Molecular simulation of membrane based separations of ethanolic electrolyte solutions

The method of molecular dynamics has been used to study osmosis, reverse osmosis and electro-osmosis in ethanolic electrolyte solutions. Our results have shown the significant role solvation forces play in reverse osmosis based separations in a wide range of polar solvents. For instance in NaBr solutions, Na+ ions, despite their rather small molecular size, were prevented from permeating the semi-permeable membrane while ethanol molecules could readily permeate the membrane. This is because of the solvation of the Na+ ions by the polar solvent and the rather high energy of desolvation (estimated at about 400 kJ/mol, compared to the hydrogen bond energy of ethanol of about 20 kJ/mol) makes the solvated ion rather stable. This in effect increases the apparent size of the Na+ ions considerably. In fact, it appears that such solvation and possibly adsorption effects do not even permit the Na+ ions to get close to the membrane walls. In the past it was thought that ions are prevented from crossing the membrane due to surface interactions between the ions and the membrane surface, which were not clearly defined or understood. In our studies there were only Lennard–Jones interactions between the ions and the membrane. We have also found that the rate of osmosis and reverse osmosis seems to be significantly affected by the selective adsorption of the various solution components on the membrane.