Physical Adsorption of Linear Hydrocarbon Quadrupoles on Graphite and MgO (100): Effects of the Compatibility of Surface and Molecular Symmetries

The process of physical adsorption finds a practical role in wide-ranging fields from catalysis, to lubrication, and even optoelectronics. Furthermore, it provides a mechanism to probe the fundamental understanding of intermolecular forces and how symmetries can play a role in the behavior of a system. Linear quadrupoles preferentially adopt square-T configurations when confined in two dimensions. This would lead the system to adopt a four-fold symmetry in the molecular lattice.

Two archetypal surfaces often studied in physisorption research are MgO (100), which has a four-fold symmetry of alternating charges, and the basal plane of graphite, which has a six-fold symmetry to its non-polar, weakly corrugated surface. These differing surface symmetries provide two test cases for comparison. In the case of MgO (100), the molecule-molecule and molecule-surface interaction are synergistic, both driving the film towards the same symmetry; whereas for graphite, the six-fold surface symmetry is incompatible with the preferred four-fold interaction symmetry of the molecules. This presents the opportunity for structurally frustrated systems to arise.

Acetylene and allene are both simple, linear, rigid hydrocarbons with large quadrupole moments of similar strength. The most distinct variations between these two molecules are size and axial rotational symmetry. These molecules, just like the surface, provide two simple, but contrasting symmetry effects. The simple point group of truly linear molecules of acetylene allow for them to lie completely flat against a surface. The 90-degree dihedral angle between the hydrogen pairs on opposing sides of allene molecules prevent them from easily being able to lie perfectly flat against the surface, creating another opportunity for broken symmetry in the molecule-surface interactions – this instance in the vertical direction rather than the two dimensional adsorption plane.

This investigation aims to study the behavioral properties of acetylene and allene films through thermodynamic, structural, and phase behavior analyses when adsorbed on both graphite and MgO. To this end, a combination of volumetric adsorption isotherms, elastic neutron diffraction, and computational modeling have been employed.