Turbidimetric and dynamic light scattering studies of mixtures of cationic polymers and anionic mixed micelles

Abstract

We have studied the formation of microscopic and macroscopic aggregate phases between strong cationic polymers and oppositely charged anionic/nonionic or anionic/zwitterionic mixed micelles at moderate to high ionic strength. Conditions for bulk phase separation, which appears to be a critical phenomenon, are determined by the ionic strength and the mole fraction of anionic surfactant ( Y), not by the concentrations of polymer or total surfactant. Polyion-micelle association is thus viewed as a coulombic phenomenon, mainly intramolecular with respect to polymer, with the mixed surfactant composition controlling the surface charge density of mixed micelles. At critical conditions for irreversible phase separation, the dependence of the square root of ionic strength on Y is found to be linear. At values of Y smaller than this critical value by 1–20%, such mixtures contain stable aggregates that may be studied by optical and hydrodynamic methods. These general features have been demonstrated for a range of cationic polymers, differing widely with respect to chemical composition or molecular weight, as well as for several anionic and nonionic (or zwitterionic) surfactants. However, a soluble aggregate regime, which is observed for the surfactant system sodium dodecyl sulfate (SDS)/ Triton X-100, is absent when the surfactant mixture is the monodisperse SDS/C 12E 6 pair. This may suggest that precipitation in the former system is delayed or inhibited by some form of micellar heterogeneity. For the system poly(dimethyldiallylammonium chloride)/sodium dodecyl sulfate/Triton X-100, QELS autocorrelation decay curves have been analyzed using “canned” software programs (Langley-Ford and Nicomp, respectively) for the characterization of size distributions. Both of these analyses indicate bimodal size distributions for the scattering particles. Two well-resolved populations appear to coexist, corresponding to unbound mixed micelles and polyion-micelle complexes, respectively.