Microions

Microions are the small, mobile ions that inhabit a solution, typically the cations and anions arising from dissolved salts. They are distinguished from macroions, which are much larger charged species such as colloidal particles or charged polymers. In aqueous media, microions freely diffuse and rearrange in response to electric fields and fixed charges on surfaces or macromolecules.

A key property of microions is their contribution to ionic strength, I, defined as one-half the sum of c_i z_i^2 over all ion species, where c_i is concentration and z_i is valence. The presence of microions screens electrostatic interactions, reducing the range of influences between charged objects. The Debye length, a measure of screening distance, decreases as ionic strength increases, meaning stronger screening at higher salt concentrations.

The behavior of microions is described by theories such as Debye-Hückel theory for dilute electrolytes and the Poisson-Boltzmann framework for systems with fixed charged objects in a sea of mobile ions. In more complete models, microions are treated within the primitive model, with macroions represented as large charged bodies and microions as point charges. The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory combines electrostatic repulsion from microions with van der Waals attraction to predict colloidal stability.

Common microions include simple inorganic ions like H+, Na+, Cl−, and multivalent species such as Ca2+ or Al3+. They can form structured counterion layers near charged surfaces, contribute to osmotic pressure, and determine conductivity and electrokinetic properties of solutions. Microions are central to electrochemistry, colloid science, and biophysics, influencing the behavior of many practical systems.