Surface diffusion of stearic acid on aluminum oxide

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Abstract
By means of infrared internal reflection spectroscopy we measured the time dependence of the surface concentration of stearic acid molecules over a well defined region of the surface. The molecules were diffusing into this region from a source comprising a compressed monolayer or bulk crystalline material outside the field of view. By these measurements in conjunction with solutions to the diffusion equations we determined the diffusion coefficient Ds of stearic acid on the (0001) plane of α‐alumina. We also determined the dependence of Ds upon the amount of water coadsorbed on the surface. When the surface was free of reversibly adsorbed water, Ds had a value of about 2 × 10−7 cm2/s. As the surface coverage of coadsorbed water increased to about a monolayer Ds increased to a maximum value of about 4 × 10−5 cm2/s. With further additions of surface water Ds then decreased and became less than 10−8 cm2/s at coverages slightly less than about two monolayers. In the region of its maximum value the temperature dependence of Ds indicated an activation energy for the diffusion process of about 26 kcal/mole.

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