Micellar solubilization in aqueous binary surfactant systems: Barbituric acids in mixed anionic + nonionic or cationic + nonionic mixtures

Abstract

The micellar solubilization of four barbituric acids, namely reposal, pentobarbital, butobarbital, and barbital, has been determined in the whole composition range of two binary surfactant systems: (I) sodium dodecylsulfate (SDS) + dodecylpolyoxyethylene 23 (POE) and (II) tetradecyltrimethylammonium bromide (TTAB) + POE. Reposal solubility was also studied in system (I) in presence of 0.2 molal NaCl. The specificities of solubilization experiments in mixed surfactant systems are discussed. It is shown that the variation of the barbituric acids' partition coefficients P between micelle and water as a function of micellar composition may be adequately represented by a relationship similar to that deduced from the regular solution approximation specialized for a three-component solution, i.e., with a single empirical coefficient. This parameter is negative for all systems studied. It is solute dependent for system (I) but not for system (II). From a comparison with the regular solution approach, it is concluded that the interaction between unlike surfactants in the micelles governs the sign and amplitude of the fitting parameter and therefore the variation of P with surfactant composition. As a consequence it is indicated that in most cases, the P values of polar hydrophobic molecules should be less in mixed surfactant systems than that calculated from the ideal additivity principle.