Surfactant–dye binding degree method for the determination of amphiphilic drugs

The surfactant–dye binding degree (SDBD) method was extended to the determination of amphiphilic drugs. This new methodology was based on the effect of amphiphilic compounds on the degree of binding of a surfactant to dye molecules. The dye induces the formation of surfactant aggregates and allows this process to be monitored from changes in their spectral features. The interaction between the anionic dye Coomassie Brilliant Blue G (CBBG) and the cationic surfactant didodecyldimethylammonium bromide (DDABr) was used for the sensitive determination of phenamic acids (meclophenamic, mephenamic, fluphenamic and niflumic acid), non-steroidal anti-inflammatory drugs. The addition of phenamic acid to the dye–surfactant mixture resulted in the formation of drug–DDABr aggregates with well-defined stoichiometries (between 1:1 and 1:3) and, hence, in decreased interactions between the dye and the cationic surfactant. Mixtures of drug–surfactant were demonstrated to behave as those made up of pure surfactant, and, therefore, the expression previously derived for determining surfactants could be used to quantify amphiphilic drugs. The proposed method permitted the determination of phenamic acids at the mg l^-1 level with the precision required for quality control (the relative standard deviation for 7 mg l^-1 of meclophenamic acid was 1.1%). Pharmaceutical preparations were analysed directly after dissolution of the samples in ethanol.