Donnan

Donnan refers to a set of related electrochemical and osmotic phenomena described by the early 20th-century chemist Frederick George Donnan. The Donnan concept arises when two solutions are separated by a semipermeable membrane that allows some ions to pass but prevents others from crossing. If some ions (usually large or charged species such as proteins or polyelectrolytes) cannot permeate the membrane, their fixed charges create an imbalance in the distribution of permeant ions on the two sides. The system then reaches a Donnan equilibrium, accompanied by a measurable electric potential difference across the membrane known as the Donnan potential. The effect is most evident for simple, monovalent ions, and it imposes constraints on ion concentrations on each side of the membrane.

Mechanistically, the impermeant ions influence ion distribution through electroneutrality and the need for equal chemical potentials of permeant ions across the membrane. For monovalent ions, this leads to relationships between the concentrations on the two sides and a finite membrane potential. In practice, the Donnan effect causes unequal ion distributions and can contribute to osmotic pressure differences across the membrane.

Applications of the Donnan concepts appear in biology, physiology, and chemistry. In cells, impermeant intracellular components contribute to ion gradients and osmotic balance. In dialysis and filtration, Donnan effects influence solute partitioning and membrane selectivity. In polymer gels and colloids with fixed charges, Donnan equilibria help explain ion distributions and swelling behavior. The term encompasses the Donnan equilibrium, the Donnan potential, and related Donnan exclusion phenomena.