POLYELECTROLYTE-INDUCED MICELLE FORMATION OF IONIC SURFACTANTS AND BINARY SURFACTANT MIXTURES STUDIED BY TIME-RESOLVED FLUORESCENCE QUENCHING

A semiempirical model for the distribution of two cationic surfactants between subphases in solutions containing a polyelectrolyte of opposite charge is presented. The model, based on ideal mixing in the surfactant aggregates, is tested in fluorescence quenching experiments with satisfying results. An experimental relation which gives the critical aggregation concentration (cac) or critical micelle concentration (cmc) for dodecylpyridinium ions (C12P+) as a function of cac (or cmc) for dodecyltrimethylammonium ions (C(12)TA(+)) under the same conditions is found. The relation, which seems to be quite general, is very useful when the mixing model is applied to results from fluorescence quenching. Binding isotherms for C(12)TA(+) in solutions of sodium polyacrylate (NaPA) at various concentrations of NaBr are reported together with estimated surfactant aggregation numbers. The aggregation numbers (60 +/- 4) are not affected by the presence of 0.01 m NaBr or by an increase in the polyelectrolyte concentration. The cac for C(12)TA(+) is found to increase both with increasing NaBr and NaPA concentrations, although stronger in the former case. We show that the interpretation of the polyion-surfactant interactions can be rationalized from an analogy with the self-assembly of free surfactant. From this perspective we estimate the ''local cmc'', i.e. the critical concentration of surfactant close to the polyion when micelle formation starts. The local cmc is found to decrease with increasing salt and polyion concentration.