A monolithic bipolar membrane having elastic properties is immersed in a polar solvent or an electrolyte, such as water. An electric field is used to control the transfer rate of neutral or ionized molecules in solution in the electrolyte through the membrane in response to an applied external electromotive force. By adjusting the external electromotive force the membrane can be controlled to selectively pass one type of molecule while rejecting other molecules in solution. A separation between molecules of two or more materials in solution in the electrolyte can thus be achieved.