Fluorescence and Conductivity Studies of Polyelectrolyte-Induced Aggregation of Alkyltrimethylammonium Bromides

Abstract

Micelle formation of alkyltrimethylammonium surfactants (CnTMAB) in aqueous sodium poly(styrenesulfonate) (NaPSS) solutions was studied by fluorescence and conductivity measurements at 25 °C. The dependence of the aggregation phenomena on the chain length of the surfactant (n = 12, 14, 16, 18) and on the ionic strength of solutions was investigated. The critical aggregation concentration (cac) decreases with increasing hydrophobicity of the surfactant but remains approximately unchanged for C16TMAB and C18TMAB. The cac values are considerably lower than the critical micellization concentration (cmc) values for the same surfactant. The decrease is in the range from about three orders of magnitude for C12EDMAB in water to approximately 10 times for C18TMAB in 0.01 M NaBr. Furthermore, the cac for C12EDMAB and C14TMAB in NaPSS was found to increase slightly with increasing NaBr concentration but much less than, for instance, in sodium poly(acrylate) solutions. No significant effect of NaBr on cac values for C16TMAB and C18TMAB was found. The relatively weak dependence of cac on the surfactant chain length and on the simple salt concentration can be explained by the formation of a very stable aggregate between the polyelectrolyte and the amphiphile via the inclusion of aromatic side groups on the polymer chain into the surface of the micelle-like surfactant aggregate. Measurements of electrolytic conductivity showed that the mobility of the surfactant ions is greatly reduced in the presence of oppositely charged polyion. The surfactant preferentially forms the complex with the polyelectrolyte. When the complexation is complete free micelles appear in the system at the apparent critical micellization concentration (cmc*), which is therefore slightly higher in polyelectrolyte solutions than the ordinary cmc in pure surfactant solutions.