Ionic diffusion and proton transfer of MgCl2 and CaCl2 aqueous solutions: an ab initio study under electric field

Abstract

ABSTRACTWe report on a series of ab initio molecular dynamics simulations on MgCl and CaCl aqueous solutions subjected to the effect of static electric fields. The diffusion properties of the solvated cationic species have been investigated both in the low-to-moderate field regime and for intense field strengths, where correlated proton transfers between the water molecules take place. Albeit the Grotthuss-like motion of the protons H dramatically affects the standard relative mobility of monovalent cations such as Li, Na, and K [Phys Chem Chem Phys 2017;19:20420], here we demonstrate that the rule ‘the bigger the cation the higher its mobility’ is preserved for divalent cations – such as Mg and Ca – even when a sustained protonic current is established by the field action. Notwithstanding the presence of charged particles anticipates the field threshold of the molecular dissociation of water from 0.35 V/Å to 0.25 V/Å, such a shift does not depend on the nominal charge the cations hold. Protons flow more easily in the MgCl solution (=2.3 S/cm) rather than in the CaCl (=1.7 S/cm) electrolyte solution because of a twofold reason. Firstly, Ca, being larger than Mg, more strongly hampers the propagation of a charge defect of the same sign (i.e. H). Secondly, we demonstrate that the mobility of Ca is sizably higher than that of Mg. This way, by spanning more efficiently the aqueous environment, Ca further inhibits the proton transfers along the H-bonded network. Finally, the protonic conduction efficiency is inversely proportional both to the ionic radii and to the nominal charge of the cations present in solution.