Thermodynamic and Neutron Scattering Investigation of Ethylene Wetting on MgO (100)

The adsorption properties of a molecular film on a solid substrate are governed by the relative strength of the molecule-substrate versus molecule-molecule interaction. The wetting properties of ethylene (C$_2$H$_4$) molecular thin films on graphite are of fundamental interest because the number of observed adlayers increases as the isothermal temperature increases with T$\leq$104K (bulk triple point). In adsorbate/substrate systems like C$_2$H$_4$/graphite, it is accepted that triple point wetting occurs. For our studies, we employed MgO nanocubes because they represent a prototypical metal oxide with a wide variety of technological uses including catalyst support. Of particular interest are wetting/layering transitions and the changes that take place in the neighborhood of the bulk triple point. We report our experimental investigation of the adsorption behavior of evidence C$_2$H$_4$ on MgO (100) using high-precision adsorption isotherms and neutron diffraction and scattering. We demonstrate the dominate role that molecule-molecule interaction plays in the wetting phenomena by comparing the behavior of ethylene on graphite and MgO. U.S. Department of Energy (DE-AC05-00OR22725) at ORNL managed and operated by UT-Battelle, LLC, and the NSF (DMR-0412231).