Thermodynamics of Li⁺-Crown Ether Interactions in Aqueous Solvent

Lithium ion-based batteries are ubiquitous in modern technology due to applications in personal electronics and high-capacity storage for electric vehicles. Concerns about lithium supply and battery waste have prompted interest in lithium recycling methods. The crown ether 12-crown-4 has been studied for its abilities to form stable complexes with lithium ions (Li⁺). In this paper, molecular dynamics simulations are applied to examine the binding properties of a 12-crown-4-Li⁺ system in aqueous solution. It was found that 12-crown-4 did not form stable complexes with Li⁺ in aqueous solution due to the binding geometry which was prone to interference by surrounding water molecules. In addition, the binding properties of sodium ions (Na⁺) to 12-crown-4 are examined for comparison. Subsequently, calculations were performed with the crown ethers 15-crown-5 and 18-crown-6 to study their complexation with Li⁺ as well as Na⁺. It was determined that binding was unfavorable for both types of ions for all three crown ethers tested, though 15-crown-5 and 18-crown-6 showed a marginally greater affinity for Li⁺ than 12-crown-4. Metastable minima present in the potential of mean force for Na⁺ render binding marginally more likely there. We discuss these results in the context of membrane-based applications of crown ethers for Li⁺ separations.