Colloidal and Coarse Lyophobic Dispersions

Abstract

Materials in the colloidal state are those in which the particles range from molecular size to coarse dispersions. The word colloid derives from the Greek kolla (glue) and was coined from the impression that colloidal substances were amorphous or glue-like rather than crystalline forms of matter. The colloidal state was recognised by Thomas Graham in 1861 and described by Wolfgang Ostwald some fifty years later as the ‘world of neglected dimensions’, a reference both to the fact that colloid science has somehow remained a Cinderella topic, and to the special world of systems in which the particles are extremely small, below about 1–10 µm in diameter. Colloids can be broadly classified as those that are lyophobic (solvent hating) and those that are lyophilic (solvent loving). The appropriate terms in aqueous media are, respectively, hydrophobic and hydrophilic. Surfactant molecules, because of their affinity for water and their tendency to associate into micelles, form hydrophilic colloidal dispersions in water. Proteins and gums also form lyophilic colloidal systems. Water-insoluble drugs or clays and oily phases will form lyophobic dispersions, the topic covered in this chapter. Lyophobic colloidal dispersions have the tendency to coalesce as they are thermodynamically unstable because of their high surface energy. Lyophilic dispersions are inherently stable.