The role of specific interaction in phase behavior of polyelectrolyte-surfactant-water mixtures.

Aqueous phase behavior and structures of phases were studied in systems containing sodium poly(styrenesulfonate), NaPSS, and complex salt CTAPSS, formed between cetyltrimethylammonium cations, CTA+, and PSS- anions. It was shown that hydrophobic interaction of the polyion styrene groups with surfactant aggregates, which supports the strong electrostatic attraction between CTA+ and PSS-, has a significant effect on phase behavior and structures. Only the disordered micellar (L1) and the ordered hexagonal (H1) phase were found that are connected over a broad two-phase region of L1-H1 coexistence. At water contents above 60 wt%, CTAPSS is easily dissolved in proportion to the amount of added NaPSS, whereas at lower water contents a large excess of NaPSS is needed to dissolve CTAPSS. Phase separation in the two-phase region is controlled by two tendencies: (i) to maximize the contact between the hydrophobic groups and micelles (assisted by hydrophobic interaction) and (ii) to form as dense phase as possible (assisted by both, electrostatic and hydrophobic interactions). Structural characteristics of soluble non-stoichiometric complexes from the L1 phase show that hydrophobic interaction contributes also to a relatively small size of PSS-induced micelles and leads to a network-like association between PSS chains in which micelles serve as cross-links.