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 “cross-linked” micelles postulated to explain the large decrease of the NMR
relaxation time T
2 taking place
upon
addition of another anionic dimeric surfactant to the same CTAB system
(Menger and Eliseev, Langmuir
1995, 11, 1855).
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