Poly(acrylic acid)-cetyltrimethylammonium bromide interactions studied using dynamic and static light scattering and time-resolved fluorescence quenching

Poly(acrylic acid) (PAA) and cetyltrimethylammonium bromide (C(16)TAB) are found to interact strongly in aqueous solution. The properties of the system are determined mainly by the parameter beta, defined as [C(16)TAB]/[PAA]; the PAA concentration is expressed as monomers. Light scattering measurements in dilute solutions show that a single PAA chain binds several C(16)TAB micelles to form a complex. At beta approximate to 1 (independent of the PAA concentration), the complex is saturated with surfactant. At higher beta, free micelles coexist with the complexes. The hydrodynamic radius of the complex is smaller than (beta < 1) or similar to (beta > 1) the radius of the surfactant-free PAA coil. Estimates of the hydrodynamic virial coefficient show that the effective repulsion between complexes increases with beta up to beta approximate to 1 where a dramatic change is brought about by the screening effect of free micelles. Time-resolved fluorescence quenching shows that the aggregation number of the micelles in the complex increases from 50 at beta = 0.35 to 90 at beta = 1. (The aggregation number for 20 mM C(16)TAB in the absence of polymer is estimated as 107.) Pyrene lifetime measurements indicate that this change is accompanied by an increase of the apparent degree of bromide ion binding to the micelles from 50% to 95% of the value for free C(16)TAB micelles. Furthermore, the decreasing influence of the polymer on the surfactant assembly with increasing beta is supported by the variation of the intramicellar quenching rate constant. Viscosity measurements confirm that no multipolymer complexes form in the studied concentration range. pH measurements show that the degree of dissociation of the polymer acidic groups increases when the polymer binds to the micelles.