Structure of an $n$-butane monolayer adsorbed on magnesium oxide (100)

Neutron diffraction has been used to characterize the structure of the solid phase of the completed monolayer of $n$ butane on the $\mathrm{MgO}\left(100\right)$ surface at low temperature. The monolayer is found to adopt a commensurate $(7\sqrt{2}\times \sqrt{2}R45°)$ structure with lattice constants $a=29.47\mathrm{\AA }$ and $b=4.21\mathrm{\AA }$, $P_{2gg}$ symmetry and four molecules in the unit cell. Excellent agreement with the experimental diffraction pattern is realized, using a Lorenztian profile to describe the line shape.

Figure 1

Butane isotherms in the temperature range 158–$182\mathrm{K}$. The inset compares an isotherm from methane and one from butane measured on the same sample to estimate the surface area of each butane molecule. It is clear that two layering transitions are evident. This behavior is similar to that observed for propane and ethane on $\mathrm{MgO}$, whereas methane forms at least seven discrete layering transitions. The butane surface coverage used in the diffraction measurements corresponds to the crest of the first step of the isotherm, equivalent to $\sim 0.08\mathrm{mmol}$, or “monolayer completion.”

Figure 2

Schematic picture of the $P_{2\mathrm{gg}}$ structure adopted by $n$ butane on $\mathrm{MgO}$, which illustrates the molecular arrangement and defines the fit parameters; $a$ and $b$ are the unit cell parameters, HB is the “herringbone” angle, and $y$ is the displacement along the $b$ axis. The centers of the four molecules within the unit cell are thus located at $(0,+y)$, $(\frac{a}{4},-y)$, $[\frac{a}{2},(\frac{b}{2}-y\left)\right]$, and $[\frac{3a}{4},(\frac{b}{2}+y\left)\right]$. The positions of the atoms are then defined by the coordinates listed in Table with the HB and axial rotations applied.

Figure 3

Experimental diffraction pattern and “best-fit” calculated pattern for $n$ butane adsorbed on $\mathrm{MgO}$. The peaks are indexed for the rectangular unit cell $a=29.47\mathrm{\AA }$, $b=4.21\mathrm{\AA }$.