Association between Poly(vinylamine) and Sodium Dodecyl Sulfate: Effects of Mixing Protocol, Blending Procedure, and Salt Concentration

Abstract

The association between a weak cationic linear polyelectrolyte, poly(vinylamine), and the anionic surfactant sodium dodecyl sulfate (SDS) has been investigated in dilute solutions, containing 20 ppm of poly(vinylamine) and surfactant up to a concentration of 8 mM. We particularly focus on the importance of the order of addition of the components and of stirring after mixing. Two mixing protocols were used, denoted “PTS” and “STP.” In the PTS method the polyelectrolyte is added to the surfactant solution, and in the STP method the surfactant is added into the polyelectrolyte solution. The results obtained demonstrate the presence of long‐lived trapped nonequilibrium states. In addition, we also address the effect of the blending procedure on association. We studied two blending methods, denoted “Blending” and “Vigorous Blending.” In the Blending method equal volumes of the polyelectrolyte and surfactant were added simultaneously to the sample tube, after which the ingredients were mixed together by turning the sample tube upside down a few times; in the Vigorous Blending method the mixing was provided by a magnetic stirrer. The results, obtained using turbidity, electrophoretic mobility, and light scattering measurements, demonstrate that Vigorous Blending facilitates flocculation at low SDS concentrations, close to the charge neutralization concentration of the system. This is interpreted as being due to additional surfactant incorporation in initially positively charged complexes during collision events. Vigorous mixing in excess surfactant produces stable dispersions consisting of small negatively charged complexes containing one polyelectrolyte and surfactant in excess of what is needed to neutralize the polyelectrolyte charges. The same results are obtained with the Blending protocol, which gives comparable particle size and polydispersity in excess surfactant and polyelectrolyte.