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Role of local order in anomalous ion diffusion: Interrogation through tetrahedral entropy of aqueous solvation shells

Banerjee, P and Bagchi, B (2020) Role of local order in anomalous ion diffusion: Interrogation through tetrahedral entropy of aqueous solvation shells. In: Journal of Chemical Physics, 153 (15).

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Official URL: https://dx.doi.org/10.1063/5.0022580


Small rigid ions perturb the water structure around them significantly. At constant viscosity, alkali cations (Li+, Na+, and so on) exhibit an anomalous non-monotonic dependence of diffusivity on ion-size, in stark violation of the Stokes-Einstein expression. Although this is a well-known problem, we find that an entropic view of the problem can be developed, which provides valuable insight. The local entropy experienced by the solute ion is relevant here, which leads to the connection with local viscosity, discussed earlier by many. Due to the strong interactions with ions, the translational and rotational entropy of solvation water decreases sharply; however, an opposite effect comes from the disruption of the tetrahedral network structure of water near the charges. We compute the tetrahedral order of water molecules (qtet) around the ion and suitably defined tetrahedral entropy S(qtet) that is a contribution to the excess entropy of the system. Our results reveal that although the structural properties of the second shell become nearly identical to the bulk, S(qtet) of the second shell is found to play an important role in giving rise to the non-monotonic ion-size dependence. The detailed study of the static and dynamic fluctuations in qtet and the number of hydration water molecules provides interesting insights into correlation between the structure and dynamics; the smallest static fluctuation of qtet for the first hydration shell water molecules of Li+ is indicative of the iceberg picture. The study of fluctuation properties of qtet and the coordination number also reveals the role of the second hydration layer and could explain the anomalous behavior of the Rb+ ion. © 2020 Author(s).

Item Type: Journal Article
Publication: Journal of Chemical Physics
Publisher: American Institute of Physics Inc.
Additional Information: The copyright of this article belongs to American Institute of Physics Inc.
Keywords: Entropy; Hydration; Molecules; Sea ice; Shells (structures); Solvation; Viscosity, Coordination number; First hydration shell; Fluctuation properties; Non-monotonic dependence; Static fluctuations; Stokes-Einstein expression; Structure and dynamics; Tetrahedral ordering, Ions
Department/Centre: Division of Chemical Sciences > Solid State & Structural Chemistry Unit
Date Deposited: 27 Jan 2021 10:27
Last Modified: 27 Jan 2021 10:27
URI: http://eprints.iisc.ac.in/id/eprint/67353

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