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43277-B5
Better Understanding the Behavior of Hexane Deposited onto Graphite Using Improved Computer Simulations

Michael W. Roth, University of Northern Iowa

I. General Comments

We met the proposed objectives regarding hexane on graphite (C6/gr) as well as tetracosane on graphite (C24/gr) . Two papers have been published, and several other studies are in progress which will lead to dissemination which credit PRF. The work done has been disseminated in 5 conference presentations with student co – authors.

In addition, the funding over the last three yeas has allowed the PI to continue collaborating with colleagues at UMC and at Penn State, to build collaborations with scientists from Europe and, most importantly, to leverage the experience to benefit local UNI undergraduate students. There will be publications and presentations crediting this source of funding over the next year, and the project will act as a seed for future work and proposals.

II. Hexane on Graphite

A published paper is reported in the appropriate electronic location.

III. Tetracosane on Graphite

A paper in press is reported in the appropriate electronic location.

In studying C24/gr, adjustment of the nonbonded scaling parameter in NAMD simulations is crucial in determining alkane flexibility and hence phase transition temperatures and dynamics. The interplay between torsional potential and the scaling of nonbonded interactions plays a crucial role in all – atom NAMD simulations. Three alkanes (C₆, C₁₂, C₂₄), are simulated and the melting temperatures compared to accepted values. Thee scaling parameter is not universal but changes with the molecular length. In fact we are able to plot the optimal scaling factor as a function of backbone length, as shown in Fig. 1. There is also a manuscript in preparation:

L. Firlej, B. Kuchta, M.W. Roth, and C. Wexler, “Mechanism of the melting transition in monolayers of alkanes adsorbed on graphite and the universality of energy scaling”, in preparation

Figure 1. Optimal scaling factor (SF) as a function of the inverse number of the carbons in the alkane backbone.

V. Alkane Percolation

A current percolation study involves an extensive series of simulations of alkanes at submonolayer coverages. Melting is directly related to lattice topology, and we concentrate on where the adsorbate forms a connected (percolating) network. In this regime, melting temperature is insensitive to density changes. We study four different species (C5, C6, C12 and C24) with the intent of relating molecular flexibility to the system’s behavior at percolation. We have completed the study for rigid molecules (C5; C6, Fig. 2) and have moved on to more flexible species (C12; C24).

Figure2. Low – temperature snapshots for pentane (top row) and hexane (bottom row) layers on graphite ant various coverages.

We continue to collaborate with Renee Diehl at Penn State regarding fullerenes on graphite at various coverages. Our fundamental results are not altered from those discussed last time but the runs are being taken out much longer in order to ensure good statistics.

V. Tetracosane Bilayers

The PI and collaborators from Missouri and Europe are currently studying C24/gr bilayer systems. The molecules in the second layer show much more thermal agitation (gauche defects and tumbling) those in the lower layer. The dynamics are very slow and the primary challenge is to run the simulations out for ca. 10 ns in order to get good statistics. We expect the results to be completed and published over the next year.  

Figure3. Final snapshot side view of a C24 bilayer. Note that the second layer shows considerably more gauche defects and signs of thermal fluctuations than does the lower layer.

VI. Tetracosane Trilayers

The PI and the SUMR Scholar also simulated C24 trilayers this summer, with the third layer standing perpendicular to the substrate and first two parallel layers. In addition, the upper layer shows interesting concavity on it sides as temperature is increased up to thermal collapse. The Scholar had some health issues this past summer and the PI wants to involve him in as much of the science as possible so the project will be completed and published over the next year. We are currently running out our simulations for much longer times in order to get better statistics.

VII. Hydrogen  The PI conducted some NAMD hydrogen on graphite simulations and the results are published; the paper is reported in the appropriate electronic location.

VIII. Natural Gas / Crude Oil

The PI and SUMR Scholar began simulating natural gas and crude oil on graphite – the first such work to our knowledge. The crude oil is a mixture of alkanes from C6 on up to C30, and natural gas contains C1 – C5. We see selective adsorbtion and interesting adsorbed molecule orientation. We are currently running simulations out for a much longer time in order to get better statistics.

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