The equilibrium set up when two solutions are separated by a membrane permeable to some but not all of the ions in the solutions. In practice, the membrane is often permeable to the solvent and small ions but not to charged entities of colloidal size or such polyelectrolytes as proteins. An electrical potential develops between the two sides of the membrane with the two solutions having varying osmotic pressure. Donnan equilibrium is named after the British chemist Frederick George Donnan (1870–1956), who developed the theory of membrane equilibrium. Donnan equilibrium is important in biology.