Complex Formation between Polyelectrolyte and Oppositely Charged Mixed Micelles: Soluble Complexes vs Coacervation

The system comprised of poly(dimethyldially1ammonium chloride) (PDMDAAC) and oppositely charged mixed micelles of Triton X-100 (TXlOO) and sodium dodecylsulfate (SDS) displays several states, including coacervate and various soluble complexes. The phase boundary for the equilibrium between soluble complexes and coacervate (“associative phase separation” according to Piculell and Lindman) for 0.4 M NaCl and Y = [SDS]/([SDS] + [TXlOO]) = 0.3 was constructed. Coacervation takes place when the total concentration of surfactants and PDMDAAC is very low and the weight ratio of PDMDAAC to TX100-SDS, W, is close to 0.09, a stoichiometry which corresponds to a 1:l charge ratio of PDMDAAC to TX100-SDS. The phase separation region region is over 2 orders of magnitude smaller than that of most polyelectrolyte/ oppositely charged surfactant systems (without nonionic surfactant). In the soluble complex region, dilution with 0.4 M NaCl is seen to disaggregate multipolymer complexes into intrapolymer complexes, but the electrophoretic mobility of the complexes remains unchanged. The electrophoretic mobility of complexes changes from negative to positive with increasing W and approaches zero at incipient coacervation. In the soluble complex region, an increase in W leads to an increase in the concentration of intrapolymer complexes, followed by formation of interpolymer complexes and coacervation. Further increase in W redissolves the coacervate. The formation of soluble complexes over a wide range of conditions supports the theoretical models of complex coacervation by Veis and by Tainaka. However, the interpolymer complex formation that is observed in this system is not considered in either theory.