The Donnan equilibrium is a property of biological membranes but is also a characteristic of semi-permeable membranes used in the food industry. The situation where it is most applicable is nanofiltration of milk and whey where the rejection of ions is critical in concentration practice.
In biological membranes, the differences in permeability to different solutes is very complex and dependent on a number of factors. In cell membranes the ability to move ions actively or by diffusion has great impact on nerve cell performance.
The movement of ions across any membrane leads to an equilibrium with a distribution of both permeable and impermeable ions. That equilibrum is the Donnan equilibrium.
One of the consequences is a membrane potential which is an electrical phenomenon. There are three factors that contribute to this charge/electrical difference, ion mobility, the ionic concentration difference and ion permeability.
Ions have different mobility due to their different sizes and charge densities. They have the ability to form hydration layers around them which add to their size and alter their movement through semi-permeable membranes. Cells in particular actively support a concentration difference which can also be described as a gradient. Inside a cell, the Na+ and Cl– concentrations are lower whilst on the other side the K+ concentration is higher. All other ions tend to be in equal concentration.