Thermodynamics of Ionic Bonding with Crown Ethers

Lithium resources are limited, and the recent increases in demand and a general desire for less energy intensive, more environmentally responsible processes have lead to increased interest in lithium recycling methods. Multiscale processes studies linking molecular properties, device design decisions and process system optimization are needed to develop effective separation systems. Our long-term interest is in improving membrane technologies used to separate Li⁺ in brines. By using polymeric membranes functionalized with unique ligands, ions can be captured via host-guest interactions. Thus, in this work, we approach the molecular properties side of this multiscale study by performing Molecular Dynamics simulations of specific hosts—crown ethers—in aqueous solutions. Three different crown ethers were selected in order to understand their binding affinity and the mechanics with which they bind to Li⁺ and Na⁺, a typical competitor ion in brines. Free energy calculations with umbrella sampling were used to compute the potential of mean force PMF of each system. We report on the relative efficacy of binding for these ions and its implications for the design of lithium separation membranes.