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Effects of van der Waals Interactions in the Adsorption of Isooctane and Ethanol on Fe(100) Surfaces.

Bedolla PO, Feldbauer G, Wolloch M, Eder SJ, Dörr N, Mohn P, Redinger J, Vernes A - J Phys Chem C Nanomater Interfaces (2014)

Bottom Line: van der Waals (vdW) forces play a fundamental role in the structure and behavior of diverse systems.Nevertheless, they do not influence the spatial configuration of the adsorbed molecules.Their effect on the electronic density is a nonisotropic, delocalized accumulation of charge between the molecule and the slab.

View Article: PubMed Central - PubMed

Affiliation: Institute of Applied Physics, Vienna University of Technology , Wiedner Hauptstraße 8-10/134, 1040 Vienna, Austria ; Austrian Center of Competence for Tribology (AC2T Research GmbH), Viktor-Kaplan-Straße 2, 2700 Wiener Neustadt, Austria.

ABSTRACT
van der Waals (vdW) forces play a fundamental role in the structure and behavior of diverse systems. Because of development of functionals that include nonlocal correlation, it is possible to study the effects of vdW interactions in systems of industrial and tribological interest. Here we simulated within the framework of density functional theory (DFT) the adsorption of isooctane (2,2,4-trimethylpentane) and ethanol on an Fe(100) surface, employing various exchange-correlation functionals to take vdW forces into account. In particular, this paper discusses the effect of vdW forces on the magnitude of adsorption energies, equilibrium geometries, and their role in the binding mechanism. According to our calculations, vdW interactions increase the adsorption energies and reduce the equilibrium distances. Nevertheless, they do not influence the spatial configuration of the adsorbed molecules. Their effect on the electronic density is a nonisotropic, delocalized accumulation of charge between the molecule and the slab. In conclusion, vdW forces are essential for the adsorption of isooctane and ethanol on a bcc Fe(100) surface.

No MeSH data available.


Charge density difference (ρdiff) of isooctaneadsorbed on the bcc Fe(100) surface at d = 2.00 Å.The charge density difference is defined as ρdiff = ρ – (ρisooctane + ρFe(100)) where ρ denotes the charge density of isooctane adsorbedon Fe(100), while ρisooctane and ρFe(100) represent the charge densities of the isolated molecule and theclean Fe(100) surface, respectively. The charge density differenceis plotted in a plane perpendicular to the surface for values between−5 × 10–4 (solid blue, deficit) and5 × 10–4 (solid red, accumulation) electrons/Å3.
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fig4: Charge density difference (ρdiff) of isooctaneadsorbed on the bcc Fe(100) surface at d = 2.00 Å.The charge density difference is defined as ρdiff = ρ – (ρisooctane + ρFe(100)) where ρ denotes the charge density of isooctane adsorbedon Fe(100), while ρisooctane and ρFe(100) represent the charge densities of the isolated molecule and theclean Fe(100) surface, respectively. The charge density differenceis plotted in a plane perpendicular to the surface for values between−5 × 10–4 (solid blue, deficit) and5 × 10–4 (solid red, accumulation) electrons/Å3.

Mentions: As a result of the dispersion forces, the nonlocal correlationbetween electrons induces the change in the adsorption energy. Theequilibrium distance is mainly determined by a balance between thelong-range attractive vdW forces and the short-range Pauli repulsion.When the isooctane molecule approaches the iron slab, the Pauli repulsioncauses a redistribution of the charge density, particularly amongthe d-orbitals of the iron ions. No charge is transferred betweenthe molecule and the iron slab during this process. The overlappingbetween the wave functions of the molecule and the slab accounts forthe movement of the electrons to higher energy states, increasingthe total energy of the system. This effect is weaker when a properdescription of nonlocal interactions is considered because the nonlocalcorrelations reduce the electron–electron repulsion (Figure 4). This allows the isooctanemolecule to reach a shorter equilibrium distance, where the magnitudeof the attractive forces is larger and, consequently, the bindingenergy increases. The calculated equilibrium distance is also affectedby the choice of the vdW density functional, since the Pauli repulsiongives rise to exchange interactions and these functionals differ inthe description of the exchange energy. For instance, the differencebetween the binding distance calculated with the optB86b-vdW functionaland the one calculated with the vdW-DF is 0.50 Å.


