Study of interaction between cetyltrimethylammonium bromide and poly(acrylic acid) by rheological measurements

Interaction between the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the anionic polyelectrolyte poly(acrylic acid) (PAA), having molecular weight by viscosity of 750 kDa, is investigated by rheological measurements. Upon addition of 1 or 0.5 wt % CTAB to the semidilute PAA solutions with concentration ranging from 0.04 to 0.1 wt %, the insoluble complex salts (the surfactant ion + the polyion) precipitate. In contrast, with addition of 2 or 3 wt % CTAB to the aforementioned PAA solutions, evidence of transition from shear-thinning to nearly Newtonian behavior is observed. The origin of this observation appears to lie in the fact of formation of more compact PAA/CTAB aggregates, because the radii of gyration, R_g, of PAA coils have been reduced by about 60% with addition of 2 wt % CTAB by static light-scattering measurements. Furthermore, pH measurements also reveal the possible formation of "ion pairs" between PAA and CTAB as the pH values of the PAA solutions decrease with addition of CTAB. That is, the pH values of the solutions containing PAA and CTAB together are less than those of neat PAA or CTAB solutions. This implies that addition of CTAB leads to more acids dissociated from the acrylic acid monomers on the poly(acrylic acid) macromolecules.