The isoelectric points of sapphire crystals and alpha-alumina powder

Streaming potential measurements and atomic force microscropy (AFM) were used to determine the zeta potentials, diffuse layer potentials and isoelectric points (ieps) of alpha alumina sapphire single crystals for four different crystallographic orientations: c-plane, 0001; a-plane, 112̄0; m-plane, 101̄0; and r-plane, 11̄02. Both types of measurements indicated that the iep of the sapphire crystals was between about pH 5.0 and 6.0 for all crystals investigated. Unfortunately the techniques utilized could not conclusively differentiate any subtle differences in iep between the four orientations studied. The iep of alpha alumina powder was found to be pH 9.4. The difference in the ieps of the two different types of alpha alumina (single crystal and powder) is attributed to the presence of different types of surface hydroxyl groups on the two different types of surfaces. Powder is likely to have a greater fraction of singly coordinated surface hydroxyl groups (that have a high pKa), while sapphire single crystals are more likely to have most surface hydroxyls multiply coordinated (which have a low pKa).