Donnan Equilibrium — Osmotic Pressure & Ion Partitioning

Macroion charge Z8

Macroion conc. c_M (mM)20

Salt conc. c_s (mM)50

Temperature T (K)298

Donnan equilibrium: charged macroions (proteins, polyelectrolytes) confined by a semipermeable membrane attract counterions and repel coions. Ion partitioning: c±_in = c_out · exp(∓ZeΨ_D/kT). Osmotic pressure Π = RT[Σc_in − Σc_out] includes both ionic and macroion contributions.