Donnan Effect:

Ion selective membranes display selective permeability towards ions of a particular sign. Like membranes are ion exchangers in sheet form, a cation exchanger being selectively permeable towards cation and an anion exchange showing the similar behaviour towards anions. The chemical structure of ion exchange membranes consists of a network of carbon atoms and styrene nuclei along with a suitable functional group attached to the styrene nuclei. If the membrane is immersed in an aqueous medium, it absorbs water and swells because of the affinity among the functional groups and water. Functional groups dissociate to yield an electrically charged fixed set chemically bound to the hydrocarbon matrix and a mobile ion carrying the opposite charge. The latter are free to move under the influence of an applied electrical field or to exchange along with other ions of same charge diffusing within the membrane from the external solution.

Cation selective membranes holds negatively charged groups fixed to the hydrocarbon matrix and a mobile cation. Anion selective membranes hold a positively charged group fixed to the hydrocarbon matrix and a mobile anion. While an ion selective membrane is in contact along with an aqueous solution of an electrolyte, those ions having the charge same to that of the fixed group on the membrane polymer matrix are designated as coions and those ions having charge same to that of the mobile ions are designated as counter ions. While the electrical equilibrium is established among the ion selective membrane and the external electrolyte solution, within the membrane phase, the charges of the fixed group is electrically balanced with those of the counter ions from the external solutions as well as its own intrinsic counter ions. A coion from the external solution is more or less excluded from the polymer matrix. This kind of exclusion is known as the Donnan Exclusion and the selectivity of ion exchange membranes arises because of Donnan effect.