Enhanced ionic interaction in narrow pores and ion pair formation

The same physical phenomenon that gives rise to the increase in the electrostatic self-energy of an ion within a narrow water-filled pore is shown to result in interionic electrical interactions within the pore that are much stronger and of longer range than those between the same ions in the same solution in bulk. Because of the much enhanced attraction between ions of opposite charge within the pore the formation of ion pairs becomes likely, even for strong electrolytes that are fully dissociated in the same solution when not spatially confined. Some predicted consequences of ion pair formation in narrow pores that may be experimentally detected are discussed. It is shown that, in a simple passive pore, due to ion pair formation, an Ussing unidirectional flux ratio exponent of less than 1 is predicted. This is usually thought to characterize a carrier rather than a pore.