Determination of dialkyldimethylammonium surfactants in sewage based on the formation of premicellar aggregates

A method is described for the determination of dialkyldimethylammonium surf actants (DDAS) in sewage, based on the formation of premicellar aggregates with sodium dodecyl sulfate and using the dye Coomassie Brilliant Blue G (C BBG) as a photometric probe. The mixed aggregates are selectively obtained at DDAS concentrations three orders of magnitude below the critical micelle concentration. As a result, DDAS can be determined at the mug l(-1) level without interference from other quaternary ammonium/pyridinium or anionic/n on-ionic surfactants. Because of the selectivity of the method, the clean-u p of sewage influents and effluents only involves extraction with chlorofor m, after formation of the DDAS-sodium dodecane sulfonic acid ion pair, evap oration of the extracts to dryness and dissolution of DDAS in 4 M hydrochlo ric acid. The recovery of DDAS ranged from 82 to 93% and from 100 to 105% i n sewage influents and effluents, respectively.