Emulsions continued emulsifying agents

An emulsion is defined as a dispersion of two immiscible liquids, one of which is finely subdivided and uniformly distributed as droplets (the dispersed phase) throughout the other (the continuous phase). A third component (or multiple additional components), the emulsifying agent(s), is necessary to help stabilize the emulsion. The emulsifying agent(s) coats the droplets and prevents droplet coalescence by either reducing the interfacial tension or by creating a physical repulsion between the droplets. The dispersed phase is occasionally also defined as the internal phase the continuous phase is occasionally also defined as the external phase or dispersion medium. Virtually all emulsions are inherently physically unstable. 

Bancroft summarized the results of emulsification experiments carried out from the late I9th until the early 20th century by a rule of thumb which stated that in order to have a kinetically stable emulsion the emulsifying agent must be soluble in the continuous phase (23). In principle, this rule finds its expression in the empirical system of the HLB values (hydrophilic-lipophilic balance) which was developed in the middle of the 20th century to allow the... 

Polymers may be made by four different experimental techniques bulk, solution, suspension, and emulsion processes. They are somewhat self-explanatory. In bulk polymerization only the monomers and a small amount of catalyst is present. No separation processes are necessary and the only impurity in the final product is monomer. But heat transfer is a problem as the polymer becomes viscous. In solution polymerization the solvent dissipates the heat better, but it must be removed later and care must be used in choosing the proper solvent so it does not act as a chain transfer agent. In suspension polymerization the monomer and catalyst are suspended as droplets in a continuous phase such as water by continuous agitation. Finally, emulsion polymerization uses an emulsifying agent such as soap, which forms micelles where the polymerization takes place. 

It is difficult to explain why toxic hydrocarbons can be made selective to carrots by the addition of a nontoxic oil but not by the addition of water. Green (7) found some correlation between the toxicity of oils and their ability to emulsify. It is commonly found that high aromatic oils are easier to emulsify than are oils with low aromatic content. It is possible that some action between the aromatic hydrocarbons and the emulsifying agent results in increased toxicity. There is some evidence that the permeability of the protoplasmic membrane is the key to carrot resistance. If this is true, the presence of the emul er or the physical properties of the emulsion might increase the cell penetration of the hydrocarbons. Work is being continued along these lines and on the fundamental reasons for differential plant resistance to oils. 

Although they are a relatively small volume product—approximately 75,000 tons produced in 1949 (126)—interest in asphalt emulsion has continued at a high level. Abraham (6) has reviewed the patent literature relative to the types of emulsifying agents used, while commercial practice has been discussed by Day (16). The most common emulsifiers are sodium or potassium soaps of tall oil, abietic acid, or Vinsol resin, or colloidal clays such as bentonite for adhesive base emulsions. Lyttleton and Traxler (53) studied the flow properties of asphalt emulsions, and Traxler (122) has investigated the effect of size distribution of the dispersed particles on emulsion viscosity. A decrease in particle size uniformity was found to be accompanied by a decrease in consistency because particles of various size assume a more loosely packed condition than do those of the same size. 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

Basically, there are three components in a water-ln-oil emulsion (1) Water, the dispersed or internal phase, (2) oil, the continuous or external phase, (3) the emulsifying agent, which stabilizes the dispersion. 

An empirical generalization that predicts that the continuous phase in an emulsion will be the phase in which the emulsifying agent is most soluble. An extension for solid particles acting as emulsifying agents predicts that the continuous phase will be the phase that preferentially wets the solid particles. See also Hydrophile-Lipophile Balance. 

Superficially, emulsion polymerization resembles suspension polymerization, but there are a number of important differences. Water is used as the continuous phase and heat transfer is very good for both suspension and emulsion polymerization. In contrast to suspension polymerization, the polymer particles produced in emulsion polymerization are on the order of 0.1 xm in diameter.33 Another important difference is the presence of an emulsifying agent or soap. At the beginning of polymerization the soap molecules aggregate together in a group of about 50-100 molecules to form what is called a micelle. Some of the... 

Emulsion A dosage form consisting of a two-phase system comprised of at least two immiscible liquids, one of which is dispersed as droplets (internal or dispersed phase) within the other liquid (external or continuous phase), generally stabilized with one or more emulsifying agents. (Note Emulsion is used as a dosage form term unless a more specific term is applicable, e.g. cream, lotion, ointment.). 

The viscosity of an emulsion is directly proportional to the continuous-phase viscosity (rjc), and therefore, all the viscosity equations proposed in the literature are written in terms of the relative viscosity (17 )- If an emulsifying agent is present in the continuous phase, as is the case with emulsions, 17 Is then the viscosity of the emulsifier solution rather than the viscosity of the pure fluid phase (i.e., oil or water alone). When an emulsion is prepared, some of the emulsifying agent becomes adsorbed at the oil-water interface this adsorption tends to lower the original concentration of emulsifier in the continuous phase and cause an associated decrease in 7], However, the amount of emulsifier adsorbed is usually very low compared with the total amount present, and therefore any decrease in concentration of the emulsifier can easily be neglected (23). 

The chemical nature and the concentration of an emulsifying agent also play a role in determining the viscosity of emulsions (37). The average particle size, particle size distribution, and the viscosity of the continuous phase (to which an emulsifier is normally added) all depend upon the properties and concentration of emulsifying agent. Also, ionic emulsifiers introduce electroviscous effects, leading to an increase in the emulsion viscosity. 

A stable emulsion is one where the globules retain their initial character and remain uniformly distributed throughout the continuous phase. The addition of emulsifying agents results in the formation of an interfacial film around each of the dispersed droplets the physical nature of this film creates a barrier that controls eventually the coalescence of the droplets that approach one another. If this film is electrically charged through the use of charged surfactants, repulsion occurs before contact is made and improvements in stability are normally seen. 

Emulsion polymerization is considered the generally accepted technique to produce dispersion (plastisol) resins and vinyl latexes today. The monomers are made dispersible by emulsifiers in a continuous water phase. The initiators or catalysts are water soluble while the emulsifying agents stabilize the emulsion formed when the system is agitated. Gellner (2 ) cites the following advantages of emulsion polymerization over other methods ... 

The term emulsification refers to the technique that involves mixing of two immiscible materials (usually liquids) in order to produce a homogeneous system. Usually, one of the two materials has an oily nature and the second is the water. In the emulsion, the liquid present in the larger proportion is called continuous phase, while the liquid in the smaller proportion, which disperses, is called dispersed phase. Depending on the dispersed phase, there are different types of emulsions, that is, oil-in-water (o/w) wherein oil is the dispersed phase, water-in-oil (w/o) wherein water is the dispersed phase, as well as multiple emulsions, like oil-in-water-in-oil (o/w/o), in which there are continuous layers of the two immiscible materials. Emulsification process includes the use of an emulsifying agent, which will be adsorbed on the interface of the two immiscible materials, in order to achieve the miscibility of them. The structure of these agents contains a polar and nonpolar part, and such molecules may be proteins, phospholipids, etc. Emulsification can also include the use of other colloidal macromolecules, which are able to form multilayer films on the interface, in order to achieve a better kinetic stabilization of the system. ... 

---