Adsorption of Al13-Keggin clusters to sapphire c-plane single crystals: Kinetic observations by streaming current measurements

The streaming current measurements were carried out to investigate whether Al13 clusters would (i) be adsorbed to the surface and (ii) remain stable. Close to the isoelectric point of the sapphire c-plane (around pH 4) it was found that the Al13 ions were adsorbed and stable in the systems for an extended period. After some time the conductivity of the solution strongly increased and the zeta-potential declined, indicating that the clusters were destroyed (certainly in solution, as indicated by the conductivity reading, but potentially also at the surface). ESI-MS measurements at various dilution factors or decreased pH also showed disintegration of the clusters resulting in a multitude of smaller Al species, in agreement with the conductivity measurements during the streaming current measurements. Additions at higher pH showed clear adsorption of the clusters but resulted in stability over the complete observation period as inferred from the conductivity measurements.