Characterization and Modeling of the Al-Oxide/Aqueous Solution Interface: I. Measurement of Electrostatic Potential at the Origin of the Diffuse Layer Using Negative Adsorption of Na+ Ions

A method of measuring electrostatic potential at the origin of the diffuse layer, ψd, from co-ion negative adsorption experiments was described and used for an amphoteric Al-oxide. It was found that the magnitude of ψd for the Al-oxide decreased with increasing electrolyte concentration. Our data showed that the zeta potential, ζ, differed from ψd for the Al-oxide. Therefore, the generally accepted assumption that ψd ≈ ζ should be further examined. We modeled our experimental data using the triple layer model. It was shown that both the surface charge density, σ0, measured from acid-base titration and ψd, determined from co-ion negative adsorption experiments, can be quantitatively described by the triple layer model using a self consistent set of parameters. A major criterion of a desired surface complexation model should be its ability to account for various properties of the interface with a self consistent set of parameters as demonstrated by describing or modeling different experimental data. Hence, further characterization of the physical and chemical nature and properties of the colloidal systems is needed to provide the additional data base for developing, testing, and modifying models.