Polyelectrolyte Surfactant Nucleation Sites
FULL PAPER
the CMC of pure C12TAB (third inflexion point), thus sug-
gesting the existence of an independent second critical ag-
gregation concentration (CAC2).
Fluorescence-quenching experiments can be used to cal-
culate the micellar aggregation numbers (N).[21,22] However,
knowledge of the concentration of free surfactant is re-
quired to obtain N, and it is frequently assumed that this
concentration is almost equal to the CMC. Although this
might be true for pure surfactant systems, in more complex
systems the situation can be completely different, as we
clearly demonstrate here. In any case, the concentration of
micelles can be directly obtained from fluorescence-quench-
ing experiments. In this study, we obtained the concentra-
tion of micelles using pyrene as a fluorescence probe and ce-
tylpyridinium as the quencher (Figure 4). The observed
When SC6HM is present in solution at 0.1 mm, its diffu-
sion coefficient cannot be accurately determined by PGSE-
NMR spectroscopy because the low concentration leads to
line broadening, probably owing to the aggregation phenom-
ena, and this restricts the application of this technique.
However, when 0.5 mm SC6HM was used, problems were
not encountered in the application of PGSE-NMR spectro-
scopy to determine the Dobs for this species (Figure 3). The
results show that the Dobs of the calixarene starts to drop at
concentrations above the CAC, thus indicating that this spe-
cies forms part of the aggregates. Besides, it can clearly be
seen that the Dobs for SC6HM decreases more smoothly
than that of C12TAB and a minimum is reached at approxi-
mately 5 mm. The lower slope observed for SC6HM seems
to indicate that the ratio C12TAB/SC6HM in the micelles is
much higher than one. In other words, several molecules of
C12TAB are required to bind one SC6HM molecule to the
aggregates. Above the minimum (ꢀ2 mm) observed for the
Dobs of C12TAB, it can be assumed that the majority of the
SC6HM is bonded to the aggregates. Thus, after that point,
the variation observed for Dobs with the addition of surfac-
tant can be assumed to be due to a variation in the aggre-
gate morphology or of the solution viscosity, or a combina-
tion of the two.
If some rough approximations are made, the diffusion
data can be used in conjunction with Equation (1) to esti-
mate the concentration of micellized and free surfactant.
Once the concentration of micellized surfactant had been
determined, the ratio of C12TAB/SC6HM in the micelles
was calculated for concentrations above the charge neutrali-
zation point, bearing in mind that under these conditions all
calixarene molecules are incorporated in the micelles (see
Supporting Information for details).
As can be observed, the results are very informative (see
Figures S5 and S6 in the Supporting Information). In the
range between charge neutralization and the CAC2, the
concentration of free surfactant increases considerably,
whereas the concentration of micellized C12TAB remains
practically constant upon increasing the total surfactant con-
centration. This suggests that after all of the SC6HM mole-
cules have micellized, the addition of surfactant to the
system results principally in an increase in free monomers.
On the other hand, within the same concentration range, the
C12TAB/SC6HM ratio (see Figures S9 and S10 in the Sup-
porting Information) roughly varies between approximately
3 and approximately 6, thereby suggesting that under these
conditions the micellar aggregates are characterized by a
high content of SC6HM, which seems to be variable. If this
estimation is correct, it implies that the concentration of mi-
cellized C12TAB does not remain strictly constant but a very
small fraction is being incorporated into the micelles. Final-
ly, above the CAC2, the micellized C12TAB/SC6HM ratio in-
crease exponentially, as one would expect.
Figure 4. Plot of the micelle concentration against the total surfactant
concentration. (black dots) [SC6HM]=0.1 mm and (gray triangles)
[SC6HM]=0.5 mm; T=258C.
trend is in good agreement with those obtained by the other
techniques: the micelle concentration increases up to a
point at which the concentration of C12TAB is slightly below
the charge neutralization point (ꢀ0.5 mm and ꢀ2 mm in the
cases of SC6HM at 0.1 and 0.5 mm, respectively) before
reaching a plateau at which the micelle concentration re-
mains constant. When the concentration of C12TAB goes
beyond the CAC2, the micellar concentration increases
again, as expected.
As can be observed, in the interval between the charge
neutralization point and the CAC2, both the concentration
of micelles and the concentration of micellized surfactant
molecules increase with the concentration of SC6HM, and
this is practically independent of the C12TAB concentration.
The results suggest that in this range the concentration of
aggregates can be controlled by the addition of calixarene.
The aggregation numbers can be calculated by combining
the concentration of micellized surfactant estimated from
the diffusion data and the concentration of micelles deter-
mined from fluorescence-quenching experiments. The re-
sults (Figure 5) seem to show that the aggregation numbers
are approximately constant (around 20ꢁ5) at low C12TAB
concentrations and start to increase gradually below the
CAC2 before reaching values of about 60, which is to say,
Chem. Eur. J. 2013, 19, 4570 – 4576
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4573