The Role of Sulphonic and Phosphoric Pendant Groups on the Diffusion of Monovalent Ions in Polyelectrolyte Membranes: A Molecular Dynamics Study

Lithium-ion consumption has risen significantly in recent years due to its use in portable devices. Alternative sources of lithium, which include the recovery from brine using the sustainable and eco-friendly electrodialysis technology, has been explored. This technology, however, requires effective cation-exchange membranes that allow the selective permeation of lithium ions. In this study, we have investigated, via molecular dynamics simulations, the role of the two common charged groups, the sulfonic and the phosphoric groups, in promoting the adsorption of monovalent ions from brine comprising Li(+), Na(+), Mg(2+), and Ca(2+) ions. The analysis of the mean square displacement of the ions revealed that Li(+) and Na(+) ions exhibit superior diffusion behaviors within the polyelectrolyte system. The O-atoms of the charged groups bind strongly with the divalent ions (Mg(2+) and Ca(2+)), which raises their diffusion energy barrier and consequently lowers their rate of permeation. In contrast, the monovalent ions exhibit weaker interactions, with Na(+) being slightly above Li(+), enabling the permeation of Li(+) ions. The present study demonstrates the role of both charged groups in cation-exchange membranes in promoting the diffusion of Li(+) and Na(+) ions, and could serve as a guide for the design of effective membranes for the recovery of these ions from brine.