Emulsions DLVO theory

Fig. XrV-6. (a) The total interaction energy determined from DLVO theory for n-hexadecane drops for a constant ionic strength - 5.0 nm) at various emulsion pH (b) enlargement of the secondary minimum region of (a). (From Ref. 39.)...

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

The theory has certain practical limitations. It is useful for o/w (od-in-water) emulsions but for w/o (water-in-oil) systems DLVO theory must be appHed with extreme caution (16). The essential use of the DLVO theory for emulsion technology Hes in its abdity to relate the stabdity of an o/w emulsion to the salt content of the continuous phase. In brief, the theory says that electric double-layer repulsion will stabdize an emulsion, when the electrolyte concentration in the continuous phase is less than a certain value. 

Figure I. DLVO Theory for explaining emulsion stability...

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

Sauce bearnaise, for example, is an O/W emulsion that is mainly stabilized by egg-yolk protein in an aqueous phase of low pH. Perram et al. [830] describe how this system is primarily stabilized by electrostatic repulsive forces, and show how DLVO theory can be used to describe the effects of pH, surface charge, ionic strength, and temperature, on the stability of this emulsion. 

Some products, like butter and margarine are stabilized by fat crystals. Salad dressings and beverage emulsions are stabilized by other emulsifiers. The stability of non-protein stabilized food emulsions, involving lower molar mass type molecules, tend to be better described by the DLVO theory than are protein-stabilized emulsions. An example of an O/W emulsifier whose emulsions are fairly well described by DLVO theory is sodium stearoyl lactylate [812],... 

Very often, the microstructure and the macroscopic states of dispersions are determined by kinetic and thermodynamic considerations. While thermodynamics dictates what the equilibrium state will be, kinetics determine how fast that equilibrium state will be determined. While in thermodynamics the initial and final states must be determined, in kinetics the path and any energy barriers are important. The electrostatic and the electrical double-layer (the two charged portions of an inter cial region) play important roles in food emulsion stability. The Derjaguin-Landau-Verwey-Oveibeek (DLVO) theory of colloidal stability has been used to examine the factors affecting colloidal stability. 

When two surfaces approach each other, two forces exist one repulsive and one attractive. Whether or not the surfaces touch and coalesce depends on the relative sizes of the two forces. This is equally true for liquids (e.g., oil droplets in an emulsion), solids (e.g., finely divided CaCOs), and films (air bubbles in a foam). The description of these interactions is stated in the DLVO theory (8). 

Application of DLVO Theory. Some of the concepts and expressions of Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory of colloid stabihty have been described in Chapter 1, or can be found in many different textbooks 4, 5). The application of DLVO theory to oil-in-water colloids with special reference to the stability of bitumen-in-water emulsions will be discussed here. 

Figure 9. The behavior of bitumen-in-water emulsions in the presence of 300 mM NaCl (right side) and 20 mM CaCU. The emulsions are behaving as predicted by DLVO theory.

Non-DLVO Forces. Although DLVO theory worked very well for the electrolyte-induced coagulation of bitumen-in-water emulsions, it cannot be applied in some cases. 

The DLVO theory, which was developed independently by Derjaguin and Landau and by Verwey and Overbeek to analyze quantitatively the influence of electrostatic forces on the stability of lyophobic colloidal particles, has been adapted to describe the influence of similar forces on the flocculation and stability of simple model emulsions stabilized by ionic emulsifiers. The charge on the surface of emulsion droplets arises from ionization of the hydrophilic part of the adsorbed surfactant and gives rise to electrical double layers. Theoretical equations, which were originally developed to deal with monodispersed inorganic solids of diameters less than 1 pm, have to be extensively modified when applied to even the simplest of emulsions, because the adsorbed emulsifier is of finite thickness and droplets, unlike solids, can deform and coalesce. Washington has pointed out that in lipid emulsions, an additional repulsive force not considered by the theory due to the solvent at close distances is also important. 

The DLVO theory does not explain either the stability of water-in-oil emulsions or the stability of oil-in-water emulsions stabilized by adsorbed non-ionic surfactants and polymers where the electrical contributions are often of secondary importance. In these, steric and hydrational forces, which arise from the loss of entropy when adsorbed polymer layers or hydrated chains of non-ionic polyether surfactant intermingle on close approach of two similar droplets, are more important (Fig. 4B). In emulsions stabilized by polyether surfactants, these interactions assume importance at very close distances of approach and are influenced markedly by temperature and degree of hydration of the polyoxyethylene chains. With block copolymers of the ethylene oxide-propylene oxide... 

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... 

The overall results show that electrostatic factors lead to the formation of first black films from non-ionic surface active agents in electrolytes and that zeta-potentials as low as 18 mV are adequate to stabilize the film assuming the film potentials are comparable with those of the emulsion drop. As shown in more detail elsewhere, the DLVO theory can be used... 

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