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

TL;DR

This study investigates how the flow behavior of oppositely charged polyelectrolytes in aqueous solutions is influenced by ion pairing and salt concentration, revealing that rheological responses can be tuned via counter-ion selection based on HSAB theory.

Contribution

It demonstrates that differences in flow behavior of polycations and polyanions are driven by specific ion pairing, not just charge, enabling targeted rheological tuning through counter-ion choice.

Findings

Polycation has roughly three times the intrinsic viscosity of polyanion despite lower molar mass.

Shear-overlap parameter exhibits a maximum for polycation and an inflection point for polyanion at low salinity.

Ion pairing based on HSAB theory explains differences in flow behavior, not just charge polarity.

Abstract

Solution viscosities of a polycation and a polyanion in NaCl-water: HSAB-guided rheology The zero-shear viscosities of poly(3-acrylamido-propyl-trimethyl-ammonium-chloride) (PAPTMAC-Cl, M ~ 7.8 kDa) and poly(styrene-sulfonate sodium) (PSS-Na, M ~ 75.6 kDa) were measured in aqueous NaCl solutions at 25 degrees C over a wide range of salt concentrations. Extrapolation to zero polymer concentration yields an intrinsic viscosity of 4 460 mL g^-1 for the polycation, i.e. roughly three times larger than that of the polyanion, although the polycation's molar mass is only one-tenth of the polyanion's. At low salinities the shear-overlap parameter S as a function of polymer concentration c exhibits a pronounced maximum for PAPTMAC-Cl, whereas PSS-Na shows a clear inflection point. With increasing NaCl concentration both curves become linear, indicating that the system has entered a regime where the solute dominates the flow behavior. The crossover concentrations (S_crov) of the polycation are systematically larger than those of the polyanion. By applying Pearson's Hard-Soft Acid-Base (HSAB) concept we find that the observed differences are not a simple consequence of opposite polymer charges. Rather they arise from the specific ion-pairing: the soft NR4+ group of PAPTMAC pairs with the hard Cl-, whereas the hard RSO3- units of PSS-Na interact with the hard Na+. This insight suggests that the rheological response of polyelectrolyte solutions can be deliberately tuned by choosing counter-ions of appropriate hardness/softness. Keywords: intrinsic viscosity, shear-overlap, polyelectrolytes, sodium chloride, HSAB theory, rheology, soft-matter.