The interaction of Cations with Solutes in an Aqueous Solution

Lithium ion batteries are important in a number of applications, and the demand for Lithium is expected to increase. In typical industrial Li⁺ extraction, brine is pumped to the surface of large reservoirs, followed by chemical treatment. This method requires large quantities of water and can pollute groundwater. There has been considerable interest in developing membranes that can be used for preferential Lithium recovery. It has been suggested that para-toluene sulfonate (PTS) can preferentially bind Li⁺ over other cations. In this work, we study the physical chemistry of Li⁺, Na⁺, and K⁺ binding to solutes in an aqueous solution via combined classical and ab initio molecular dynamics simulations. The solutes we consider are PTS, a trimer of PTS, and 4-(trifluoromethyl)benzenesulfonate (TBS). From the analysis of radial distribution functions and the potential of mean force (PMF), we find that the sulfonate group in the cations has the strongest spatial correlation with Li⁺ followed by Na⁺ and then K⁺. The magnitude of PMF, however, is only of the order of the thermal energy. The correlation between the sulfonate group and Li⁺ can be enhanced by polymerizing the PTS molecule, providing a possible means of extracting Li⁺ from the aqueous phase. For PTS, K⁺ is more strongly correlated with the carbon atoms in the benzene ring than the sulfonate group. This might provide a resolution to the puzzling difference between the behavior of sulfonated polystyrene polymers in the presence of K⁺ and Li⁺ counterions.