Molecular sieving of tetraalkylammonium cations on cation exchange systems in DC electric field

Ion exchange membranes and ion exchange resin particles are widely used in electro-membrane separation processes such as electrodeionization and electrodialysis. The internal structure of these ion exchange systems implicitly determines their properties, which, in turn, strongly affect the efficiency of the separation process. Here, we indirectly investigate the internal structure of a heterogeneous cation exchange membrane and cation exchange resin particles with the use of homologous series of tetraalkylammonium chlorides. The tetraalkylammonium cations form a series of cations of various sizes amendable for sieving on the studied systems. This series is used for determination of the cation of critical size. The performed experimental study focuses oninvestigating characteristic features of current-voltage curves measured for individual tetraalkylammonium chloride solutions and their effect on water splitting reaction and electroconvection. Our experimental data show that while tetramethyl-, tetraethyl-, and tetrapropylammonium cations can easily be exchanged by both the membranes and the resin particles, tetrapentyl- and tetrahexylammonium cations are poorly exchanged by both studied systems due to their size. Tetrabutylammonium cation, possessing the Stokes diameter of around 1 nm, turns out to be the cation with the critical size. We observe a strong effect of the counterion size on the occurrence of electroconvection that is pertinent to the overlimiting region of the ion exchange systems. While small counterions allow intensive electroconvection to develop, large counterions have a damping effect on the electroconvective motion. The measurement of the pH changes in our systems during chronoamperometric experiments indicates that water splitting takes place in none of them. Our results classify counterions into two groups based on their ability to pass through theinvestigated cation exchange systems. The first group includes counterions easily exchanged by the ion exchange system. The other group contains cations that cannot be exchanged due to their size. The counterions from the second group are possible foulants with increased potential for adsorption onto the cation exchange systems due to strong electrostatic attractions. Our technique can be used for the determination of critical sizes of other cation exchange systems