After completing this chapter, the reader should be able to:
► Describe the difference between homogenous and heterogeneous dispersions.
► Describe classifications of dispersed systems, based on the phases of the components and based on the size of the dispersed particles.
► Describe the properties and list examples of colloidal dispersions and coarse dispersions.
► Describe the natural forces involved in the movement and interaction of dispersed particles.
► Describe the clinical importance and mechanisms of destabilization of pharmaceutical dispersions.
► Apply knowledge of the parameters involved in dispersion destabilization to recognize, and prevent or resolve, the formulation, preparation, or patient use challenges associated with dispersed dosage forms.
Pharmaceutical dispersions are systems where one substance is dispersed within another substance. In this sense, these would include homogenous molecular dispersions—the true solutions discussed in Chapter 3. However, the dispersions discussed in this chapter differ from true solutions in that they are heterogeneous systems—as exemplified by suspensions and emulsions—where the dispersed phase, typically a particle of some type, is physically distinguishable from the medium in which it is dispersed. In pharmacy, dispersions are found in a wide variety of dosage forms and in nearly all routes of drug administration. Examples range from solutions of very large molecules (macromolecules) such as albumin and polysaccharides, to liquid suspensions of “nano”-sized crystals (nanocrystals) and of “micro”-sized droplets (microemulsions), to coarse (larger particle) emulsions and suspensions. Having physically distinct phases endows dispersions with properties that are different from true solutions, and it's these properties, such as particle settling and aggregation, that are the main concepts discussed in this chapter.
Terms used to describe dispersion components include internal or noncontinuous phase to describe the dispersed phase component (the particles) and the external or continuous phase to describe the dispersion medium. As for the nature of the dispersion mixture, the different phases can be the same state of matter, such as with an oil droplet-in-water (o/w) emulsion, or different states of matter, such as a suspension of solid crystals in water. How dispersions are classified can also vary, with the two main approaches to classification being (1) the nature of the dispersion's internal and external phases (e.g., solid, liquid, or gas) and (2) the size range of its dispersed particles (colloidal versus coarse).
DISPERSED SYSTEMS CLASSIFIED BY THEIR PHASES
Table 8-1 lists several examples of dispersions classified by the states of matter that comprise the dispersed phase and the dispersion medium. In addition to the macromolecular dispersions, liquid-in-liquid emulsions, and solid-in-liquid suspensions mentioned, they include solid-in-solid suspensions (e.g., some suppositories), semisolid emulsions (e.g., creams), and gas-in-liquid foams. Adding to the spectrum of available dispersions are those that have more than two phases. Common examples of these include the typical over-the-counter hydrocortisone cream, in which the solid hydrocortisone crystals are suspended in an oil-water emulsion ...