Thermodynamics of Li+–Crown Ether Interactions in Aqueous Solvent

Abstract

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 (\ce{Li+}). In this paper, molecular dynamics simulations are applied to examine the binding properties of a 12-crown-4—\ce{Li+} system in aqueous solution. It was found that 12-crown-4 did not form stable complexes with \ce{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 (\ce{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 \ce{Li+} as well. 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 \ce{Li+} than 12-crown-4. Metastable minima present in the potential of mean force for \ce{Na+} render binding marginally more likely there. We discuss these results in the context of membrane based applications of crown ethers for \ce{Li+} separations.

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“…The selectivity of crown ethers is driven by the size of the cation relative to the crown ether cavity and the strength of the interaction is related to the decrease in free energy associated with forming the complex. 27,28 It was found that stronger interactions led to increased partitioning and lower salt diffusivities. 29 Interestingly, strong interactions can lead to selective separations through partition driven gating; here, molecules with greater partition coefficients preferentially enter a confined pore thereby preventing other molecules from entering and subsequently diffusing through the membrane.…”