Nanoscale Supramolecular Ordering in Gel−Surfactant Complexes: Sodium Alkyl Sulfates in Poly(diallyldimethylammonium Chloride)

Studies of slightly cross-linked polycationic gels interacting with anionic surfactants have been performed by using random copolymers of poly(diallyldimethylammonium chloride) (PDADMACl) and polyacrylamide (PAAm) with varying content of PDADMACl and degree of cross-linking. Gel samples which had been fully swollen in water were placed in aqueous solutions of sodium alkyl sulfates (octyl (SOS), decyl- (SDCS), dodecyl (SDS), tetradecyl (STS), and hexyl (SHS) sulfates). The degree of the sample volume contraction depends on the PDADMACl content. The collapsed gel−surfactant complexes were studied using synchrotron small-angle X-ray scattering. All studied samples containing PDADMACl exhibited pronounced supramolecular nanostructures. The gel−SDCS complex exhibited a cubic structure with a periodicity (7.75 nm) of approximately 4 times the surfactant molecular length, while the gel−SDS, gel−STS, and gel−SHS complexes showed hexagonal supramolecular ordering with a periodicity of approximately 2 times the surfactant molecular length. The d spacing of the longest periodicity in the complexes was dependent on the PDADMACl content and the surfactant. The d spacing generally increased with decreasing PDADMACl (charge) content and increasing number of carbon atoms in the surfactant alkyl chain.