Osmotic Properties of Crystalline Inorganic Precipitation Membranes: Study of Barium Sulfate Membranes

Abstract

BaSO4 precipitation zones with membrane properties (“conditioned state”: characteristic values of membrane potential, non‐ohmic electric current‐voltage characteristic) are generated by interdiffusion of iso‐osmotic aqueous solutions of BaCl2 and Na2SO4 within cellophane skins. In the “conditioned state” they are found to be impermeable to glucose, sucrose, and raffinose. Osmotic differences of these substances between the bulk phases as well as osmotic differences of BaCl2 and Na2SO4 cause an osmotic volume flow across the precipitation zone from which the hydrodynamic permeability of the precipitation membrane imbedded within cellophane skin can be calculated. The hydrodynamic permeability of a cellophane skin containing a BaSO4 precipitation zone in its “conditioned state” is smaller by a factor of more than 10 compared with that of an untreated cellophane skin. A precipitation zone looses its characteristic properties (“deconditioned state”) if the ions forming the precipitate (Ba++, SO4–) are removed from the bulk phases. This triggers changes of the properties of the surface of the BaSO4 crystallites and Ostwald ripening processes set in. Such a precipitation zone becomes permeable to glucose, sucrose, and raffinose. In the “deconditioned state” the hydrodynamic permeability of precipitations zones approaches nearly the value of an untreated cellophane skin. Adding again Ba+ + and SO4– ions to the bulk phase regenerates the electrochemical and osmotic properties of the precipitation zone. Low angle scattering patterns of synchroton radiation of freeze dried cellulose skins containing a BaSO4 precipitation zone in its “conditioned state” are analyzed in terms of a random structure model. It is found that the correlation length of local electron density fluctuations has a value of about 6 nm. This reflects the tight packing of BaSO4 crystallites forming a precipitation zone with membrane properties.