Emulsions complex dispersions

The number of the constituent phases of a disperse system can be higher than two. Many commercial multiphase pharmaceutical products cannot be categorized easily and should be classified as complex disperse systems. Examples include various types of multiple emulsions and suspensions in which solid particles are dispersed within an emulsion base. These complexities influence the physicochemical properties of the system, which, in turn, determine the overall characteristics of the dosage forms with which the formulators are concerned. 

FFF is an analytical technique well deserving of a more widespread application at least in its most developed variants Fl-FFF, Th-FFF and S-FFF. Especially for complex colloidal or particulate matter, emulsion and dispersion technology, there are many advantages of FFF which certainly justify the implementation and research in most analytical laboratories dealing with such problems. 

Conditions can be deduced from the energy balance mentioned above under which the rewetting tension is positive, a precondition for the mechanism shown in Fig. 1.12. It should be said that re-wetting is the first step in the complex process of removal of hydrophobic layers from a solid. The oil droplets formed must also be sufficiently stabilised by the surfactant to prevent them coalescing. This takes place in many cleaning processes. When a water-insoluble substance is to be dispersed in water we can distinguish between thermodynamically stable and unstable dispersions. Thermodynamically unstable dispersions are the usual emulsions or dispersions of solids. Solubilisation systems and optically transparent emulsions, so-called micro-emulsions, are in a metastable state where drop growing by collision and coalescence cannot be completely suppressed. These systems are frequently called thermodynamically stable. 

Emulsions are dispersions of one liquid in another liquid, most commonly water-in-oil or oil-in-water. The total interfacial area in an emulsion is very large, and sinee flie interfacial area is associated with a positive free energy (the interfacial tension), the emulsion system is thermodjmam-ically unstable. Nevertheless, it is possible to make emulsions with an excellent long-term stability. This requires the use of emulsifiers that accumulate at the oil/water interface and create an energy barrier towards flocculation and coalescence. The emulsifiers can be ionic, zwitterionic, or nonionic surfactants, proteins, amphiphilic polymers, or combinations of polymers and surfactants. The structure of the adsorbed layer at the water/oil interface may be rather complex, involving several species adsorbed directly to the interface as well as other species adsorbing on top of the first layer. 

Uses Emulsifier, stabilizer, texturizer for foods, beverages, chewing gum protective coating on fruits aerating agent starch complexing agent dispersant, stabilizer for fats carrier, solvent for colors and fat-sol. antioxidants emulsifier in cosmetic and pharmaceutical creams, lotions, ointments emulsion stabilizer, dispersant, solubilizer for pharmaceutical tablets... 

CMG crystallizes as an a-like crystal form, which is stable. It forms a milky emulsion or dispersion in water, but owing to the complex composition it forms no ordered mesomorphic structures. 

Polyurethane emulsions and dispersions in water may also be made by incorporating polar or hydrophilic groups as described in Chapter 9. They are fully reacted, containing urethane and urea groups, are of high molecular weight, and may contain cosolvent. The chemistry of these is complex and beyond the scope of this book. They are used for the same applications as the solution polymers, and as primers for a range of plastic surfaces. 

Emulsions with complex multilayered stmctures are referred to as multiple emulsions. A well-known example is a double emulsion, in which microdroplets that enclose even smaller droplets are suspended in a continuous liquid phase [1-3]. Figure 21.1 shows the two main types of double emulsions water-in-oil-in-water (W/O/W) emulsions, in which a water-in-oil (W/0) emulsion is dispersed in an aqueous phase (Figure 21.1a) and oil-in-water-in-oil (O/W/O) emulsions, in which an oil-in-water (0/W) emulsion is dispersed in an oil phase (Figure 21.1b). Usually, both hydrophilic and lipophilic surfactants are required to stabilize these multilayered dispersions. 

Complex Dispersions Such as Emulsions and Other Formulations... 

Emulsifiers (also known as emulgents) are surfactants enabling the formation of emulsions (especially dispersions of fat in various products). In addition to their ability to form an emulsion, emulsifiers have the ability to interact with other food ingredients. The emulsifier may be an aerating agent, starch complexing agent and/or crystallisation inhibitor. In flours they act as conditioners... 

Food emulsions and foams are complex dispersions that constitute a major part of the food products that are consumed daily. The correct elaboration of such colloidal systems determines the functional properties of the final product such as texture or long-term stability. Accordingly, the optimization of the food product depends fundamentally on the comprehension of the structural characteristics of its components. Foams and emulsions are dispersions of air and liquid in another immiscible liquid, respectively. Formation of these dispersions is subject to the presence of amphiphilic molecules, which tend to place themselves at the air-water interface (foams) or the oil-water interface (emulsion). Thus, they constitute molecular barriers that stabilize the dispersion. The composition and structure of these molecular barriers determine ultimately the behavior of foams and emulsions. 

Complex Coacervation. This process occurs ia aqueous media and is used primarily to encapsulate water-iminiscible Hquids or water-iasoluble soHds (7). In the complex coacervation of gelatin with gum arabic (Eig. 2), a water-iasoluble core material is dispersed to a desired drop size ia a warm gelatin solution. After gum arabic and water are added to this emulsion, pH of the aqueous phase is typically adjusted to pH 4.0—4.5. This causes a Hquid complex coacervate of gelatin, gum arabic, and water to form. When the coacervate adsorbs on the surface of the core material, a Hquid complex coacervate film surrounds the dispersed core material thereby forming embryo microcapsules. The system is cooled, often below 10°C, ia order to gel the Hquid coacervate sheU. Glutaraldehyde is added and allowed to chemically cross-link the capsule sheU. After treatment with glutaraldehyde, the capsules are either coated onto a substrate or dried to a free-flow powder. 

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

Sihcone products dominate the pressure-sensitive adhesive release paper market, but other materials such as Quilon (E.I. du Pont de Nemours Co., Inc.), a Werner-type chromium complex, stearato chromic chloride [12768-56-8] are also used. Various base papers are used, including polyethylene-coated kraft as well as polymer substrates such as polyethylene or polyester film. Sihcone coatings that cross-link to form a film and also bond to the cellulose are used in various forms, such as solvent and solventless dispersions and emulsions. Technical requirements for the coated papers include good release, no contamination of the adhesive being protected, no blocking in roUs, good solvent holdout with respect to adhesives appHed from solvent, and good thermal and dimensional stabiUty (see Silicon COMPOUNDS, silicones). 

Citric acid is utilized in a large variety of food and industrial appHcations because of its unique combination of properties. It is used as an acid to adjust pH, a buffer to control or maintain pH, a chelator to form stable complexes with multivalent metal ions, and a dispersing agent to stabilize emulsions and other multiphase systems (see Dispersants). In addition, it has a pleasant, clean, tart taste making it useful in food and beverage products. 

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