gegenions

Gegenions are ions of opposite charge to a charged species in solution. In English-language literature the term is typically rendered as counterions, while Gegenion is most often encountered in German-language texts or in discussions that translate between languages. Gegenions surround charged macromolecules and surfaces, forming an ion atmosphere that partially neutralizes and screens the electric field created by the charge.

In polyelectrolyte chemistry and biophysics, Gegenions balance the charge of polymers such as DNA, proteins, and synthetic polyelectrolytes, as well as colloidal particles. Their presence affects stability, conformation, and interactions with other species. The distribution of Gegenions results from a balance of electrostatic attraction, ion size and valence, hydration, and entropy. The concept of counterion condensation describes how Gegenions partially neutralize highly charged chains, influencing properties like stiffness, persistence length, and folding. Theoretical treatments often employ Debye–Hückel theory, Poisson–Boltzmann equations, or related approaches to model ion atmospheres.

Common Gegenions include cations such as Na+, K+, Ca2+, and Mg2+ when the charged species is anionic. They can be probed and manipulated through osmotic measurements, electrophoretic mobility, scattering techniques, and spectroscopic methods, revealing their roles in screening, binding, and bridging interactions.

Nomenclature note: while Gegenion remains a valid historical and cross-linguistic term, most English-language discussions prefer the term counterion. The concept, however, is widely used to describe the opposite-charge ions that accompany and regulate charged systems in solution.