Aggregation numbers and fluorescence quenching kinetics of simple single-tailed amphiphilic vesicles of alkyltrimethylammonium bromides in water

Simple single-tailed amphiphiles (STAs) can form vesicles in their single-component aqueous solutions, under the mediation of solid surfaces, particularly through a dehydration-rehydration (DHRH) process. However, knowledge about the characteristics of the STA vesicle-systems thus obtained is still limited. In the current work, the vesicles formed by simple STAs, alkyl trimethylammonium bromides (C_<i>n</i>TABs), including dodecyl, tetradecyl, and cetyl ones (DTAB, TTAB, and CTAB), <i>via</i> a DHRH process were chosen. The aggregation numbers of the STA vesicles (<i>N</i>_v) were first estimated, and their fluorescence quenching kinetics was then investigated using the time-resolved fluorescence quenching (TRFQ) technique, which were compared with those of conventional vesicle systems. In particular, the mole fractions of C_<i>n</i>TAB existing in the form of micelles (<i>f</i>_m) in the vesicle systems were estimated using the TRFQ data. The <i>N</i>_v values of DTAB, TTAB, and CTAB were found to be <i>ca.</i> 7.33 × 10⁴, 8.33 × 10⁴, and 10.6 × 10⁴, respectively, which are similar to those of conventional vesicles. In addition, the values of fluorescence quenching kinetic parameters obtained for the STA vesicle-systems are also similar to those of conventional vesicles. For the STA vesicle systems, vesicles coexist with micelles, but the <i>f</i>_m significantly decreases with an increase in chain length. DTAB molecules exist mainly in the form of micelles (with <i>f</i>_m ∼ 0.82), while TTAB and CTAB exist mainly in the form of vesicles (with <i>f</i>_m ∼ 10^-5). Overall, the properties of STA vesicles are similar to those of conventional vesicles. This work deepens our understanding on the features of simple STA-based vesicles.