Complexation of a Thermoresponsive Brush-Type Polyelectrolyte with an Oppositely Charged Surfactant: Effect of Temperature and Surfactant Concentration

Abstract

Responsive drug delivery vectors can be designed using oppositely charged polyelectrolyte-surfactant complexes. As a model, we created a brush-type copolymer (PECop), combining alginate and Poly(N-isopropylacrylamide) (PNIPAAm), whose side chains respond to temperature. Aggregation of PECop with the cationic surfactant dodecyltrimethylammonium bromide (DTAB) was examined versus surfactant concentration and temperature. We used surface tension, electrophoretic mobility, zeta potential, potentiometry, light scattering, and atomic force microscopy to analyze the complexes. PECop/DTAB complexes form spherical, monodisperse aggregates in certain surfactant ranges, even though the copolymer itself is polydisperse. The binding isotherms combine features of oppositely charged polyelectrolyte/surfactant systems and hydrophobically modified polymers. Compared to alginate alone, PECop binds six times more DTAB at 1 mM surfactant concentration. Temperature responsiveness depends on surfactant concentration (cs). The surfactant triggers progressive collapse of polymer chains, maximized at cs = 2.8 mM, where thermo-responsiveness is lost. For cs 10 mM, size increases above LCST. This inversion in thermal response with rising surfactant concentration suggests changing aggregate structure, offering new avenues for drug delivery system design using these polymer-surfactant complexes.