Phase behavior of polyion-surfactant ion complex salts: Effects of surfactant chain length and polyion length

The aqueous phase behavior of a series of complex salts, containing cationic surfactants with polymeric counterions, has been investigated by visual inspection and small-angle X-ray scattering (SAXS). The salts were alkyltrimethylammonium polyacrylates, C(x)TAPA(y), based on all combinations of five surfactant chain lengths (C-6, C-8, C-10, C-12, and C-16) and two lengths of the polyacrylate chain ( 30 and 6 000 repeating units). At low water contents, all complex salts except C(6)TAPA(6000) formed hexagonal and/or cubic Pm3n phases, with the hexagonal phase being favored by lower water contents. The aggregate dimensions in the liquid crystalline phases changed with the surfactant chain length. The determined micellar aggregation numbers of the cubic phases indicated that the micelles were only slightly aspherical. At high water contents, the C(6)TAPA(y) salts were miscible with water, whereas the other complex salts featured wide miscibility gaps with a concentrated phase in equilibrium with a ( sometimes very) dilute aqueous solution. Thus, the attraction between oppositely charged surfactant aggregates and polyions decreases with decreasing surfactant chain length, and with decreasing polyion length, resulting in an increased miscibility with water. The complex salt with the longest surfactant chains and polyions gave the widest miscibility gap, with a concentrated hexagonal phase in equilibrium with almost pure water. A decrease in the attraction led to cubic-micellar and micellar-micellar coexistence in the miscibility gap and to an increasing concentration of the complex salt in the dilute phase. For each polyion length, the mixtures for the various surfactant chain lengths were found to conform to a global phase diagram, where the surfactant chain length played the role of an interaction parameter.