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

Abstract The ion aggregation trend for nucleation in MgCl 2 and MgCl 2 –LiCl solutions has been investigated by density functional theory (DFT) and molecular dynamics (MD) simulations. The DFT results show that the solvent-separated Mg 2+ –Cl − conformers of the mixed LiCl·MgCl 2 ·7H 2 O clusters are more stable than their contact-associated Mg 2+ –Cl − isomers in the dilute MgCl 2 –LiCl aqueous solution. The contact-associated Mg 2+ −Cl − structures become dominant in the concentrated MgCl 2 and MgCl 2 –LiCl solutions, while the Cl − ions can be more frequently found in the second shell than in the first shell of the Mg 2+ ions. The aggregation characteristics indicate that the solvent-separated Mg 2+ –Cl − association is more likely correlated to the aggregation process for nucleation in MgCl 2 and MgCl 2 –LiCl solutions, whereas the contact-associated Mg 2+ –Cl − structures may be correlated to the supersaturation of the MgCl 2 aqueous solution or MgCl 2 –LiCl brines. The evolution of the clusters shows that the solvent-separated Mg 2+ –Cl − mixed clusters can markedly enlarge, while this is not the case for contact associated Mg 2+ –Cl − mixed clusters during the simulation. The MD simulations reveal that the trends of solvent-separated Mg 2+ –Cl − association and contact Li + –Cl − association around Mg 2+ ions become synchronized in MgCl 2 –LiCl brines with a low Mg/Li ratio, in which LiCl·MgCl 2 ·7H 2 O double salt can be formed.