Thermodynamic and Modeling Study ofn-Octane,n-Nonane, andn-Decane Films on MgO(100)

Abstract

The thermodynamic properties of n-octane, nnonane, and n-decane adsorbed on the MgO(100) surface were investigated using high-resolution, volumetric adsorption isotherms in the temperature ranges of 225−295, 245−280, and 250−310 K, respectively. Two distinct molecular layers were observed in all isotherms. The heat, differential enthalpy, differential entropy, and isosteric heat of adsorption were determined. The temperature dependence of the two-dimensional compressibility was used to identify that two-phase transitions occur for the first and second layers, respectively. The average area per molecule of 137 ± 5,128 ± 5, and 202 ± 5 Å 2 for octane, nonane, and decane, respectively, suggests that the carbon backbone is preferentially oriented parallel to the MgO(100) surface. A substantial set of molecular dynamics (MD) temperature and coverage runs for all three molecules using the COMPASS force field were used to calculate both the minimum energy configurations for individual molecules and near-completed layers on the MgO(100) surface. These calculations support the thermodynamic evidence of the carbon backbone oriented parallel to the surface and additionally suggest a preferential alignment of the molecule along the ⟨11⟩ and ⟨10⟩ directions in the surface (100) plane. The MD simulations were used to evaluate the distribution of the molecules perpendicular to the MgO(100) surface as a function of temperature and nominal surface coverage. Evidence is also recorded that suggests that the interface broadens, the orientational order decreases, and liquid-like layers appear as the temperature is increased. As observed previously with the behavior of heptane on MgO(100), an entropic contribution to the free energy is apparent as a result of the odd number of carbon atoms in the nonane backbone. The near surface layer of all three molecules appears to remain orientationally ordered (i.e., noticeably more planar) and stabilized (i.e., more solid-like) at substantially higher temperatures when the second and third layers are observed to be liquid-like. This observation is consistent with earlier experimental observation of rare gases and alkanes on graphite that indicate that the near-surface layer melts at temperatures above the bulk melting temperature.

Adsorption of Pentane and Hexane Thin Films on the Surface of Graphite(0001)

“…Unfortunately, no such anomalies were observed for pentane and hexane on graphite, likely because they appear at temperatures below where these measurements were made. It was possible to locate potential phase transitions in our earlier adsorption studies of pentane and hexane on MgO(100) , and hexagonal boron nitride because the binding energies were lower and hence exhibited higher equilibrium vapor pressures overall across the same temperature ranges.…”

Mechanochemistry of Physisorbed Molecules at Tribological Interfaces: Molecular Structure Dependence of Tribochemical Polymerization

“…45 Molecular dynamics simulations of long-chain linear alkanes on the MgO(100) surface found that the carbon backbone is preferentially oriented parallel to the surface at low temperature but the orientational disorder and liquid-like layer appear as the temperature is increased to room temperature. 46 Also, the disjoining pressure measurements of wetting films of n-octane and n-decane on aluminum oxide showed that the films have molar volumes identical with those of the bulk liquids. 47 These studies suggest that the comparison of ΔV* with the liquid molar volume is reasonable.…”

“…Computational and experimental sum frequency generation studies of α-pinene adsorbed on silica revealed a rotationally fluidic state; the energy difference between two distinct adsorption geometries is smaller than the thermal energy at room temperature . Molecular dynamics simulations of long-chain linear alkanes on the MgO(100) surface found that the carbon backbone is preferentially oriented parallel to the surface at low temperature but the orientational disorder and liquid-like layer appear as the temperature is increased to room temperature . Also, the disjoining pressure measurements of wetting films of n -octane and n -decane on aluminum oxide showed that the films have molar volumes identical with those of the bulk liquids .…”

Liquid Adsorption of Organic Compounds on Hematite α-Fe2O3 Using ReaxFF

“…This assumption has also been proved to be unsuitable for the study of polymers of intrinsic microporosity (PIM) designed to have a rigid and contorted backbone which swells upon adsorption of carbon dioxide at high pressures . Nevertheless, nonporous solids are often considered inert, regardless of the density and polarity of the liquid. − In this work, we study the effect that organic molecules of different polarity have on the structure of a crystal surface as well as the effect on the adsorbed-phase properties that results from neglecting changes in the solid structure. We focus our attention on the adsorption of small organic molecules such as ethanol (C 2 H 5 OH), toluene (C 6 H 5 –CH 3 ), and iso-octane (C 8 H 18 ), which are commonly used as solvents in the chemical industry, on hematite (α-Fe 2 O 3 ) which is often formed on the surface of iron-containing alloys , that are extensively used for the construction of equipment and mechanical structures.…”