Emulsions stability theory

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

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],... 

Upon mixing two immiscible liquids, one of the two liquids (i.e., the dispersed phase) is subdivided into smaller droplets. The surface area and the interfacial free energy increase, and the system is then thermodynamically unstable. Without continuous mixing, the droplets will be stabilized throughout the dispersion medium by dissolving the surface-active agent. There are several theories for the stabilization of emulsions but a single theory cannot account for the stabilization of all emulsions. 

In the interfacial tension theory, the adsorption of a surfactant lowers the interfacial tension between two liquids. A reduction in attractive forces of dispersed liquid for its own molecules lowers the interfacial free energy of the system and prevents the coalescence of the droplets or phase separation. Therefore the surfactant facilitates the stable emulsion system of the large interfacial area by breaking up the liquid into smaller droplets. However, the emulsions prepared with sodium dodecyl (lauryl) sulfate separate into two liquids upon standing even though the interfacial tension is reduced. The lowering of the interfacial tension in the stabilization of emulsions is not the only factor we should consider. 

To quantify the increase of a due to pressure, a mean bubble diameter has been estimated using Taylor s stability theory [7] on bubble deformation and break-up in sheared emulsions. According to this theory, bubble size in a sheared emulsion results from a balance between viscosity and surface tension forces. The dimensionless number that describes the ratio of these forces is called the capillary number Q. For large bubble deformations, the maximum stable bubble diameter in a shear flow is expressed as [8] ... 

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... 

It is usually observed that mixtures of surfactants form more stable emulsions than do single surfactants. This may be because complex formation at the interface results in a more rigid stabilising film. Certainly where complex films can be formed, such as between sodium lauryl sulfate and cetyl alcohol, the stability of emulsions prepared with such mixtures is high. Theory has not developed to... 

Some progress toward an understanding of these systems is also possible by considering the influence of the presence of water within the oil drops on the interaction between the oil drops and by consideration of the influence of the size of the internal water droplets on their internal stability and on the possibility of coalescence with the external aqueous phase. It is premature to consider all this in detail as the application of colloid stability theory to simpler emulsions has not been particularly successful (37). For type A w/o/w emulsions, the approach of Void (38) may perhaps be used if the oil layer is thought of as the homogeneous adsorbed layer. 

From the survey of the above literature, it is concluded that only a limited work is done on such type of problems. In the present study, the stability of emulsion has been discussed in the light of Derjaguin, Landau, Vervey and Overbeek theory (8) using Deoxyribonucleic acid and ribonucleic acid as flocculants for the emulsion stabilized by the drug sulphapyridine. 

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... 

The stability of colloidal emulsions—also due to the unexplained sinic-ture—is very difficult to foresee, as particularly in mixtures for total parenteral nutrition (TPN) there are many interactions. In a. series of anictes Washington attempted to put the prediction of stability of emulsions in TPN regimes on a rational basis (183-189). To explain the stability of emulsions the following effects and theories are referred to ... 

We have investigated theoretically film-thickness stability and structure formation inside a liquid film by Monte Carlo numerical simulations and analytical methods, using the Omstein-Zemicke (0-Z) statistical mechanics theory (21-24). The formation of longrange, ordered microstructures (giving rise to an oscillating force) within the liquid film leads to a new mechanism of stabilization of emulsions (3,4,25). In addition to the effective volume of micelles or other colloidal particles and polydispersity in micelle size, the film size is also found to be flic main parameter governing emulsion stability (15). 

JA Kitchener, PE Musselwhite. The Theory of Stability in Emulsions. Emulsion Science. New York Academic Press, 1968. 

It is clearly evident that emulsions are very complicated systems. Progress has been made on theoretical studies attempting to clarify the complexities of these systems. However, the majority of predictions of the type and stability of emulsions derives more from empirical observation than from theory. Emulsion formulation is still considered to be an art rather than a scientific method in many circles of industry (11). 

