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Ionic diffusion and proton transfer in aqueous solutions of alkali metal salts

Abstract

We report on a series of ab initio molecular dynamics investigations on LiCl, NaCl, and KCl aqueous solutions under the effect of static electric fields. We have found that although at low-to-moderate field intensity regimes the well-known sequence of cationic mobilities μ(K+) > μ(Na+) > μ(Li+) (i.e., the bigger the cation the higher the mobility) is recovered, from intense field strengths this intuitive rule is no longer verified. In fact, the field-induced water molecular dissociations lead to more complex phenomena regulating the standard migration properties of the simplest monovalent cations. The water dissociation threshold is lowered from 0.35 V/Å to 0.25 V/Å by the presence of charged species in all samples. However, notwithstanding a one-stage process of water ionization and proton conduction takes place at 0.25 V/Å in the electrolyte solutions where "structure maker" cations are present (i.e., LiCl and NaCl), the KCl aqueous solution shows some hindrance in establishing a proton conductive regime, being characterized by the same proton conduction threshold of neat water (i.e., 0.35 V/Å). In addition, it turns out that protons flow easier in the LiCl (σp = 3.0 S/cm) solution and then - in descending order - in the NaCl (σp = 2.5 S/cm) and KCl (σp = 2.3 S/cm) electrolyte solutions. The protonic conduction efficiency is thus inversely proportional to the ionic radii of the cations present in the samples. Moreover, Cl- anions act as a sort of "protonic well" for high field intensities, lowering further the overall proton transfer efficiency of the aqueous solutions. As a consequence, all the recorded protonic conductivities are lower than for neat water (σp = 7.8 S/cm), which strongly indicates that devices exploiting the proton transfer ability should be designed so as to minimize the presence of ionic impurities.

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Publication details

The article was received on 31 May 2017, accepted on 12 Jul 2017 and first published on 12 Jul 2017


Article type: Paper
DOI: 10.1039/C7CP03663A
Citation: Phys. Chem. Chem. Phys., 2017, Accepted Manuscript
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    Ionic diffusion and proton transfer in aqueous solutions of alkali metal salts

    G. Cassone, F. Creazzo, P. V. Giaquinta, J. Sponer and F. Saija, Phys. Chem. Chem. Phys., 2017, Accepted Manuscript , DOI: 10.1039/C7CP03663A

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