolyte content in the aqueous phase, a zeta potential of 30 mV is found to be sufficient to establish an energy maximum (energy barrier) to ensure emulsion stability. For emulsions containing... 

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). 

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