polyion

A polyion is a macromolecule that carries multiple ionic charges along its structure. In solution, these repeating charged units give rise to polyelectrolyte behavior, making polyions highly responsive to changes in pH, ionic strength, and solvent quality. Polyions are typically classified as polyanions (negatively charged) or polycations (positively charged), depending on the sign of their ionizable groups.

The most common examples of polyions include biological polymers such as DNA, which is a polyanion, and synthetic polymers like poly(acrylic acid) or polystyrene sulfonate (polyanions) or certain polyamines and poly(dimethylamino) polymers (polycations). The charged backbone causes electrostatic repulsion between segments of the chain, often resulting in extended conformations in good solvents. The behavior of polyions in solution is influenced by factors such as ionic strength, pH, solvent dielectric constant, and the density of charges along the chain. Counterions are attracted to and partially condense around the polyion, reducing its effective charge through phenomena such as counterion condensation.

Characterization of polyions involves determining charge density, molecular weight, and conformational behavior. Techniques include acid–base titration, rheology to measure viscosity, electrophoretic mobility, light scattering for size and shape, and various spectroscopic methods. In applications, polyions are used in water treatment for coagulation and flocculation, in drug and gene delivery, and as components of hydrogels and responsive materials. The study of polyions intersects chemistry, physics, and materials science, reflecting their complex interplay between charge, structure, and solvent environment.