Some data and simple models for the silanated glass-electrolyte interface
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The interface between silanated glass and aqueous KCl electrolyte solutions was studied by zeta-potential and contact angle measurements. Variation of the pH in the zeta-potential measurements yielded a sharp isoelectric point (IEP) at about pH 4, which was independent of the KCl concentration. This particular pH corresponds to zero electrokinetic potential for many hydrophobic surfaces encompassing Teflon, polyethylene and others.
The results are interpreted in terms of hydroxide adsorption at the silanated glass-aqueous interface. A previously developed model for such inert surface—electrolyte interfaces is applied to the experimental data. The model parameters are within the range of those previously obtained for comparable surfaces.
Discrepancies were observed in pH dependent contact angle measurements, which were done using both the plate and the drop method: advancing contact angles were always higher than receding contact angles. Advancing contact angles using the drop method yielded no clear tendency as a function of pH and lower values than those obtained by the plate-method. Overall the contact angle data are presented but deemed difficult to interpret in terms of pH dependence and relation to zeta-potentials. A preliminary description of the contact angle data is attempted via the Lippman-Young equation for the different contact angle data sets based on the model potentials. The model results are discussed.
Zeta-potential measurements were also performed as a function of salt content within a narrow pH range. As would be expected the presence of a 2:1 electrolyte (BaCl2) results in lower absolute values when compared to the KCl case.
KeywordsHydrophobicity Contact angle Zeta-potential Hydroxide ion adsorption
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