On the potential of ionic liquids to recover metals from the Martian regolith: Computational insights into interfacial interactions

Abstract Ionic liquids (ILs) have been considered as suitable media for the extraction of metals and other elements from the Martian regolith. The complex mineralogy of the Martian regolith and the innumerable possible combinations of cation/anion pairs in available ILs pose challenges to the selection of most effective ILs to extract desired regolith elements. In this work, we utilized molecular dynamics simulations to fundamentally investigate the interactions between three major element oxides in the composition of the Martian regolith simulants, represented as alumina, hematite, and silica, and one basic and one acidic IL, i.e., 1-ethyl-3-methylimidazolium acetate ([emim][Ac]) and 1-ethyl-3-methylimidazolium hydrogen sulfate ([emim][HSO4]), respectively. We used potential of mean force (PMF) calculations to generate the free energy profiles of the cations and anions of both ILs on the oxide surfaces. Additionally, we generated their number density profiles on these surfaces to examine the structural features, including ion layering, at the IL-oxide interfaces. Our results indicate that both ILs have less favorable interactions with the silica surface and, hence, are less stable on it than on the metal oxide surfaces. Between the two ILs, [emim][Ac] shows more favorable energetics than [emim][HSO4] with the hematite surface. However, clear distinction cannot be made between the two ILs regarding their interactions with the alumina surface. Our calculated net interfacial energies of the IL-oxide systems further confirm our findings. The current work provides a general methodology to perform an initial screening of different ILs with respect to their potentials to selectively extract metals and other elements from the Martian regolith.