  This paper discusses the little-known fact that Donnan's equilibrium criteria
established over 100 years ago neglected the coupling between ion
concentrations and the osmotic pressure. Such coupling can be treated based on
general thermodynamic considerations including the solvent equilibrium, leading
to a membrane potential that consists of not only the classical Donnan
potential term but also an additional term due to the osmotic pressure, and the
existence of a membrane potential even when the impermeable species are not
charged. This coupled treatment is in conflict with the well-established
Poisson-Boltzmann equation and Nernst-Planck equation, but is consistent with
the extension of these equations including the solvent effects by Freise and
Schlogl, enables us to view the electrical double layer equilibrium as Donnan
equilibrium.
