Binding of polycarboxylic acids to cationic mixed micelles: Effects of polymer counterion binding and polyion charge distribution

Mixed micelles of cetyltrimethylammonium chloride (CTAC) and n-dodecyl hexaoxyethylene glycol monoether (C12E8) bind to polyanions when the mole fraction of the cationic surfactant exceeds a critical value (Y-c). Y-c corresponds to a critical micelle surface charge density at which polyelectrolyte will bind to this colloidal particle. Turbidimetric titrations were used to determine Y-c for such cationic-nonionic micelles in the presence of acrylic acid and acrylamido-2-methylpropane sulfonate homopolymers (PAA and PAMPS, respectively) and their copolymers with acrylamide, as function of pH, ionic strength, and polyelectrolyte counterion. In 0.20 M NaCl, Y-c for PAA is found to be remarkably insensitive to pH, i.e., virtually independent of the apparent polymer charge density xi(app). On the other hand, the expected inverse relationship between Y-c and xi(app) is observed either for PAA when NaCl is replaced by TMACl (tetramethylammonium chloride), or when xi(app) is manipulated using acrylic acid/acrylamide copolymers at high pH. The effective charge density of PAA is thus seen to be suppressed by specific sodium ion binding, indicating that the influence of salts on the interaction of polycarboxylic acids with colloidal particles may differ qualitatively from their effect on the analogous behavior of strong polyanions. Comparisons between homo- and copolymers of acrylic acid were carried out also to test the hypothesis that the "mobility" of charges on PAA at moderate pH (degree of ionization less than unity) could make this "annealed" polymer exhibit the behavior of a more highly charged one. The results, while consistent with this expectation, were obscured by the likely effect of copolymer sequence distributions. (C) 1998 Academic Press.