(484j) Thermodynamics of Mineral Extraction from Martian Regolith Using Ionic Liquids: Insights from Molecular Dynamics Simulation

Ionic liquids (ILs) have unique physico-chemical properties, such as extremely low vapor pressure, that have rendered these solvents suitable for in-situ extraction of minerals from Martian regolith. With NASA’s “Journey to Mars” program identifying a need for the development of novel materials for the extreme Martian environment, recent efforts have confirmed ILs as one such class of materials with numerous space applications. The complex minerology of the Martian regolith, which is composed of silica, hematite, alumina, and other metal oxides, and the innumerable possible combinations of cation/anion pairs in ILs pose challenges to the prediction of selectivity and separation performance of these ILs with respect to select regolith components. In this work, we utilized different computational approaches, through molecular dynamics simulation, to fundamentally investigate the interactions between the IL solvent and Martian regolith main components, as well as the solvation free energy (SFE) at infinite dilution of solute component in two acidic ILs, 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) and 1-Ethyl-3-methylimidazolium hydrogen sulfate ([EMIM] [HSO4]). We used a hybrid OPLS-AA and CVFF force field for the simulations that we validated by comparing the simulated ILs density and other physical properties to the experimental values. In the first approach, Potential of Mean Force (PMF) calculations were conducted to analyze the free energy changes of IL cations and anions in a physical adsorption process (physisorption) on silica, hematite, and alumina solid surfaces. The resulting free energy curves provide valuable insights into solvent-solute interactions. In the second approach, the SFEs were calculated using the free energy perturbation (FEP) method. In total, we investigated the SFE of 15 potential solutes originating from the regolith, , including oxides, metal cations, and silicate anions, such as Fe³⁺, Al³⁺, Ca²⁺, Cl^-, [SiO₃]^2-, [SiO₄]^4-, etc. The SFEs were calculated at two temperatures to elucidate the temperature effect on solute solubilities in the IL. Multiple simulations were replicated for solute/IL combinations to analyze the standard deviations of the SFEs. The current work provides a general methodology to screen a list of IL solvents for selective extraction of minerals from the Martian regolith.