20420 | Phys. Chem. Chem. Phys., 2017, 19, 20420--20429 This journal is © the Owner Societies 2017 Cite this: Phys. Chem. Chem. Phys., 2017, 19, 20420 Ionic diffusion and proton transfer in aqueous solutions of alkali metal salts Giuseppe Cassone, * a Fabrizio Creazzo, b Paolo V. Giaquinta, c Jiri Sponer a and Franz Saija d 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 m(K + ) 4 m(Na + ) 4 m(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 (s p = 3.0 S cm 1 ) solution and then – in descending order – in the NaCl (s p = 2.5 S cm 1 ) and KCl (s p = 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 effi- ciency of the aqueous solutions. As a consequence, all the recorded protonic conductivities are lower than that for neat water (s p = 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. I. Introduction Most of the properties and anomalies describing the behaviour of water are somehow related to the hydrogen bonded (H-bonded) network. 1–3 Albeit the features of H-bonds have been investigated and depicted by an impressive amount of research, the way in which some external conditions – such as the inclusion of ionic species – affect the three-dimensional H-bonds arrangement is wrapped up in a high degree of uncertainty. If, on one hand, the presence of solvated ions cannot be avoided even in ultra-pure water samples, on the other hand, the lack of scientific consensus about the ion-induced micro- scopic effects on the water structure is representative of the practical challenges faced when investigating electrolyte solutions. 4,5 However, the indisputable role played by a few atomic charged species both in biology (i.e., Na + ,K + , Cl  , Mg 2+ , Ca 2+ , etc.) 6–8 and in industry (e.g., Li + batteries) 9 requires impelling and massive scientific efforts. In fact, besides the well-known Hofmeister series, 10 hydrated ionic species finely rule the selectivity of cell membranes, 6,7 which is thus responsible for complex processes such as nerve pulse generation. On the other hand, aqueous solutions represent the prototype of electro- lytic batteries. In all cases, a subtle balance between electrostatics, quantum mechanics (i.e., partial orbital sharing), and thermodynamics governs the delicate behaviour of the hydration process. The complexity of the problem is witnessed, inter alia, by the fact that there is no general consensus on the spatial extent of the effects induced by the inclusion of an ion in bulk water. 11–13 Recent ab initio calculations 14 have shown that the presence of a chaotrope species such as Cl  does not have any effect on the orientation of water dipoles beyond the first hydration shell, whereas detectable perturbations – perhaps extremely small and unable to affect biological phenomena – have been observed in the polarizability of the water molecules at longer distances. a Institute of Biophysics, Czech Academy of Sciences, Kra ´lovopolska´135, 61265 Brno, Czech Republic. E-mail: cassone@ibp.cz, sponer@ncbr.muni.cz b Universite ´ d’Evry val d’Essonne-Universite ´ Paris-Saclay, Blvd. F. Mitterand, 91025 Evry, France. E-mail: fabrizio.creazzo@univ-evry.fr c Universita ` degli Studi di Messina, Dipartimento di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Contrada Papardo, 98166 Messina, Italy. E-mail: paolo.giaquinta@unime.it d CNR-IPCF, Viale Ferdinando Stagno d’Alcontres 37, 98158 Messina, Italy. E-mail: saija@ipcf.cnr.it Received 31st May 2017, Accepted 12th July 2017 DOI: 10.1039/c7cp03663a rsc.li/pccp PCCP PAPER Published on 12 July 2017. Downloaded by University Library Zurich on 10/21/2022 6:24:08 PM. View Article Online View Journal | View Issue