88 Aristeidis Papagiannopoulos and Stergios Pispas
polyelectrolyte-surfactant complexes are fairly monodisperse (compared
to the initial polyelectrolyte aggregates) and they contain well-defined
hydrophobic domains (proof of complex coacervate phases) as shown by
pyrene fluorescence probing. The system under study is proved to have
tunable complexation properties not only by its dependence on surfactant
content, but also by the effect of dilution upon already formed complexes,
which shows a gradual decrease in the hydrophobic domains and a
variation of the complexes size.INTRODUCTION
Charged macromolecular species in aqueous solutions offer a great
variety of possibilities for complex self-assembled morphologies. Interactions
between oppositely charged polyelectrolytes result to complex coacervate
core-shell nanoparticles, while polyelectrolytes and oppositely charged
surfactants in aqueous solutions may form aggregates that contain dense
complex coacervate phases [1]. These nano-formulations are attractive for
biological applications e.g., drug delivery [2] and industrial use as in foods and
paints [3]. Addition of surfactants to polyelectrolyte solutions affects the
swelling of polyelectrolyte networks [4] and the conformation of the
polyelectrolyte chains under the important role of electrostatic forces. The
amphiphilicity-driven aggregation of surfactants in solution is modified by the
presence of polyelectrolyte chains and this in effect tunes the aggregation of
the polyelectrolytes. Light scattering [1] has been extensively used (among
other methods as small angle scattering, microscopy and nuclear magnetic
resonance) to characterize this kind of systems, because of its high sensitivity
in changes in aggregation state (scattered intensity) and variations in size
(from several nm up to several hundreds of nm).
The morphology of the polyelectrolyte-surfactant complexes except from
their concentrations in solution depend on individual species characteristics
i.e., polymer molecular weight, rigidity, architecture and charge ratio and
surfactant polar head and chain length [3, 5]. Gradual binding of oppositely
charged surfactants to polyelectrolyte aggregates normally causes a collapse of
the chains and to additional multi-chain complexation as precipitation of the
dense coacervate phase approaches. In general phase separation takes place
when polyelectrolyte/surfactant charge ratio approaches stoichiometry [6],
but this is not a strict rule pointing to the fact that hydrophobic interactions
are also important. Binding of surfactants on polyelectrolytes becomes
cooperative above a certain surfactant concentration termed critical