Stabilizing effect, solid particles

It was pointed out in Section XIII-4A that if the contact angle between a solid particle and two liquid phases is finite, a stable position for the particle is at the liquid-liquid interface. Coalescence is inhibited because it takes work to displace the particle from the interface. In addition, one can account for the type of emulsion that is formed, 0/W or W/O, simply in terms of the contact angle value. As illustrated in Fig. XIV-7, the bulk of the particle will lie in that liquid that most nearly wets it, and by what seems to be a correct application of the early oriented wedge" principle (see Ref. 48), this liquid should then constitute the outer phase. Furthermore, the action of surfactants should be predictable in terms of their effect on the contact angle. This was, indeed, found to be the case in a study by Schulman and Leja [49] on the stabilization of emulsions by barium sulfate. 

In practice, tliere are various ways by which ( ) can be detennined for a given sample, and tire results may be (slightly) different. In particular, for sterically stabilized particles, tire effective hard-sphere volume fraction will be different from tire value obtained from tire total solid content. 

Heydenreich, A.V., Westmeier, R., Pedersen, N., Poulsen, H.S., and Kristensen, H.G., Preparation and purification of cationic solid lipid nanospheres effects on particle size, physical stability and cell toxicity, International Journal of Pharmaceutics, 2003, 254, 83-87. 

The effects of improved wettability, entropic repulsion, and sterical hindrance undoubtedly play a role in stabilizing dispersed solid particles by block or graft copolymers. However, since the dispersions of titanium dioxide in toluene stabilized by carboxylated styrene-butadiene block copolymers are so much more stable than dispersions stabilized by carboxylated homopolymers under otherwise identical conditions, we must assume that an additional factor comes into play when block copolymers are used. The model in Figure 1 is an attempt to explain this additional... 

Powders often have a stabilizing effect on emulsions [548], To understand the responsible effect we have to remember that a particle assumes a stable position in the liquid-liquid interface if the contact angle is not zero (see section 7.2.2). Upon coalescence of two drops the solid particles would have to desorb from the interface. This is energetically unfavorable. A common example of the stabilizing contribution of solid particles are margarine and butter. Both are water-in-oil emulsions. The water droplets are stabilized by small fat crystals. 

Example 12.5. The stabilizing effect of powders was impressively demonstrated by making liquid marbles in air [549], Liquid marbles (Fig. 12.11) are obtained by making a small amount of water (typically 1 mm3) roll on a very hydrophobic powder. The powder particles go into the interface and completely coat it so that, after spontaneous formation of the spherical drop, only the solid caps of powder particles come into contact with the solid support. 

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. 

No systematic studies of the use of silicone surfactants as emulsifiers have yet been published. Silicone polyoxyalkylene copolymers with relatively high molecular weight and a high proportion of silicone are effective water-in-silicone oil emulsifiers and a recent study of these copolymers suggests that they stabilize emulsions by a solid-particle mechanism [68]. This type of silicone surfactant has been used to prepare transparent water-in-oil emulsions (often with an active ingredient in the internal phase) for use as deodorants or antiperspirants as well as cosmetics and other personal care products. Their use as drug delivery vehicles has also been claimed. These copolymers can also be used to prepare multiple emulsions not requiring a two-pot process. 

Fine solid particles adsorb at interfaces and can provide long-term kinetic stability of emulsions and foams.1"5 For more effective stabilization the particles must be much smaller than the dispersed droplets.1,2 For the production of microemulsions, nano-sized particles are therefore of particular interest. 

Fat crystallization has been extensively studied in bulk fats and, to a lesser extent, in emulsified fats. It has been shown that the crystallization behavior of a fat will proceed quite differently, depending on whether it is in bulk or emulsified form (4,5). Authors have examined the effect of the state of dispersion on the crystallization mechanisms (nucleation, crystallization rate) and polymorphic behavior (6-11) of partial- and triglycerides in bulk and emulsified form. Understanding the mechanisms of emulsion nucleation and crystallization is one of the first steps in understanding the destabilization of emulsions and partial coalescence, e.g., stabilization of liquid fat emulsions by solid particles (fat) or control of the polymorphic form of crystals during the process of partial coalescence to control the size of aggregates and textural properties. 

This process involves extraction of fine particles from an aqueous phase into an oil phase. The effectiveness of this technique, as shown in Figure 2, is based on the stability of emulsion droplets with solid particles. If a particle is partially wetted by two immiscible liquids the particle will concentrate at the liquid-liquid interface. The thermodynamic criteria for distribution of solids at the interface of two immiscible liquids is the lowering in the interfacial free energy of the system when particles come in contact with two immiscible liquids. (12) If ygw, yWQ and ygp are the interfacial tensions of solid-water, water-oil and solid-oil interfaces respectively, and if ygQ > y + ygw then the solid particles are preferentially dispersed within the water phase. However, if ygw > ywq + ygQ, the solid is dispersed within the oil phase. On the other hand, if yWQ > ygQ + ysw, or if none of the three interfacial tensions is greater than the sum of the other two, the solids in such case will be distributed at the oil-water interface. 

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