From Associative to Segregative Phase Separation: The Phase Behavior of Poly(acrylate)/Dodecyltrimethylammonium Complex Salts in the Presence of NaBr and NaCl.

We report here a systematic study on the phase behavior of dodecyltrimethylammonium-poly(acrylate) (CTAPA) complex salts upon addition of simple salts (NaBr and NaCl). Complex salts of poly(acrylate) of two molar masses were employed (2000 and 450,000 g mol). Systems containing different surfactant mesophases were observed, whose structures were elucidated by small-angle X-ray scattering analyses. Overall, we observe a transition from associative to segregative phase separation intercalated with monophasic regions. In terms of surfactant association, a transition from micellar cubic phases to an isotropic micellar solution was observed, where these two phases occur in equilibrium with a dilute phase as a consequence of associative phase separation. However, as the salt concentration increases, segregative phase separation is observed, and scattering analyses of the surfactant-rich phase indicate the growth of micellar aggregates. These results are discussed in terms of the electrolyte effect screening the interactions between the oppositely charged surfactant and polymer and as a result of salting-out effects of the different electrolytes that prevail at higher concentrations.