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 dete rmination of ionic amphiphilic substances based on the mixed-micelle method ology. 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 st udied. The linear concentration range achieved for ATAS based on the format ion of pseudo-nonionic complexes was 0.1-0.6 mu M, which permits these surf actants 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 effec ts were observed for these systems. As a result, the critical micelle conce ntration for the mixture was significantly lower than those for the anionic and cationic surfactants alone, improving thus the detection limit for ATA S. The response was independent of the length of the hydrophobic chain in t he cationic surfactant as the Likely result of electrostatic interactions g overning the incorporation of ATAS into SDS micelles. The relative standard deviation within- and between-assays for 0.3 mu M of dodecyltrimethylammon ium bromide was 3.3% and 3.9%, respectively. (C) 1999 Published by Elsevier Science B.V. All rights reserved.