Naslov
Is water structure at oxide electrolyte interfaces causing interfacial charge?

Autori
Lützenkirchen, Johannes ; Zimmermann, Ralf ; Preočanin, Tajana ; Küttner, David ; Filby, André ; Rabung, Thomas ; Schild, Dieter ; Plaschke, Markus ; Geckeis, Horst ; Werner, Carsten ; Kallay, Nikola

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Faraday discussion 141: Water- From Interfaces to the Bulk, Poster Abstracts / - Edinburgh : Faraday division, RCS, 2008, P04-P04

Skup
Faraday discussion 141: Water- From Interfaces to the Bulk

Mjesto i datum
Edinburgh, Ujedinjeno Kraljevstvo, 27.08.2008. - 29.08.2008

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
water; surface reaction; surface charge; sapphyre

Sažetak
The current view of charging of oxide electrolyte interfaces is direct protonation and/or deprotonation of surface functional groups. We have performed various experiments to study the charging of several crystal planes of sapphire. All measurements were performed carefully to exclude impurities both inorganic (carbonate, silicate lowering the isoelectric point) and organic (organic films giving rise to a hydrophobic surface). Streaming potential/current measurements were performed at several crystal planes of sapphire (a-Al2O3) using the Microslit Electrokinetic Set-up. Zeta potentials (pertaining to a plane of small but unknown separation from the surface plane) were derived from those measurements using the Smoluchowski equation. In agreement with previous reports in the literature, isoelectric points around pH 4 were obtained. We compare those results to zeta potentials obtained for interfaces between aqueous electrolyte solutions and hydrophobic surfaces and find agreement between the data. Both the isoelectric points and the absolute values of the zeta potentials as a function of pH at a given ionic strength coincide. Surface potential measurements were also performed at the crystal planes using the Single-Crystal-Electrode approach. Surface potentials (pertaining to the surface plane) are nearly zero over a wide pH range. For the 001 plane of sapphire, this can be explained in terms of surface functional groups on this plane. High zeta potentials are incompatible with this interpretation. Consequently, the zeta potentials must arise from another charge building mechanism. Phenomenologically, we explain the observations by an (on average) oriented film of water molecules at the oxide surface, which will cause a similar preference for hydroxide adsorption as is the case for hydrophobic surfaces. Data on other crystal planes of sapphire show similar behaviour. Our interpretation of the experimental data would be a strong challenge for the current view of oxidic surfaces, where the surface charge and potential in electrolyte solutions is entirely attributed to direct (chemical) adsorption or desorption of protons (or hydroxide ions) to surface functional groups.

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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