Determination of cationic surfactants in pharmaceuticals based on competitive aggregation in ternary amphiphile mixtures

The surfactant to dye binding degree (SDBD) method was extended for the first time to the determination of cationic amphiphiles. For this purpose, Cresyl Violet (CV) and sodium dodecylsulphate (SDS) were selected as dye and reactant surfactant, respectively. This chemical system was used for the determination of cationic surfactants in pharmaceuticals. The approach was based on the competition established between the dye and cationic analytes to form mixed aggregates with the anionic surfactant (SDS–CV and SDS–analyte), which resulted in an increase of the amount of SDS required to reach a given SDS–CV binding degree. The feasibility of the proposed method to determine quaternary ammonium surfactants belonging to different structural groups (alkyldimethylbenzylammonium chlorides, alkyltrimethylammonium bromides and alkylpyridinium chlorides) in a wide variety of pharmaceutical formulations (solutions, creams and powders) was proved. The analytical features of the SDBD method (versatility, high precision and selectivity, ruggedness, rapidity, simplicity and low cost) made it an advantageous alternative to the conventional methods used in cationic surfactant quality control.