Types of Emulsions

Macroemulsions

On the basis of the nature of the dispersed phase and the dispersion medium, two types of macroemulsions can be distinguished: (i) If the continuous phase is an aqueous solution, and the dispersed phase an oil, the system is called an o/w emulsion. Such an o/w emulsion is generally formed if the aqueous phase constitutes more than 45% of the total weight, and if a hydrophilic emulsifier is used. (ii) Conversely, if the aqueous phase is dispersed within the oil, the system is called a w/o emulsion. w/o emulsions are generally formed when the aqueous phase constitutes less than 45% of the total weight and if a lipophilic emulsifier is used. Generally, o/w emulsions are more popular than w/o emulsions in the pharmaceutical field, especially when they are designed for oral administration. In the cosmetic industries, lotions or creams are either of the o/w or w/o type, depending on their applications. o/w emulsions are most useful as water-washable drug bases and for general cosmetic purposes, while w/o emulsions are employed more widely for the treatment of dry skin and emollient applications. It is important for a pharmacist to know the type of emulsion since it can significantly affect its properties and performance. There are several methods to determine the emulsion type, as summarized in Table 4 (121).

Multiple Emulsions

Multiple emulsions are more complex systems (25,122,123). If a simple emulsion is further dispersed within another continuous phase, a triple emulsion is obtained, which can again be dispersed within a further continuous phase, etc. Figure 7 shows as an example the two preparation steps for a water-in-oil-in-water (w/o/w) emulsion: In the first step, an aqueous phase is added to an oily phase containing a lipophilic surfactant.

Table 4 Methods to Determine the Type of a Macroemulsion (w/o or o/w)

Test

Method/observation

Phase dilution test

Dye solubility test

Conductivity test

Place two emulsion droplets on a glass slide and add a droplet of one component to each emulsion droplet, stir, and observe under a microscope. This test is based on the principle that an emulsion can only be diluted with the liquid that constitutes the continuous phase A colored dye soluble in only one component is added to the emulsion. If the color spreads throughout the whole emulsion, the phase in which the dye is soluble is the continuous phase. Immerse a pair of electrodes connected to an external electric source in the emulsion. If the external phase is water-containing electrolytes, a current passes through the emulsion. If the oil is the continuous phase, the emulsion fails to carry the current

Abbreviations: W/O, water in oil; o/w, oil in water.

Oily phase + W/O emulsion lipophilic surfactant

Oily phase + W/O emulsion lipophilic surfactant

Aqueous phase + W/O/W emulsion hydrophilic surfactant

Figure 7 Schematic presentation of a two-step preparation procedure used to obtain a water-in-oil-in-water (w/o/w) emulsion.

Aqueous phase + W/O/W emulsion hydrophilic surfactant

Figure 7 Schematic presentation of a two-step preparation procedure used to obtain a water-in-oil-in-water (w/o/w) emulsion.

Upon mixing, a w/o emulsion is formed. In the second step, this w/o emulsion is poured into a second aqueous phase containing a hydrophilic surfactant. Upon mixing, the multiple w/o/w emulsion is formed.

Multiple emulsions can be end products or serve as intermediate products, for instance, during the preparation of drug-loaded microparticles (124-126). The use of multiple emulsions, such as w/o/w, o/w/o, w/o/o, and w/o/o/o emulsions, can, for example, help to reduce drug loss into an outer aqueous phase and, thus, increase the resulting drug encapsulation efficiency.

Microemulsion

Already in the early 1940s, Hoar and Schulman introduced the concept of microemulsions (127). However, the term "microemulsion" was only proposed in 1959 (128). Since then it has been redefined on various occasions. In 1981, Danielsson and Lindman (129) defined a microemulsion as "a system of water, oil, and amphiphile, which is a single optically isotropic and thermodynamically stable liquid solution." A three-phase diagram illustrating the area of existence of microemulsions is presented in Figure 8 (130). The phase equilibria, structures, applications, and chemical reactions of microemulsion have been reviewed by Sjoblom et al. (131). In contrast to macroemulsions, microemulsions are optically transparent, isotropic, and thermodynamically stable (132,133). Micro-emulsions have been subject of various investigations, because their unique properties

Lamellar

Surfactant

Lamellar

Surfactant

Figure 8 A hypothetical pseudoternary phase diagram of an oil/surfactant/water system with emphasis on microemulsion and emulsion phases. Within the phase diagram, existence fields are shown where conventional micelles, reverse micelles, or water-in-oil (w/o) microemulsions and oil-in-water (o/w) microemulsions are formed along with the bicontinuous microemulsions. At very high surfactant concentrations two-phase systems are observed. Source: From Ref. 130.

Figure 8 A hypothetical pseudoternary phase diagram of an oil/surfactant/water system with emphasis on microemulsion and emulsion phases. Within the phase diagram, existence fields are shown where conventional micelles, reverse micelles, or water-in-oil (w/o) microemulsions and oil-in-water (o/w) microemulsions are formed along with the bicontinuous microemulsions. At very high surfactant concentrations two-phase systems are observed. Source: From Ref. 130.

allow for a wide range of potential practical applications (134-139). However, there is still substantial controversy concerning the exact nature of these systems and the appropriateness of the terminology. Terms such as "transparent emulsions," "micellar solutions," "solubilized systems," and "swollen micelles" have all been applied to the same or similar systems. Nonetheless, emulsions and microemulsions may be differentiated on the basis of particle size: microemulsions contain particles in the nanometer size range (typically 10-100 nm), whereas conventional emulsions (or "macroemulsions" or "coarse emulsions") contain particles in the micrometer range.

Liposomes

Liposomes are vesicular lipid systems of a diameter ranging between 50 nm and a few mm. They are composed of membrane-like, lipid layers surrounding aqueous compartments. The lipid layers consist of phospholipids, making liposomes biocompatible and biodegradable. The phospholipids have a hydrophilic head and a lipophilic tail. Different preparation methods for liposomes have been described by Vemuri et al. (140), leading to different types of vesicular structures (Fig. 9) (141). Nowadays, liposomes are widely used for drug delivery and drug targeting (142-147).

In Situ-Forming Microparticle Systems

The ISM systems were first introduced by Bodmeier and coworkers (148-152). They are included in this chapter as special disperse systems because these formulations are based on o/o or o/w emulsions. Being liquids, the ISM systems can easily be injected intramuscularly or subcutaneously and subsequently form microparticles within the human body upon contact with physiological fluids. The ISM system is composed of a drug and biodegradable polymer, which are codissolved in a water-miscible, biocompatible solvent. This solution is emulsified into an external phase, either oily or aqueous, containing an emulsion stabilizer to form o/o or o/w emulsions. Upon contact with aqueous physiological fluids, the polymer solvent dissipates, leading to polymer

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