Self-assembled structures of anionic hydrophobically modified polyacrylamide with star-shaped trimeric and hexameric...

The self-assembly of a 1% hydrophobically modified and 30% hydrolyzed polyacrylamide (C12PAM) with cationic star-shaped oligomeric surfactants has been investigated by isothermal titration microcalorimetry, turbidimetry, ζ potential, scanning electron microscopy, and (1)H NMR techniques. The oligomeric surfactants are composed of quaternary dodecyldimethylammonium ions with three or six hydrophobic chains connected by a polyamine spacer at the headgroup level, abbreviated as DTAD and PAHB, respectively. DTAD/C12PAM and PAHB/C12PAM mixed systems undergo the same aggregate transitions with increases in surfactant concentration from soluble networklike aggregates to precipitated denser and more cross-linked structures and then to soluble spherical aggregates. The networklike aggregates are generated at very low surfactant concentration. However, at the corresponding surfactant concentration without C12PAM, DTAD cannot form aggregates and PAHB forms only networklike aggregates with a very loose structure. The strong electrostatic and hydrophobic interaction of DTAD and PAHB with C12PAM and the hydrophobic interaction between the alkyl chains of DTAD and PAHB themselves evidently promote the formation of networklike aggregates. As the surfactant concentration increases, cationic surfactants become excessive. The molecular configuration is changed by the stronger hydrophobic association among the DTAD and PAHB molecules and the enhanced electrostatic repulsion between the mixed aggregates. Thus, the networklike aggregates transfer to spherical aggregates.