Physical Adsorption of Ethylene on MgO(100): Effects of Substrate Ionicity and Symmetry on Wetting

The wetting behavior of ethylene adsorbed on MgO(100) was investigated from 83{ 135 K using high resolution volumetric adsorption isotherms. Layering transitions for ethylene on MgO(100) are observed below the bulk triple point at 79.2 K and 98.4 K, respectively. From these isotherms, thermodynamic quantities associated with physical adsorption are determined. It was found that the average area occupied by ethylene on MgO(100) is around 22.6 squared angstroms per molecule. Using the two dimensional isothermal compressibility, the location of two potential phase transitions are identified at 108.6 K and 116.5 K for the first and second layer, respectively. The potential monolayer melting transition at 65 K is reported along with a proposed phase diagram. Neutron scattering data and molecular modeling results provide microscopic insight into the observed physical behavior using structure and dynamics. The results are compared to ethylene adsorption on graphite, a classic example of novel adsorption behavior. These results exhibit striking similarities to the classic example and strongly support the dominant role of molecule-molecule interactions over molecule-substrate interactions during the growth of 2D ethylene films.