Solid-Stabilized Double Emulsions

RGURE 6.21. Microscopic images of double emulsions stabilized by two types of silicapar-ticles of different hydrophobicity. (Top) W/OAV with triglyceride oil (scale bar = 50 am). (Bottom) OAV/O with toluene (scale bar = 20 am). (Reproduced with permission from [40] and [41].) 

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While the electric double layer on a solid surface is relatively well understood and theories are able to account for colloidal stability and coagulation kinetics quite well, there has been much less success in understanding the double-layer structure at liquid-liquid or liquid-gas interfaces. This is despite the fact that the stability of emulsions or dispersion of particles and... 

Two oil drops covered with these par-rides will now repel each other due to the double layers on the solid particles. Suitable solids for stabilizing oil emulsions are, among others bentonite, Al(OH)3, Si02 ... 

Figure 2.22. (a) Disjoining pressure vs. thickness isotherm for an emulsion film stabilized by 0.1% BSA, ionic strength of 10 mol/1 NaCl, oil phase = hexadecane. The dots are the experimental data, dashed line is the double-layer contribution to the total disjoining pressure, and the solid line is the best fit done supposing additivity of the double-layer and steric forces, (b) Force vs. distance profiles for ferrofluid emulsions stabilized with mixed BSA-Tween-20 adsorption layers. The total concentration of the Tween-20 is kept constant = 5CMC, pH = 5.8. (Adopted from [78].)... 

Two additional stabilizing influences will be summarized next that of viscoelastic films and that of solid-particle films. In general, where electrical surface charge is an important determinant of stability, it is easier to formulate a very stable O/W emulsion than a W/O emulsion because the electric double layer thickness is much greater in water than in oil. (This is sometimes incorrectly stated in terms of greater charge being present on droplets in an O/W emulsion.) However, there are ways to effectively stabilize W/O emulsions. 

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. 

In the second place the interaction of the double layers of two droplets of an emulsion will determine the stability of the emulsion A complete description of this effect offering still more mathematical difficulties than the interaction of two spherical double layers as treated in 4 will not be aimed at, as a superficial consideration will already be sufficient to demonstrate that the energy of interaction between two droplets of an emulsion is much smaller than that between solid particles This explains why emulsions of two pure liquids are never stable (c/ chapter VIII, 11, p 336) and why the addition of emulsifiers is necessary to prepare stable emulsions Let us first consider the meeting of... 

The existence of electrical charges at any interface will give rise to electrical effects, which will, in many cases, determine the major characteristics of that interface. Those characteristics will affect many of the properties of a multicomponent system, including emulsion and foam formation and stability, solid dispersions, and aerosols. The theoretical and practical aspects of electrical double layers are the subject of a vast amount of literature and for that reason have not been addressed in any detail so far. Such details can be found in bibliographic references cited for this chapter. 

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