Assessing polyelectrolyte effective charge
Polyelectrolytes are distinguished by their linear charge density, a parameter responsible for their many functions in nature and their many technological applications. Despite the fundamental and practical importance of this parameter, quantitative understanding of polyelectrolyte charge remains rudimentary, primarily due to a lack of experimental methods capable of capturing polyelectrolyte's actual or effective charge under diverse conditions.
Capillary electrophoresis study revealed how polyelectrolyte effective charge correlates with polymer structure and solvent polarity. The study focused on ionenes, polyelectrolytes with quaternary ammonium groups linked in the backbone by spacers of defined uniform or alternating length. Traditional ionenes with hydrophobic aliphatic spacers as well as novel, inherently hydrophilic ionenes with oxyethylene spacers were synthesized, characterized, and examined by capillary electrophoresis. Complete ionization of ionenes and their good solvency in water and mixtures of water with lower dielectric constant solvents (methanol, acetonitrile) enabled measurements of ionene effective charge density as both backbone charge spacing and solvent dielectric constant varied. Two fundamental trends were uncovered: (i) As charge spacing decreased, effective charge rose linearly with backbone charge density to saturation at a critical value of backbone charge density, as expected from the standard (Manning) counterion condensation theory; (ii) As solvent dielectric constant decreased, deviating from the existing theory, charge saturation occurred not at a critical backbone charge density but at a critical dielectric constant. The latter trend suggests a new type of counterion condensation, one driven by ion-pairing. In support of the ion-pairing hypothesis, the onset of condensation correlated with counterion size. Traditionally associated with small molecule electrolytes, ion-pairing has not been previously regarded as an origin of polyelectrolyte effective charge lowering.
Manifesting the universality of our findings with respect to polyelectrolyte chemistry and sign of charge, analogous charge behaviors were found with quaternized poly(vinyl pyridine), poly(styrene sulfonate), and acrylamide-acrylamidomethylpropanesulfonate copolymers. By comparing polyelectrolytes with sulfonated and quaternary ammonium functionality, we deduced a universal dimensionless criterion for the onset of ion-pairing on polyelectrolytes. This study is the first to report polyelectrolyte effective charge lowering by an ion-pairing mechanism, but the results should have broad implications for the polyelectrolyte field.