Ion partitioning across a semipermeable membrane with fixed charges
Donnan equilibrium occurs across membranes impermeable to large fixed charges (proteins, polyelectrolytes). Mobile ions (Na⁺, Cl⁻) redistribute so that electrochemical potential μᵢ is equal on both sides. The resulting Donnan potential φ_D = (RT/F)ln([Cl⁻]ᵢₙ/[Cl⁻]ₒᵤₜ) drives osmotic pressure Π = RT(Σcᵢₙ − Σcₒᵤₜ). Critical for cell swelling, cartilage mechanics, and electrochemistry.