Analytical potential of mixed micelle-based methodology for the determination of ionic surfactants

Application of mixed micelle-based methodology was extended to the determination of C10–C16 alkyl sulphate surfactants which have critical micelle concentrations (cmc) in the range 10−12–10−4M. The cmc is decreased by about two or three orders of magnitude by the presence of sodium chloride (2.2 M) and hydrochloric acid (0.7 M) in the titration medium, which permits these surfactants to be determined at the same concentration levels as with monomer-based methods. The sensitivity of mixed micelle-based methodology is shown to depend on both the cmc of the analyte and its value relative to the cmc of the titrant. The linear concentration range achieved for alkyl sulphate surfactants is 0.05–2 mg l−1. Responses are independent of the molecular weight for surfactants containing more than 10 carbon atoms in the alkyl chain. A selectivity study revealed that no inorganic ions (e.g. Cl−, SO42−, NO3−, Ca2+, Mg2+, Fe3+, etc.) interfere because their electrolyte effect is insignificant due to the high concentration of added salts. Dissolved organic compounds such as fenol, aniline and o-dianisidine are also tolerated at high concentrations because of the little effect of solubilization on micellization. Additional advantages of mixed micelle-based methodology over most analytical methods using monomeric anionic surfactants include experimental convenience (no extraction is needed in the determination step) and expeditiousness (mixed micelles are formed instantaneously).