Mixed micellization of cetyltrimethylammonium bromide and an anionic dimeric (Gemini) surfactant in aqueous solution

The mixed micellization between cetyltrimethylammonium bromide (CTAB) and the anionic dimeric surfactant disodium 1,11-didecyl-3,6,9-trioxaundecane)-1,11-disulfate (2) has been investigated by electrical conductivity (Krafft temperature, T_K, critical micellization concentration, cmc, and micelle ionization degree), spectrofluorometry (cmc, micelle polarity), time-resolved fluorescence quenching (micelle aggregation numbers), and transmission electron microscopy at cryogenic temperature (microstructure of the system) in water + 50 mM NaBr. Most measurements were performed at a CTAB concentration of 5 mM, a mole fraction of 2 X = 0.091, and 30°C. Some measurements were performed in pure water for assessing the effect of NaBr. The presence of 2 enlarged the range of temperature in which a 5 mM CTAB suspension in water + 50 mM NaBr clarifies. The initial micellization step which probably leads to micelles of 2 nearly fully neutralized by CTA⁺ ions was detected by spectrofluorometry with pyrene as fluorescent probe and took place at a CTAB concentration, C, below 0.1 mM. In both water and water + 50 mM NaBr, the electrical conductivity method was not sensitive to this micellization step. However, it detected a cmc at a much higher value of C which closely corresponds to a free CTA⁺ concentration equal to the cmc of CTAB alone. This second cmc is not detected by spectrofluorometry. The aggregation numbers of the mixed micelles are larger than those of CTAB micelles even at X as low as 0.02. At X = 0.091, the apparent aggregation number is very large, with a lower bound value of 410. Electron microscopy shows in this system the presence of vesicles and of very large aggregates, revealing that the system is close to precipitation under the experimental conditions used. Overall the results do not support the existence of the "crosslinked" micelles postulated to explain the large decrease of the NMR relaxation time T₂ taking place upon addition of another anionic dimeric surfactant to the same CTAB system (Menger and Eliseev, Langmuir 1995, 11, 1855).