Accelerated molecular dynamics simulation of the thermal desorption of n -alkanes from the basal plane of graphite

Kelly E. Becker, Kristen Ann Fichthorn

Research output: Contribution to journalArticle

40 Citations (Scopus)

Abstract

We utilize accelerated molecular dynamics to simulate alkane desorption from the basal plane of graphite. Eight different molecules, ranging from n -pentane to n -hexadecane, are studied in the low coverage limit. Acceleration of the molecular dynamics simulations is achieved using two different methods: temperature acceleration and a compensating potential scheme. We find that the activation energy for desorption increases with increasing chain length. The desorption prefactor increases with chain length for molecules ranging from pentane to decane. This increase subsides and the value of the preexponential factor fluctuates about an apparently constant value for decane, dodecane, tetradecane, and hexadecane. These trends are consistent with data obtained in experimental temperature-programed desorption (TPD) studies. We explain the dependence of the preexponential factor on alkane chain length by examining conformational changes within the alkane molecules. For the shorter molecules, torsional motion is not activated over experimental temperature ranges. These molecules can be treated as rigid rods and their partial loss in translational and rotational entropies upon adsorption increases as chain length increases, leading to an increasing preexponential factor. At their typical TPD peak temperatures, torsions are activated in the longer adsorbed chain molecules to a significant extent which increases with increasing chain length, increasing the entropy of the adsorbed molecule. This increase counteracts the decrease in entropy due to a loss of translation and rotation, leading to a virtually constant prefactor.

Original languageEnglish (US)
Article number184706
JournalJournal of Chemical Physics
Volume125
Issue number18
DOIs
StatePublished - Nov 20 2006

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Thermal desorption
Alkanes
Graphite
alkanes
Molecular dynamics
graphite
desorption
Chain length
molecular dynamics
Molecules
Desorption
Computer simulation
molecules
simulation
Entropy
pentanes
entropy
Temperature
temperature
Torsional stress

All Science Journal Classification (ASJC) codes

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

Cite this

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abstract = "We utilize accelerated molecular dynamics to simulate alkane desorption from the basal plane of graphite. Eight different molecules, ranging from n -pentane to n -hexadecane, are studied in the low coverage limit. Acceleration of the molecular dynamics simulations is achieved using two different methods: temperature acceleration and a compensating potential scheme. We find that the activation energy for desorption increases with increasing chain length. The desorption prefactor increases with chain length for molecules ranging from pentane to decane. This increase subsides and the value of the preexponential factor fluctuates about an apparently constant value for decane, dodecane, tetradecane, and hexadecane. These trends are consistent with data obtained in experimental temperature-programed desorption (TPD) studies. We explain the dependence of the preexponential factor on alkane chain length by examining conformational changes within the alkane molecules. For the shorter molecules, torsional motion is not activated over experimental temperature ranges. These molecules can be treated as rigid rods and their partial loss in translational and rotational entropies upon adsorption increases as chain length increases, leading to an increasing preexponential factor. At their typical TPD peak temperatures, torsions are activated in the longer adsorbed chain molecules to a significant extent which increases with increasing chain length, increasing the entropy of the adsorbed molecule. This increase counteracts the decrease in entropy due to a loss of translation and rotation, leading to a virtually constant prefactor.",
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Accelerated molecular dynamics simulation of the thermal desorption of n -alkanes from the basal plane of graphite. / Becker, Kelly E.; Fichthorn, Kristen Ann.

In: Journal of Chemical Physics, Vol. 125, No. 18, 184706, 20.11.2006.

Research output: Contribution to journalArticle

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