Pseudo-nonionic complexes as a new approach to the determination of ionic amphiphilic substances

Mixtures of anionic and cationic surfactants that form aggregates of pseudo-nonionic character and single-phase micellar solutions in the anionic- or cationic-rich range, were used to develop an effective approach to the determination of ionic amphiphilic substances based on the mixed-micelle methodology. Various alkyltrimethylammonium surfactants (ATAS) and sodium dodecyl sulphate (SDS) were tested as analytes and titrant, respectively. Nonionic-cationic aggregates made up of Triton X-100 and ATAS were comparatively studied. The linear concentration range achieved for ATAS based on the formation of pseudo-nonionic complexes was 0.1-0.6 mu M, which permits these surfactants to be determined more sensitively than with both nonionic-cationic aggregates and monomer-based methods. Because of the opposite charge of the surfactants forming the pseudo-nonionic aggregates, strong synergism effects were observed for these systems. As a result, the critical micelle concentration for the mixture was significantly lower than those for the anionic and cationic surfactants alone, improving thus the detection limit for ATAS. The response was independent of the length of the hydrophobic chain in the cationic surfactant as the Likely result of electrostatic interactions governing the incorporation of ATAS into SDS micelles. The relative standard deviation within- and between-assays for 0.3 mu M of dodecyltrimethylammonium bromide was 3.3% and 3.9%, respectively. (C) 1999 Published by Elsevier Science B.V. All rights reserved.