Complex Formation between Weak Anionic Polyelectrolyte and Medium Cationic Surfactant

This paper presents the results of an experimentally study about the formation of complexes between a weak anionic polyelectrolyte, a copolymer of acrylamide and sodium acrylate, Praestol 2515 and a medium cationic surfactant, hexadecyl trimetyl ammonium bromide (HTAB). The critical micellar concentrations, c(cm), for anionic polyelectrolyte and cationic surfactant have been computed from the superficial tension - concentration curve. Experiences took place in a system of fixed polymer concentration, c(p), with increasing amount of surfactant. Two concentrations, critical aggregation concentration, c(ac) and critical saturation concentration, c(as) are essential for understanding and studying the interactions between polymer and surfactant and they are determined experimentally. In solutions that have c(p) < c(cm), by mixing the polymer with HTAB associated in micelles some complexes are formed that have the same values of c(ca) and c(cs). In the solutions that have c(p) > c(cm), other complexes are formed with different values of the c(ca) and c(cs). The changes in the rheological behaviour of the complex solutions formation were also experimentally studied. The Praestol 2515 polymer solution is Bingham pseudoplastic type, without a rheological characteristic dependent on time or viscoelastic properties. By adding HTAB surfactant some complexes are formed with apparent viscosity and flow threshold lower than those of the pure polymer solution as long as c(HTAR) is greater than c(cs) and greater than those of the pure polymer solution when c(HTAB) is less than c(cs). For all cases, the flow index remains practically constant. The rheogrames show that the complexes are stable and resistant to moderate shear stress and therefore they can be used also in processes that involve intensive shear rate flow. For the solid - liquid interface processes are recommended the complexes formed between polymers solution with c(p) < c(cm) and surfactants solution with c > c(cs).