Flocculation in mixtures of cationic polyelectrolytes and anionic surfactants

The phase behavior of mixtures of a cationic polyelectrolyte ('Jaguar') and an anionic surfactant (either sodium dodecyl sulfate or sodium lauryl ether sulfate) has been studied. For a given polyelectrolyte (and added NaCl) concentration, with increasing surfactant concentration, three phase regions were identified. The first region is a single homogeneous phase. Within this region, at some surfactant concentration, above the critical aggregation concentration, stable open-network 'particles' form, typically approximately 100 nm in size, which are net positively charged. However, as the surfactant concentration is increased further, these particles aggregate and form a two-phase system, i.e. a separated gel phase, containing a high percentage of water, co-existing with an aqueous surfactant phase. At some higher surfactant concentration still, the particles become sufficiently negatively charged that they re-stabilize. Beyond this surfactant concentration, therefore, the system reverts to being a single, homogeneous phase. Within the two-phase (aggregated particle) region an iso-electric point for the particles has been observed at a certain surfactant concentration, by electrophoresis. Furthermore, in this aggregated, gel-phase region, there appear to be typically approximately 2-4 surfactant molecules associated with each cationic site of the polymer chains. It is postulated that association of the anionic surfactant molecules occurs within the polyelectrolyte chains, binding them together, to form the particles referred to. These associated surfactant structures have been referred to here as 'internal' micelles. A crude estimate has been made, based on turbidity/time measurements, that there may be up to approximately 1000 polymer chains in each primary particle, bound together by the internal surfactant micelles. Small-angle light scattering studies of the aggregating particles indicate a fractal dimension for the aggregates, which would correspond to a diffusion-limited aggregation process.