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.

A highly ordered supramolecular structure is formed in the polyelectrolyte-surfactant complex between the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic network of poly(diallyldimethylammonium chloride) (PDADMACl). From small-angle X-ray scattering (SAXS), the complexes between PDADMACl and SDS were shown to exhibit a hexagonal type of microstructure which is different from that of pure SDS. A d spacing of 3.7 nm corresponding to the interdistance between SDS aggregates in the gel network was obtained. The intensity of the diffraction peaks and the degree of order increased with increasing initial SDS concentration and charge content of the PDADMACl gels. The diffraction peaks were broadened when the concentration of SDS in the external solution phase was higher than its critical micelle concentration (cmc). The SAXS profiles were unexpectedly independent of the degree of cross-linking of the PDADMACl gels in the range of 0.5-2%.

The interaction of slightly cross-linked positively charged gels of poly(diallyldimethylammonium chloride) (PDADMAC1) with sodium salts of chloride, bromide, iodide, and acetate in aqueous media has been investigated. At the critical concentration of sodium iodide, a phase transition of the network to the collapsed state was observed. A wide hysteresis accompanied this transition. The presence of other salts did not initiate the network collapse in the investigated concentration range. A decrease in the charge density of the cationic network by copolymerization of DADMAC1 with acrylamide resulted in the disappearance of the phase transition. In this case, the usual contraction of polyelectrolyte gels in the presence of the mentioned salts was observed. The rate of volume change increased sharply with increasing concentration of sodium iodide. For some cases, two well distinct regions in the kinetic behavior were observed: a region of fast polyelectrolyte contraction and a region of slow collapse which, to our knowledge, had not been observed previously. The experimental results can be explained by the formation of ion pairs and multiplets (ionomer effect) for the sufficiently charged networks in the collapsed state. As a result, the collapsed ionomeric state of the network becomes competitive with the swollen polyelectrolyte state at high enough fractions of the ion-containing segments because of the tendency of the ion pairs to associate. A potential barrier which separates the ionomeric and the polyelectrolyte states of the gel can be used to explain the two-step kinetics of the collapse.