Thermodynamic and Neutron Structural Studies of Hydrocarbon Adsorption on MgO Nanocubes

Presented is a series of thermodynamic and neutron scattering experiments used to probe the nature of the hydrocarbon - MgO (100) interaction. High-resolution volumetric isotherms are used to probe the wetting, layer transitions, heats of adsorption, two-dimensional compressibilities, and identify possible phase transitions of thin films of benzene, ethane, and n - pentane on MgO powders. Neutron diffraction experiments were used to probe the structure and identify any phase transitions within monolayer films of ethane, butane and n -pentane.

Benzene adsorption, while showing no visible layering steps, shows a shift in the shape of the isotherm for p/p_o > 0.7. The change in isotherm behavior can be analyzed through use of the BET constant from fits to the experimental data. Ethane and n -pentane both show incomplete wetting with three and two distinct steps in their respective isotherms. Phase transitions from a two-dimensional liquid to a two-dimensional vapor were determined by examining the trend in the two-dimensional compressibilities for the first two layers of ethane (127 ± 1K) and the second layer of pentane (183.8 ± 4K). Neutron diffraction was used to study the structure of the ethane monolayer and determine that the structure of a monolayer of butane forms a commensurate 7√2 x√2R45° structure with p2gg symmetry and 4 molecules in the unit cell. By use of temperature dependent diffraction scans, the melting of the ethane (60K ≤ T_m ≤ 75K) and pentane (99K ≤ T_m ≤ 107K) were observed.

Combining the data sets from these two experimental techniques, preliminary two- dimensional phase diagrams can be proposed, the first step in characterizing the overall nature of the hydrocarbon-MgO interaction.