Mechanisms of ionic liquids on the enhancement of interfacial transport of lithium ions in crown ether system

Lithium and lithium isotopes are important for clean energy development. By introducing ionic liquid, the extraction performance of crown ethers for lithium ions can be significantly improved. However, the enhancement mechanism of ionic liquids in the system is not clear, which directly limits the selection of suitable ionic liquids for various applications. In this work, the synergy mechanisms of different ionic liquids with crown ether during the extraction process were investigated by molecular dynamics simulations and verified by experimental results. There were two molecular mechanisms to capture lithium ions for ionic liquids in the crown ether system: co-extraction and assisted extraction. It was generally considered that the cation exchange mechanism was responsible for the extraction of lithium ions in the crown ether-ionic liquids system. Here, a novel perspective was revealed that one [Li·H2O]+ complexed with crown ether and then the chelate cation coordinated with the anion of the ionic liquid to form a neutral molecule based on the simulation of the whole extraction process with molecular dynamics that favored the transport of Li+ from aqueous solution to organic solution. In the system without ionic liquids, Li+-crown ether chelates were distributed only at the interface and did not enter the organic phase which is supported by very low extraction efficiency. It was attributed that charge equilibrium cannot be achieved when Li+-crown ether chelates enter into organic solution in the absence of ionic liquids. Overall, a new insight into the interfacial transport mechanisms of lithium ions was proposed, in which the anion of ionic liquids played a key role in capturing lithium ions at the interface of aqueous solution and organic solution.