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

ABSTRACT

We report on a series of ab initio molecular dynamics simulations on MgCl${}_2$ and CaCl${}_2$ 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${}^{2+}$ and Ca${}^{2+}$ – 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${}_2$ solution (${\sigma }_p$=2.3 S/cm) rather than in the CaCl${}_2$ (${\sigma }_p$=1.7 S/cm) electrolyte solution because of a twofold reason. Firstly, Ca${}^{2+}$, being larger than Mg${}^{2+}$, more strongly hampers the propagation of a charge defect of the same sign (i.e. H${}^{+}$). Secondly, we demonstrate that the mobility of Ca${}^{2+}$ is sizably higher than that of Mg${}^{2+}$. This way, by spanning more efficiently the aqueous environment, Ca${}^{2+}$ 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.

KEYWORDS:

• Aqueous solutions
• ab initio molecular dynamics
• proton transfer

Disclosure statement

No potential conflict of interest was reported by the authors.