The interaction of ionic/nonionic surfactant micelles with strong polyelectrolytes has been studied by turbidimetric titrations. There exists a critical micellar fraction of ionic head groups, Yc, below which no interactions can be detected. At Y > Yc, the solute components form either soluble aggregates, complex coacervate, or amorphous precipitate, depending, in part, on the polymer:surfactant stoichiometry. The dependence of Yc on the square root of the ionic strength, P/2, appears as a linear phase boundary, which is rather insensitive to polymer concentration or total surfactant concentration. For several polyelectrolyte/mixed-micelle systems, we have measured the surface potential of the micelle ('F0) at values of Y and I corresponding to the polyion-micelle complex formation phase boundary. In the system poly-(dimethyldiallylammonium chloride)/sodium dodecyl sulfate, C12E06, the values of 'P0 measured from the pKt of the probe, dodecanoic acid, are constant (ca. -10.8 mV) along the phase boundary. For the same polymer-surfactant system, \P0 measured with the fluorescence probe hexadecylhydroxycoumarin increases with Yc from -18 to -24 mV along the phase boundary. For the system of reversed charge, sodium polystyrenesulfonate/dodecyldimethylamine oxide, in which the cationic surfactant head group itself acts as a potential probe, the surface potential varies from +12 to +43 mV along the phase boundary. The foregoing results, taken together with the absence of an effect of polymer chain length on Yc, suggest that critical conditions for complex formation are dictated by the cooperative adsorption of a relatively short sequence of polyion segments to the colloid surface. It appears likely that this process occurs when a critical potential is reached at the locus of the closest approach of the polymer chain to the micelle surface. The measured potential at critical conditions increases with the distance between this steric boundary and the mean position of the solubilized probe.
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