Counterions

Counterions are ions of opposite charge to a given charged species that accompany it in order to maintain electrical neutrality. They are central to the description of electrolytes, polyelectrolytes, colloids, and ionic solids. In a simple salt like sodium chloride dissolved in water, the solution contains Na+ and Cl- ions; each ion is accompanied by counterions of the opposite charge that balance the net charge in any region of the solution. In a broader sense, for a negatively charged polymer such as DNA, the surrounding positive ions (e.g., Na+, Mg2+) serve as its counterions, neutralizing charge along the polymer and influencing its conformation and interactions.

Counterions are typically more mobile than the charged host species in liquid media and contribute to ionic strength and electrostatic screening. The concept is a cornerstone of theories of electrostatics in solutions, such as the electrical double layer, where counterions accumulate near a charged surface to screen its field. In polyelectrolyte solutions, counterion condensation can occur, in which a fraction of counterions stays closely associated with the charged chain, effectively reducing the chain's net charge.

In solid-state chemistry, counterions balance the charges in ionic compounds and defect structures; in doped materials, the introduction of extra counterions can change conductivity and electronic properties. Overall, counterions play a fundamental role in charge balance, structure, solvation, and transport across a wide range of chemical systems.