A mechanism for water flow in bipolar membranes

Abstract

When water splitting occurs in a bipolar membrane a steady state is reached in which the water which leaves the interface of the anion and cation exchange regions in the form of hydrogen and hydroxyl ions, is replaced by fresh water from the boundary solutions. Since the boundary solutions may comprise concentrated acid and alkali while the water at the interface is deionised, the flow of the water towards the interface occurs against an osmotic pressure difference which may exceed 100 atmospheres. There must therefore be other forces on the water which combine to exceed the osmotic force and oppose it. The net water flow depends on the gradients in osmotic, Maxwell and hydrostatic pressures. In this article we show that the difference in Maxwell pressure for water between the outside solutions and the interface may exceed the osmotic pressure difference between the regions. We point out that the hydrostatic pressure at the interface can be higher than in the external solutions.