Determination of dialkyldimethylammonium surfactants in consumer products and aqueous environmental samples using the mixed micelle-based methodology

The determination of dialkyldimethylammonium surfactants (DDAS) based on th e measurement of the critical micelle concentration (CMC) of mixed sodium d odecylsulphate (SDS)-DDAS aggregates in a basic medium ([NaOH]=4.8x10(-3) M ) is proposed. The dye Coomassie Brilliant Blue G (CBBG) was used as a phot ometric probe for the rapid determination of CMCs. Formation of CBBG-DDAS a nd DDAS-SDS premicellar aggregates of well-defined stoichiometrics at catio nic and anionic surfactant concentrations far below their CMCs is demonstra ted. Increased SDS concentration in the titration medium results in the for mation of DDAS-SDS mixed micelles. The strong interaction between the oppos ite charged head group of DDAS and SDS permits these cationic surfactants t o be determined at the ng ml(-1) level with a nearly uniform response for a ll the DDAS tested (12-18 alkyl carbons). The relative standard deviation f or 1.10 mug ml(-1) ditetradecyldimethylammonium bromide (DTDAB) was 1.5%. T he mixed-micelle based methodology was applied to the determination of DDAS in softeners and aqueous environmental samples (river water and laundry ef fluents) with average recoveries ranged from 87.1 to 100.6 and from 96.3 to 104.0, respectively.