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Issue 17, 2012
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Hydrated-ion ordering in electrical double layers

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Abstract

In this work we revisit the surface forces measured between two atomically flat mica surfaces submerged in a reservoir of potassium nitrate (KNO3) solution. We consider a comprehensive range of concentrations from 0.08 mM to 2.6 M. The significantly improved resolution available from the extended surface force apparatus (eSFA) allows the distinction of hydration structures and hydrated-ion correlations. Above concentrations of 0.3 mM, hydrated-ion correlations give rise to multiple collective transitions (4 ± 1 Å) in the electrical double layers upon interpenetration. These features are interpreted as the result of hydrated-ion ordering (e.g. layering), in contrast to the traditional interpretation invoking water layering. The hydrated-ion layer adjacent to the surface (i.e. outer Helmholtz layer) is particularly well defined and plays a distinctive role. It can be either collectively expelled in a 5.8 ± 0.3 Å film-thickness transition or collectively forced to associate with the surface by external mechanical work. The latter is observed as a characteristic 2.9 ± 0.3 Å film-thickness transition along with an abrupt decrease of surface adhesion at concentrations above 1 mM. At concentrations as low as 20 mM, attractive surface forces are measured in deviation to the DLVO theory. The hydration number in the confined electrolyte seems to be significantly below that of the bulk. A 1–3 nm thick ionic layer solidifies at the surfaces at concentrations >100 mM, i.e. below bulk saturation.

Graphical abstract: Hydrated-ion ordering in electrical double layers

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Publication details

The article was received on 25 Jan 2012, accepted on 28 Feb 2012 and first published on 22 Mar 2012


Article type: Paper
DOI: 10.1039/C2CP40255F
Citation: Phys. Chem. Chem. Phys., 2012,14, 6085-6093
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    Hydrated-ion ordering in electrical double layers

    R. M. Espinosa-Marzal, T. Drobek, T. Balmer and M. P. Heuberger, Phys. Chem. Chem. Phys., 2012, 14, 6085
    DOI: 10.1039/C2CP40255F

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