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Charging of Water at Inert Surfaces

Electrokinetic measurements of gas bubbles (argon or nitrogen) in aqueous electrolyte solutions show negative surface charge (charge of stagnant interfacial water layer) in the pH region above 3, and positive charge in the pH region below 3 [1]. This finding was interpreted as preferential accumulation of OH– ions at the surface, with respect to H3O+ ions. Similar findings were observed with other inert (hydrophobic) materials, such as hydrocarbon oils, Teflon, ice, diamond, etc. [2]. The fast and simple method for measurement of electrokinetic properties of bubbles in aqueous electrolyte solution was developed. It was found that charged bubbles in a contact with the ceramic frit of reference electrode affected the measured electrode potential. This “contact potential” follows the electrokinetic ζ-potential of bubbles, i.e. it depends on pH in the similar way (it is zero near pH ≈ 3) and decreases with electrolyte concentration) [3]. The method of evaluation of ζ- potentials from “contact potentials” is proposed. Surface tension data for air/water interface were interpreted by comprehensive thermodynamic treatment that does not require assumption of constant surface composition. The theoretical analysis explains a minimum of surface tension at pH ≈ 3 and enables estimation of equilibrium parameters for charging the water layer at inert surfaces [4]. It was assumed that water at hydrophobic surfaces, such as silver halides, exhibits similar properties as inert surfaces. Accordingly, surface properties of silver halides were examined by surface potential and electrokinetic measurements [5]. It was found that pH does not influence the surface potential, but in acidic solution the isoelectric points of silver halides are shifted to higher pAg values. This finding supports the hypothesis of different affinities of H3O+ and OH– ions within the interfacial water layer near hydrophobic surfaces.

inert surfaces ; electrokinetics ; air water interface ; electrical interfatial layer ; surface charge ; surface potential

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