Polyelectrolyte-Surfactant Complex Nanoparticles
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A variety of different colloidal systems can be produced by interaction between polyelectrolytes and oppositely charged surfactants. These complexes have attracted considerable interest because of their importance for both science and industrial applications.
By interaction of two oppositely charged polyelectrolytes, polyelectrolyte–polyelectrolyte complexes (PECs, or so-called symplexes) are formed. These PECs can be loose aggregates or well-defined nanoparticles. In a similar way polyelectrolytes form complexes with oppositely charged surfactants and polyelectrolyte–surfactant complexes (PSCs) are obtained. Here the mechanism of complex formation is not only based on electrostatic interactions and entropic factors, but also on the hydrophobic interaction between the surfactant tails. At surfactant concentrations below the critical micelle concentration (CMC), complex formation of polyelectrolytes with oppositely charged surfactants occurs. With increasing surfactant chain lengths, the formation of surfactant patches along the polyelectrolyte chain can be expected, because owing to hydrophobic interactions the preferred complexation binding site is the next closest position to an occupied site. This results in a zipperlike mechanism of polyelectrolyte–surfactant complex formation. Typically three-dimensional networks form at concentrations above the CMC, where polyelectrolyte chains bridge discrete micelles without penetrating them. At even higher surfactant concentrations, when all charges of the polyion are neutralized, polyelectrolyte–surfactant complexes usually precipitate from solution. These bulk materials have a liquid crystalline-like mesostructure and display long-range order on the nanometer scale. The interaction between polyions and oppositely charged surfactants is a cooperative process in which the ionic head groups of the surfactant bind to the polyion, while the hydrophobic surfactant tails segregate into a hydrophobic microphase. Similar to amphiphilic block copolymers, this microphase separation occurs without macroscopic demixing.
One problem with most such polyelectrolyte–surfactant complexes is that they are usually water-insoluble solids or semisolids and not suitable for particle applications. Within the last few years, different techniques of PSC nanoparticle synthesis have been developed to overcome this problem. These complexes are, unlike PSC bulk materials, redispersable in water or organic solvents as discrete nanoparticles without dissociation. For synthesis of PSC nanoparticles, polyelectrolytes that exhibit additional molecular entities have to be used, which provide solubility even after stoichiometric complexation and neutralization of all charges. In this way, particles are obtained that exhibit spherical core–shell structures, cylindrical or disk-shaped morphologies, or hollow spheres like vesicles. Whereas some of these morphologies appear to be equilibrium structures, others are nonequilibrium structures. The size, solubility, and morphology of PSC nanoparticles depend strongly on the molecular structure of the polyelectrolyte, the block ionomer lengths, the block length ratios, the structure and type of surfactant, and sometimes also on the way of preparation. Potential applications for this relatively new class of materials are drug and gene delivery, amphiphilic particles, and depot systems.