Time evolution of monomers and aggregates of a polymethine dye probe the dynamics of model vesicles and micelles

An anionic polymethine dye (a fluoro-oxonol abbreviated FDID), which forms monomers and oligomers according to the surrounding medium, is used in this work to probe the stability and dynamics of bicationic colloidal nanostructures, namely micelles and vesicles. Using pure water as the reference solvent (medium 1), five aqueous colloidal media of the lipid–detergent system DDAB–DTAC (didodecyldimethylammonium bromide–dodecyltrimethylammonium chloride) were analyzed: pre-micelle DTAC media (2); pure DTAC micelles (3); DDAB–DTAC micelles (4); DDAB–DTAC vesicles (5); and pure DDAB vesicles (6). In systems 4–6, containing the double-chained DDAB, FDID showed a slow-dynamics disaggregation, H-aggregates → monomers. The kinetics of this transition was followed by running UV–vis absorption spectra along time. The transition lifetimes τ increased in the order τmedium 3 ≪ τmedia 4,5 < τmedium 6, reflecting the much faster mobility of the single-chained DTA+ long ion, as compared to that of the more hydrophobic, double-chained, DDA+. This paper showed that FDID can probe the exchanging dynamics of the DDA+/DTA+ long cations between the water phase and the colloidal nanostructures.