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Issue 31, 2017
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Ionic diffusion and proton transfer in aqueous solutions of alkali metal salts

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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 in 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, 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−1 to 0.25 V Å−1 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−1 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, which is characterized by the same proton conduction threshold of neat water (i.e., 0.35 V Å−1). In addition, it turns out that protons flow easily in the LiCl (σp = 3.0 S cm−1) solution and then – in descending order – in the NaCl (σp = 2.5 S cm−1) and KCl (σp = 2.3 S cm−1) 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, further lowering the overall proton transfer efficiency of the aqueous solutions. As a consequence, all the recorded protonic conductivities are lower than that for neat water (σp = 7.8 S cm−1), which strongly indicates that devices exploiting the proton transfer ability should be designed so as to minimize the presence of ionic impurities.

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

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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., 2017,19, 20420-20429
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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, 19, 20420
    DOI: 10.1039/C7CP03663A

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