Effects of van der Waals Interactions in the Adsorption of Isooctane and Ethanol on Fe(100) Surfaces.

Bedolla PO, Feldbauer G, Wolloch M, Eder SJ, Dörr N, Mohn P, Redinger J, Vernes A - J Phys Chem C Nanomater Interfaces (2014)

Charge density difference (ρdiff) of isooctaneadsorbed on the bcc Fe(100) surface at d = 2.00 Å.The charge density difference is defined as ρdiff = ρ – (ρisooctane + ρFe(100)) where ρ denotes the charge density of isooctane adsorbedon Fe(100), while ρisooctane and ρFe(100) represent the charge densities of the isolated molecule and theclean Fe(100) surface, respectively. The charge density differenceis plotted in a plane perpendicular to the surface for values between−5 × 10–4 (solid blue, deficit) and5 × 10–4 (solid red, accumulation) electrons/Å3.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4126734&req=5

fig4: Charge density difference (ρdiff) of isooctaneadsorbed on the bcc Fe(100) surface at d = 2.00 Å.The charge density difference is defined as ρdiff = ρ – (ρisooctane + ρFe(100)) where ρ denotes the charge density of isooctane adsorbedon Fe(100), while ρisooctane and ρFe(100) represent the charge densities of the isolated molecule and theclean Fe(100) surface, respectively. The charge density differenceis plotted in a plane perpendicular to the surface for values between−5 × 10–4 (solid blue, deficit) and5 × 10–4 (solid red, accumulation) electrons/Å3.
Mentions: As a result of the dispersion forces, the nonlocal correlationbetween electrons induces the change in the adsorption energy. Theequilibrium distance is mainly determined by a balance between thelong-range attractive vdW forces and the short-range Pauli repulsion.When the isooctane molecule approaches the iron slab, the Pauli repulsioncauses a redistribution of the charge density, particularly amongthe d-orbitals of the iron ions. No charge is transferred betweenthe molecule and the iron slab during this process. The overlappingbetween the wave functions of the molecule and the slab accounts forthe movement of the electrons to higher energy states, increasingthe total energy of the system. This effect is weaker when a properdescription of nonlocal interactions is considered because the nonlocalcorrelations reduce the electron–electron repulsion (Figure 4). This allows the isooctanemolecule to reach a shorter equilibrium distance, where the magnitudeof the attractive forces is larger and, consequently, the bindingenergy increases. The calculated equilibrium distance is also affectedby the choice of the vdW density functional, since the Pauli repulsiongives rise to exchange interactions and these functionals differ inthe description of the exchange energy. For instance, the differencebetween the binding distance calculated with the optB86b-vdW functionaland the one calculated with the vdW-DF is 0.50 Å.

Bottom Line: van der Waals (vdW) forces play a fundamental role in the structure and behavior of diverse systems.Nevertheless, they do not influence the spatial configuration of the adsorbed molecules.Their effect on the electronic density is a nonisotropic, delocalized accumulation of charge between the molecule and the slab.

View Article: PubMed Central - PubMed

Affiliation: Institute of Applied Physics, Vienna University of Technology , Wiedner Hauptstraße 8-10/134, 1040 Vienna, Austria ; Austrian Center of Competence for Tribology (AC2T Research GmbH), Viktor-Kaplan-Straße 2, 2700 Wiener Neustadt, Austria.

ABSTRACT
van der Waals (vdW) forces play a fundamental role in the structure and behavior of diverse systems. Because of development of functionals that include nonlocal correlation, it is possible to study the effects of vdW interactions in systems of industrial and tribological interest. Here we simulated within the framework of density functional theory (DFT) the adsorption of isooctane (2,2,4-trimethylpentane) and ethanol on an Fe(100) surface, employing various exchange-correlation functionals to take vdW forces into account. In particular, this paper discusses the effect of vdW forces on the magnitude of adsorption energies, equilibrium geometries, and their role in the binding mechanism. According to our calculations, vdW interactions increase the adsorption energies and reduce the equilibrium distances. Nevertheless, they do not influence the spatial configuration of the adsorbed molecules. Their effect on the electronic density is a nonisotropic, delocalized accumulation of charge between the molecule and the slab. In conclusion, vdW forces are essential for the adsorption of isooctane and ethanol on a bcc Fe(100) surface.

No MeSH data available.