Casein or egg-yolk proteins are used as emulsifiers in another category of O/W food emulsions [34,126]. A key difference here is that in these caseinate-stabilized oil emulsions, the casein forms essentially monolayers and there are no casein micelles or any calcium phosphate. Such emulsions are thought to be stabilized more by electrostatic repulsive forces and less by steric stabilization [126]. Similarly, mayonnaise, hollandaise, and beamaise sauces, for example, are O/W emulsions mainly stabilized by egg-yolk protein [34,129], Here, the protein-covered oil (fat) droplets are stabilized by a combination of electrostatic and steric stabilization [129]. Perram et al. [130] described the application of DLVO theory to emulsion stability in sauce beamaise. 

A simple geometric theory for the stabilization of emulsions is that of the oriented wedge, in which the adsorbed surfactant molecules are assumed to form a uniform structure of wedges around the emulsion droplet. If an emulsion of 1000-nm-diameter droplets is stabilized by a surfactant whose head group occupies a surface area of 0.45 nm, what must be the cross-sectional area of the hydrophobic tail for maximum effectiveness ... 

The unique density dependence of fluid properties makes supercritical fluids attractive as solvents for colloids including microemulsions, emulsions, and latexes, as discussed in recent reviews[l-4]. The first generation of research involving colloids in supercritical fluids addressed water-in-alkane microemulsions, for fluids such as ethane and propane[2, 5]. The effect of pressure on the droplet size, interdroplet interactions[2] and partitioning of the surfactant between phases was determined experimentally[5] and with a lattice fluid self-consistent field theory[6]. The theory was also used to understand how grafted chains provide steric stabilization of emulsions and latexes. 

In Chapter 3, the solution and surface properties of a relatively new class of material, namely, polymeric surfactants, are illustrated in some detail using Flory-Huggins theory and current polymer-adsorption theory. This is followed by a discussion of the phenomenon of steric stabilization of suspended particles and how it is affected by the detailed structure of the stabilizing polymeric species. It concludes with a discussion of the stabilization of emulsions by interfacial and bulk theological effects, and presents closing comments on multiple emulsions. 

Theory of steric stabilization of emulsions the role of the relative ratio of adsorbed layer thickness to the droplet radius. 

Now, if we think that the increase of the interfacial area involved in the emulsification process is of a 10 order of magnitude and that even the most efficient surface-active agent can reduce the interfacial tension by a factor of 5-10, we cannot understand why emulsifiers can stabilize the emulsions on the base of surface-tension calculations. In order to answer this question, the modem theory of Deijaguin, Landau, Verwey, and Overbeek (DLVO) will be used, but from a qualitative point of view because this chapter is devoted to the formulation job. 

Eccleston [19] reviewed the emulsion stability factors on the basis of the HLB and of the DLVO theory. He emphasized the Friberg school results which show that the presence of liquid crystals stabilizes the emulsions by delaying the film-thinning process and, consequently, by reducing the rate of coalescence. He also mentioned stabilization by the formation of a gel network (surfactant-fatty alcohol-water system). Dahms [20] explained the role of fatty alcohol as a viscosity modifier on the basis of lamella-phase generation. 

The degree of stability of many dispersions cannot be explained solely on the basis of Fa and Fr. Elworthy and Florence [56] have treated the stability of emulsions of chlorobenzene and anisole stabilized with a series of synthetic polyoxyethylene ethers in the light of colloid theory and have shown that electrical stabilization alone cannot explain the stability observed. The nature of this other force which is invoked to explain discrepancies between theory and experiment is not fully worked out. Nevertheless, much interest has been shown in this alternative mechanism of stabilization, which for non-ionic emulsions appears to play the major role [64]. Results have indicated that the thickness and degree of solvation of adsorbed layers is critical [65]. Thus, the particular conformation and length of the polyoxyethylene chains of non-ionic surfactants at interfaces is likely to be an important factor in the stabilization of emulsified droplets. 

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