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-trioxau ndecane)-1,11-disulfate (2) has been investigated by electrical conduc tivity (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 temperatur e (microstructure of the system) in water + 50 mM NaBr. Most measureme nts were performed at a CTAB concentration of 5 mM, a mole fraction of 2 X = 0.091, and 30 degrees C. Some measurements were performed in pu re 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 lead s to micelles of 2 nearly fully neutralized by CTA(+) ions was detecte d by spectrofluorometry with pyrene as fluorescent probe and took plac e 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 th is micellization step. However, it detected a cme at a much higher val ue of C which closely corresponds to a free CTA(+) concentration equal to the cmc of CTAB alone. This second cmc is not detected by spectrof luorometry. The aggregation numbers of the mixed micelles are larger t han 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 o f 410. Electron microscopy shows in this system the presence of vesicl es and of very large aggregates, revealing that the system is close to precipitation under the experimental conditions used. Overall the res ults do not support the existence of the ''cross-linked'' micelles pos tulated to explain the large decrease of the NMR relaxation time T-2 t aking place upon addition of another anionic dimeric surfactant to the same CTAB system (Menger and Eliseev, Langmuir 1995, 11, 1855).