Ion association characteristics in MgCl2and CaCl2aqueous solutions: a density functional theory and molecular dynamics investigation

In this work, ion association characteristics in MgCl2 and CaCl2 aqueous solutions have been investigated using density functional theory (DFT) and molecular dynamics (MD) simulation at various temperatures and pressures. Results all indicate that the ion association is more obvious in MgCl2 solution than in CaCl2 solution, due to the hydration characteristics of Mg2+ and Ca2+. Although ion association can occur in concentrated MgCl2 solution, contact ion pair (CIP) structures and solvent-shared ion pair (SSIP) with single Cl− dissociated (SSIP/s) ([MgCl(H2O)6Cl]0) are quite unstable. CIP and SSIP/s conformers are almost iso-energetic even in CaCl2 solution, and the lifetime of CIP conformer is much longer than that of SSIP/s conformer in concentrated CaCl2 solution. However, the possibility of CIP conformer found in concentrated CaCl2 solution is obviously lower than its SSIP/s isomers. MD simulations exhibit that ion association characteristics do not change as pressure ranges from 0.5 to 20.0 bar in MgCl2 solution while ion association in CaCl2 solution does change as pressure rises. Moreover, ion association decreases with temperature increasing in MgCl2 solution, but the situation is reversed in CaCl2 solution.