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

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.

💡 Research Summary

This study investigates the rheological behavior of two oppositely charged polyelectrolytes—poly(3‑acrylamido‑propyl‑trimethyl‑ammonium‑chloride) (PAPTMAC‑Cl, M≈7.8 kDa) and poly(styrene‑sulfonate sodium) (PSS‑Na, M≈75.6 kDa)—in aqueous NaCl solutions over a broad range of salt concentrations at 25 °C. Using a Ubbelohde capillary viscometer, the authors measured relative viscosities (η_rel = η_solution/η_solvent) for polymer concentrations from 0.005 to 0.075 wt % and NaCl concentrations from 0 to 1 M. Traditional Huggins analysis fails for polyelectrolytes because of a zero‑over‑zero problem at infinite dilution; therefore the authors adopt a logarithmic approach, fitting ln η_rel versus concentration with a three‑parameter equation (α, β, γ). These parameters capture hydrodynamic interactions and allow calculation of a generalized intrinsic viscosity {η} (Eq. 8), which represents the hydrodynamic specific volume at any concentration, not just at infinite dilution.

Key experimental findings: the intrinsic viscosity