Dispersions are mixtures of two or more components (phases), one of which is present in a prevailing quantity (dispersing phase).

There may be solids dispersed in gases (aerosols), solids in liquids (colloidal suspensions), solids in solids (glasses, alloys, rocks), liquids in gases (fog), liquids in liquids (emulsions) or gases in liquids (foam).

What mainly identifies dispersions is the heterogeneity of the phases and the size of the dispersed phases, often higher than those of colloidal particles (that is, with a diameter greater than one nanometer).

In a dispersion, dispersed particles tend to come together spontaneously so as to lower the energy of the system. To prevent this from happening, it is necessary to block their approach by using, for example, the electric charge of the surface deriving from the presence of specific ions (of the particles) or from the presence of substances that – adsorbed to the surface – provide it with an affinity for the dispersal phase and thus give rise to the formation of a solvated film that surrounds the dispersed particles preventing them from coming into contact.