Release of polyionizable compounds from submicrometer oil-in-water emulsions.

A novel, general, theoretical equation that describes the release of polyionizable compounds from submicrometer emulsions was derived and evaluated. The model accounts for simultaneous partitioning, interfacial activity, and adsorption to surfactant for n independent drug species and j surfactant species in a single emulsion system. Under the appropriate conditions, this model collapses to a previously described model for submicrometer emulsions. The model was tested for submicrometer emulsions containing a model dibasic compound (quinine sulfate). Drug release behavior was observed at pH 3.00, 5.07, and 7.50 in the presence and absence of an anionic surfactant (sodium lauryl sulfate). At each pH, the proposed theory describes the observed release behavior. At all pHs studied, a substantial amount of quinine was adsorbed to the surface of the oil droplet. Presumably, this behavior was largely the result of electrostatic attraction between ionized drug and sodium lauryl sulfate at the oil-water interface. Adsorption constants were evaluated for complexation of cationic and dicationic species of quinine to sodium lauryl sulfate, and the adsorption constant of the cationic species was larger, presumably because of the ionic nature of the surfactant.