Effects of Salt on Polyelectrolyte-Micelle Coacervation

Turbidity, dynamic light scattering, and electrophoretic mobility were used to study the effects of added salt on coacervation in the system composed of the strong cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) and oppositely charged mixed micelles of Triton X-100 (TX100) and sodium dodecyl sulfate (SDS). The phase behavior in the range of ionic strengths from 0.05 to 0.60 M includes regimes of soluble complex formation, coacervation, and precipitation. The corresponding phase boundaries are determined from differential turbidity curves. The shift of the phase boundaries to higher ratios of SDS:TX100 with increase in salt concentration is explained on the basis of electrostatic screening. The width of the coacervation region is found to increase with ionic strength. These observations are consistent with previous reports of the “salt suppression” and “salt enhancement” of coacervation. In the coacervation region, the electrophoretic mobility is found to be close to zero. At higher and lower ionic strengths, soluble complexes are positively or negatively charged, respectively. It is suggested that the principal factor governing coacervation in this system is electroneutrality of the polyion-micelle complex which in turn depends on the charge and number of bound micelles.