Features distinguishing the flow behavior of polyelectrolytes with opposite charges in aqueous solutions

Abstract

Solution viscosities of poly((3-(acrylamido)propyl)trimethylammonium chloride) (PAPTMAC-Cl 7.8) and of poly(styrenesulfonate sodium) (PSS-Na 75.6) were measured in water containing different amounts of NaCl at 25 °C; the numbers after the abbreviation of the polymers state their Mw in kDa. The intrinsic viscosity of the polycation in water amounts to 4460 mL/g and is about three times larger than that of the polyanion, despite the fact that its mass is only one tenth that of the polyanion. At low salinities the shear-overlap parameters Σ as a function of composition c (mass per volume) pass maxima for PAPTMAC-Cl 7.8, but exhibit points of inflection for PSS-Na 75.6. At sufficiently large salinities these dependencies become linear in both cases. Points of inflection in Σ (c) indicate crossover condition and separate the solvent dominated from the solute dominated flow regimes. For the polycation, the Σcrov values are always larger than for the polyanion. The analysis of these results in the light of the HSAB (Hard Soft Acid Base) approach of Pearson yields strong indications that the observed dissimilarities between the two types of polyelectrolytes are not caused by the opposite charges attached to the macromolecules, but result from the combination of the soft NR4+ with the hard Cl– ions in the case of PAPTMAC-Cl 7.8 and of the hard RSO3– with the hard Na+ ions in the case of PSS-Na 75.6. This insight opens up new opportunities for the tailoring of rheological properties for polyelectrolytes in saline solutions.