Salt effect on complex formation of neutral/polyelectrolyte block copolymers and oppositely charged surfactants

The salt effect on the complex formation of poly(acrylamide)-block- poly(sodium acrylate) (PAM-b-PAA) as a neutral-anionic block copolymer and dodecyltrimethylammonium bromide (DTAB) as a cationic surfactant at different NaBr concentrations, C_NaBr, was investigated by turbidimetric titration, steady-state fluorescence spectroscopy, and dynamic light scattering. At C_NaBr < 0.25 M, DTAB molecules may form micelle-like aggregates on PAM-b-PAA chains to form a PAM-b-PAA/DTAB complex above the critical surfactant concentration C_critical for the onset of complex formation. In the region of relatively high turbidity, a larger complex is likely to form a core-shell structure, of which the core is a dense and disordered microphase made of surfactant micelles connected by the PAA blocks. The corona was a diffuse shell of PAM chains, and it ensured steric stability. At C_NaBr= 0.25 M, a higher electrostatic intermicellar repulsion and intercomplex repulsion induced by a large amount of bound DTAB micelles may lead to a redissolution of large colloidal complexes into intrapolymer complexes. Moreover, a salt-enhancing effect on the complex formation was observed in the PAM-b-PAA/DTAB system; the critical surfactant concentration decreased with increasing salt concentration at C_NaBr < 0.10 M. The salt-enhancing effect is due to the larger increase of interaction in comparison to the screening of the interaction.