Surface charge and surface adsorption on an alpha-alumina powder were investigated in a 99.5/0.5 wt% ethanol/water suspension. A model is proposed in which ethanol molecules are dissociatively adsorbed as ethoxide ions and protons to Lewis acid and base sites on the surface. These ions can then desorb separately from the surface. The surface, therefore, acts as a catalyst for the autoprotolysis of the solvent and creates its own ionic atmosphere which cannot be predicted directly from chemistry in the bulk of the solvent. 3.5 μmol/m2 of HCl is reversibly adsorbed to the surface by replacement of ethoxide ions adsorbed to surface acid sites by chloride ions. 2.4 μmol/m2 of KOH is adsorbed to the surface by replacement of surface adsorbed protons with potassium ions. More rapid desorption of negative ions from the surface leaves the particles with a net positive surface charge except when the concentration of negative ions in solution is sufficient to supress this desorption and net surface charge goes to zero. Surface charge is found to be a function only of the activity of the negative ions in solution at the surface. No significant negative surface charge was measured under any conditions here. Equilibrium constants for surface adsorption and charge density as a function of surface activity of ethoxide and chloride ions are calculated. No effect of adsorbed potassium ions on the surface potential was found.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Surfaces, Coatings and Films
- Colloid and Surface Chemistry