Molecular simulation of diffusion behavior of counterions within polyelectrolyte membranes used in electrodialysis

A molecular dynamics simulation was performed to explore the diffusion behavior of counter-ions within polyelectrolyte membranes via Materials Studio software, which could provide a theoretical basis for the screening and performance prediction of selective ion-exchange membranes (IEMs). The mean square displacements of Li+/Mg2+ in sulfonic and phosphatic polyelectrolyte systems were calculated, and the sulfonic group was determined as a higher preference fixed charge group. The conformational variation of the polyelectrolyte chain with the concentration of counter-ion was assessed via the radius of gyration and, in combination with the pair correlations function. The interaction of counter-ions with sulfonic groups in cation exchange membranes (CEMs) was also determined. The results showed that Mg2+ ions were easily adsorbed on sulfonic groups in CEMs, shielding the negative charge of the polyelectrolyte, which resulted in the coiling of the molecular chain; hence, the radius of gyration decreased. By replacing the adsorbed Li+ ions, more Mg2+ ions were preferentially bound on the sulfonic groups with the increase in Mg2+ ions in the aqueous solution, which is consistent with the ion-exchange isotherms of the Selemion CSO membrane. By calculating the interaction energy of the counter-ions toward the polyelectrolyte and via a permeation experiment involving various cations, we found the difference in interaction energy between monovalent and divalent ions was remarkable, indicating that the interaction of counter-ions toward fixed charge groups in polyelectrolyte mainly determined the ionic diffusion performance. Raising the energy barrier on the interface was determined as an efficient method for the modification of selective IEMs.