Some oppositely charged polyelectrolyte (PE) surfactant mixtures show a remarkable increase in viscosity near charge equilibrium, while other very similar systems do not show any appreciable effect. The exact structural prerequisites to achieve a significant increase in viscosity are still unclear. In previous work, we investigated the structure and dynamics of oppositely charged polyelectrolyte surfactant complexes (PESCs) formed from sodium dodecylsulfate (SDS) and the cationically modified hydroxyethyl cellulose JR 400, which does enhance the viscosity around charge equilibrium enormously. Here, we study PESCs consisting of SDS and the cationic PE polydiallyldimethylammonium chloride (PDADMAC) which do not significantly increase the viscosity of solutions under similar conditions. Apparently very similar ingredients in PESCs can lead to largely different macroscopic behavior. Using small-angle neutron scattering, rheology, and neutron spin-echo spectroscopy, we gain insights into the system's mesoscopic structure and dynamics. Different from that in SDS/JR 400, no intermixed aggregates are formed but instead a cylindrical core−shell structure. In it, the PDADMAC is much less strongly bound and possesses much higher dynamics which explains the much lower viscosity.

By combining oppositely charged polydiallyldimethylammonium chloride (PDADMAC) and sodium polyacrylate (NaPA), interpolyelectrolyte complexes (IPECs) can be formed in aqueous solution. Such IPECs are studied for rather short NaPA and under variation of the Mw of PDADMAC. The focus is on elucidating the effect of having a hydrophobic modification of the NaPA, which is introduced by having 10 mol% of the monomeric units substituted by ones carrying a dodecyl alkyl chain. This modification renders the complexes more hydrophobic, which is seen in the fact that precipitation of the complexes occurs at a lower mixing ratio and the biphasic region is also wider. The structures of the soluble IPECs are studied by a combination of light and neutron scattering (SANS). It is observed that the complexes formed possess typical radii of gyration of ≈30–40 nm, which become somewhat smaller with increasing length of the PDADMAC chain. The SANS data can be described well with the Beaucage model for complexes, where locally small hydrophobic domains of cylindrical shape are formed, whose persistence length decreases with increasing content of NaPA in the complexes. In contrast no such structures are seen for NaPA without the hydrophobic modification. The cylindrical domains are then arranged within larger‐sized clusters of 30–40 nm, which become more compact with reduced length of the PDAMAC chains. The structure of the IPECs is largely determined by the presence of the hydrophobic modification of the NaPA and is further controlled by the length of the hydrophobic modification. Such IPECs of controlled structure, relatively small size, and containing hydrophobic domains are potentially interesting as delivery systems due to having domains of variable polarity.