Microscopic insight into the ion aggregation characteristics in aqueous MgCl2 and MgCl2–LiCl solutions: Implications for Mg2+/Li+ separation

The ion aggregation trend for nucleation in MgCl2 and MgCl2–LiCl solutions has been investigated by density functional theory (DFT) and molecular dynamics (MD) simulations. The DFT results show that the solvent-separated Mg2+–Cl− conformers of the mixed LiCl·MgCl2·7H2O clusters are more stable than their contact-associated Mg2+–Cl− isomers in the dilute MgCl2–LiCl aqueous solution. The contact-associated Mg2+−Cl− structures become dominant in the concentrated MgCl2 and MgCl2–LiCl solutions, while the Cl− ions can be more frequently found in the second shell than in the first shell of the Mg2+ ions. The aggregation characteristics indicate that the solvent-separated Mg2+–Cl− association is more likely correlated to the aggregation process for nucleation in MgCl2 and MgCl2–LiCl solutions, whereas the contact-associated Mg2+–Cl− structures may be correlated to the supersaturation of the MgCl2 aqueous solution or MgCl2–LiCl brines. The evolution of the clusters shows that the solvent-separated Mg2+–Cl− mixed clusters can markedly enlarge, while this is not the case for contact associated Mg2+–Cl− mixed clusters during the simulation. The MD simulations reveal that the trends of solvent-separated Mg2+–Cl− association and contact Li+–Cl− association around Mg2+ ions become synchronized in MgCl2–LiCl brines with a low Mg/Li ratio, in which LiCl·MgCl2·7H2O double salt can be formed.