Polarity of Aluminium oxide

Activated alumina is amorphous or crystalline alumina, which has been partially or completly dehydrated and has a large surface area per unit mass. Activated alumina is made from hydrated alumina, namely, Al2O3.nH2O, where n = 1~3, by calcining to get n closeto 0.5. It is a white or tan-colored material of chalky appearance. Stable crystalline forms are usually not thought of as adsorbents because they have very low surface areas. However, transitional forms, such as gamma and beta alumina, have high surface areas, typically between 200 and 400 m²/g. They contain pores with diameters largely in the range 2–5 nm. Being extremely hydrophilic materials, they find application mainly in the removal of water from acid, gas, or organic solvent streams.

Activated alumina has a high polarity which is suitable for mixtures of saturated and unsaturated hydrocarbons. To avoid tailing peaks for unsaturated components, some controlled surface deactivation is necessary. Aluminium oxide is a polar column chromatography adsorbent that can be separated by polar interactions. Therefore, the mobile phase used must be a non-polar solvent. Silica gel and alumina are both polar adsorbents, but silica gel is less polar than alumina.

Surface facetting and polarity of alumina

Atomic-resolution electron microscopic images of ruby show (0001) polar surface facets developing under electron irradiation. Attempts to understand the “dark-line” contrasts associated with the surface atom profiles, following comparison of the experimental images with computer modelling, leads to the realization that aluminium atom and oxygen atom surface terminations may be distinguished. Hence surface polarity, as well as details of surface step structure and mobility, may be determined. Positive surface polarity is preferred.

References:

[1] L.A. BURSILL, David J S, Peng Ju Lin. Surface facetting and polarity of alumina[J]. Ultramicroscopy, 1987. DOI:10.1016/0304-3991(87)90166-5.