Donnan membrane equilibrium is not directly applicable to distributions of ions and water in gels or cells

Equilibration of ions and water with a charged gel does not follow the simple equations of the classical Gibbs-Donnan membrane equilibrium. Partition of ions between the gel and the external solution show specific effects, which require that activity coefficients are different in the two compartments. Highly hydrated ions, such as Na⁺ and H⁺ are accumulated into the gel water, whereas less highly hydrated ions, such as K⁺ and NH₄⁺ accumulate in the external water. This selectivity is the obverse of that found for gels containing low-density, expanded water. Water in a charged gel equilibrated with solutions of MgCl₂ was found to be more dense than bulk water at the same temperature. It is proposed that gels imbibe water to maximize the entropy of the system. Ions and water then equilibrate under those constraints. The chemical potential of water in the two compartments equalizes by an increase in density in the compartment of higher osmolality (the charged gel) and a decrease in density in the compartment of lower osmolality (the external solution). Electrolytes equilibrate so that macroscopic electroneutrality is conserved, and the chemical potential of an electrolyte is the same in each compartment. Because activity coefficients are different in the two compartments this results in asymmetric distributions of ions.Because real gels usually contain both charged and hydrophobic regions of surface, populations of water molecules of different density coexist even in very small pores. This accounts for the common failure to detect this phenomenon experimentally.