Interaction, Stability, and Microenvironmental Properties of Mixed Micelles of Triton X100 and n-Alkyltrimethylammonium Bromides:  Influence of Alkyl Chain Length

Micellar properties of binary surfactant systems of Triton X100 (TX100) with three different n -alkyltrimethylammonium bromides ( n = 12 (DTAB), 14 (TTAB), and 16 (CTAB)) were investigated by the fluorescence probe technique. The critical micelle concentration (cmc) values of the corresponding mixtures in the whole range of composition were obtained by the pyrene 1:3 ratio method. In order to estimate the interaction between the surfactants in the mixed micelles, the cmc data were treated by using the conventional regular solution approach for mixed micelles. It was found that whereas the interaction parameter values (β 12 ) remained fairly constant for the TX100−TTAB and TX100−CTAB systems, the TX100−DTAB system was not well modeled by using that theory. However, in all cases the results showed deviation from ideal behavior. The stability of the mixed micelles was also discussed in the light of Maeda's treatment ( J. Colloid Interface Sci. 1995, 172, 98), and the observed differences between the three systems were justified on the basis of a certain steric factor due mainly to the presence of the phenyl group of TX100 in connection with the length of the monomer of the cationic component. The micropolarity of the mixed aggregates was examined by the pyrene 1:3 ratio index in micellar solutions of concentration well above the cmc. It was observed that the increasing participation of the ionic component induces the formation of more closed micelles with a more dehydrated structure. The polarized fluorescence measurements, by using diphenylbutadiene as a probe, were interpreted by using the wobbling in cone model. It was found that the order parameter decreases as the participation of the cationic surfactant in the micelle increases, indicating the formation of a less ordered structure than that of pure TX100 micelles. Data obtained in this investigation allowed to establish a clear correlation between micellar stability and microenvironmental properties of the mixed aggregates.