94 Aristeidis Papagiannopoulos and Stergios Pispas
Figure 2. Hydrodynamic radius from mixed solutions of fixed SCPI concentration
(5.128x10-^4 g/ml) as a function of DTMAB concentration. Inset: polydispersity 휇훤^22
corresponding to the DLS measurements of the main Figure.
In the first complexation regime the surface charge of the nanoparticles
has an abrupt increase (Figure 3). The aforementioned neutralization of the
SCPI charges is one cause. Another one is the further collapsing of the
aggregates because of the introduced hydrophobic character of the surfactants,
which brings charged SCPI units close to each other with a possibility for
them to condense their counterions and form dense hydrophobic nano-phases
[12, 16]. In the second complexation regime, ζ-potential increases mildly [17]
suggesting a loose binding between aggregates without significant internal
rearrangements.
The morphology of the polyelectrolyte/surfactant complexes at the nano-
level was explored by fluorescence spectroscopy. Ratio I 1 /I 3 which is
sensitive to the local polarity of the pyrene environment [18] is near 1.2
for all the solutions containing SCPI/DTMAB complexes (Figure 4). This
value corresponds to a well defined hydrophobic environment capable to
accommodate the pyrene probe [11]. Apparently the range of concentrations
presented is well above the critical aggregation concentration of the system.
The presence of surfactant makes the formation of hydrophobic domains
within the complexes possible since in the case of no surfactant I 1 /I 3 is close
to 1.8 which is the expected value of pyrene inside a mainly hydrophilic
environment [19]. Hence the discussed increase of hydrophobic content due to