Biomedical Engineering Reference
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between the surfactant alkyl tails and the hydrophobic moieties of the polymer. The
viscosity of the solutions goes through a pronounced maximum as the surfactant con-
centration is increased at a constant polymer concentration. This effect is important as it
enhances the viscosity under conditions where the HM polymer itself would not show
very effective thickening properties.
The effect is observed in many cases, for example with HM modi
ed neutral poly-
saccharides such as ethyl(hydroxyethyl) cellulose (HMEHEC) (Nyström et al., 1995 ),
and anionic and cationic HMHECs, in the presence of anionic surfactant (SDS) (Beheshti
et al., 2008 ) or polyelectrolytes containing a low fraction of hydrophobic groups. HM
polyelectrolytes can even associate with surfactants of the same charge; in this case, the
attractive hydrophobic interactions overcome electrostatic repulsions between the poly-
mer backbone and the surfactant ionic heads (Iliopoulos et al., 1991 ). The most com-
prehensive studies are known for mixtures of oppositely charged surfactants and
polyelectrolytes.
Magny et al.( 1994 ) made a detailed analysis of the case of mixtures of cationic
surfactants CTAC (dodecyltrimethylammonium chloride) and anionic HM polyacrylic
acid (HMPAA). A steady-state
uorescence
probe, was used to detect the formation of micelles or hydrophobic aggregates. The
fluorescence method, with pyrene as a
fluorescence emission spectrum of pyrene shows several so-called vibronic peaks, the
intensity of which is a sensitive indicator of the polarity of the pyrene microenvironment.
The ratio of the
first and third vibronic peaks (I 1 /I 3 ) is a sensitive indicator of the
formation of micelles or hydrophobic aggregates because the pyrene probe preferentially
lies close to, or inside, these microdomains. For instance, the ratio I 1 /I 3 varies from 2 in
pure water to 1.5 in a micellar solution CTAC. The end of the sharp transition of I 1 /I 3 with
increasing surfactant concentrations is taken as the CMC and is, in general, in good
agreement with other determinations of micelle formation.
Owing to binding interactions, the critical aggregation concentration (CAC) of the
surfactant differs from the CMC value (Magny et al., 1994 ). In the presence of the
unmodi
ed polyelectrolyte, the transition of I 1 /I 3 shifts to much lower surfactant concen-
trations (lower CAC). The introduction of hydrophobic groups in
uences the shape of the
fluorescence curve and further decreases the CAC. The pyrene probe monitors the gradual
increase in the hydrophobicity of the microdomains containing mixed micelles: the
transition is stretched over one order of magnitude of surfactant concentrations and the
amplitude of the transition decreases. Here, presumably, initially small aggregates of
surfactants are formed, involving alkyl chains attached to the polymers, and the structure
of the aggregates gradually changes with increasing surfactant concentration. Polymer
hydrophobicity (degree of modi
cation or length of the alkyl group) decreases the CAC of
the surfactant by two orders of magnitude. As surfactant concentration approaches the
CAC, a large increase of viscosity is seen ( Figure 6.11 ). Upon further addition of surfactant,
viscosity decreases again and the system suddenly approaches phase separation.
One possible mechanismwhich explains how such large changes in the viscosity occur
is shown in Figure 6.12 . This is the transition from inter-chain mixed aggregates to intra-
chain binding of the surfactant into mixed micelles containing the polymer hydrophobes.
At
low surfactant concentrations, close to the CAC, mixed micelles containing
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