Complex Formation between Polyelectrolyte and Oppositely Charged Mixed Micelles: Static and Dynamic Light Scattering Study of the Effect of Polyelectrolyte Molecular Weight and Concentration

Dynamic light scattering, static light scattering, and turbidimetry were used to investigate the effect of polyelectrolyte molecular weight and concentration on complex formation between a strong polyelectrolyte, poly(dimethyldially1ammonium chloride) (PDMDAAC), and oppositely charged mixed micelles of Triton X-100 (TX100) and sodium dodecyl sulfate (SDS). The hydrodynamic radius (Rh) of the complexes is about twice those of the PDMDAAC from which they are formed, while the radius of gyration (R,) of the complexes remains essentially unchanged. This suggests that the complex formed is compact. With increasing polyelectrolyte concentration, intrapolymer complexes transform to interpolymer complexes. The interpolymer complexes are not stable and in time phase separate (coacervate). Although the polyelectrolyte molecular weight has only a small effect on the structure of the complex in the intrapolymer complex region, the intra- to interpolymer complex transition depends strongly upon the molecular weight of the polyelectrolyte. The polyelectrolyte concentration at which the transition occurs decreases with increasing polyelectrolyte molecular weight. If the polyelectrolyte molecular weight is low enough, this sharp transition is not observed, and no associative phase separation is observed. of ionic surfactant brings the polyelectrolyte close to charge neutralizati~n.~ ~~~~ Therefore, phase separation effects result in the restriction of most studies of strong poly- electrolytes with oppositely charged surfactants to sur- factant concentrations below or not much higher than the cmc. Dubin and co-w~rkers~~~~ have demonstrated that the strong electrostatic interaction could be attenuated by using mixed micelles of nonionic and ionic surfactants. Thus Dubin and Oteri2* showed that the interaction between poly(dimethyldiallylammonium chloride) (PD- MDAAC) and mixed micelles of sodium dodecyl sulfate (SDS) and Triton X-100 (TX100) takes place only when a critical molar ratio Y, of SDS to TXlOO has been reached, where Y is defined as Y = (SDSI/((SDSl + (TXlOOI), and is proportional to the average mixed micelle surface charge density. In addition to the phase transition at Y, corresponding to the reversible formation of soluble polyelectrolyte-mixed micelle complexes, Dubin and Ot- eri2s observed a second phase transition, corresponding to bulk phase separation, at a higher Yvalue, denoted by Yp. Yp usually exceeds Y, by about 50-100%. The region between Y, and Yp represents a range of micelle surface charge density where soluble, reversible complexes could form. At moderate or high polyelectrolyte concentration, a maximum in the turbidity versus Y curve is observed between Y, and Yp. The soluble complex formation as well as its dependence upon ionic strength, polyelectrolyte concentration, and surfactant concentration has been studied by Dubin and co-workers using a variety of techniques.