Surface diffusion of stearic acid on aluminum oxide

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 D_s of stearic acid on the (0001) plane of α‐alumina. We also determined the dependence of D_s upon the amount of water coadsorbed on the surface. When the surface was free of reversibly adsorbed water, D_s had a value of about 2 × 10⁻⁷ cm²/s. As the surface coverage of coadsorbed water increased to about a monolayer D_s increased to a maximum value of about 4 × 10⁻⁵ cm²/s. With further additions of surface water D_s 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 D_s indicated an activation energy for the diffusion process of about 26 kcal/mole.