Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. II. Dynamics

Enevoldsen, Ann Dorrit; Hansen, Flemming Yssing; Diama, A.; Taub, H.; Dimeo, R. M.; Neumann, D. A.; Copley, J. R. D.

doi:10.1063/1.2464092

Cited by 17 publications

(23 citation statements)

References 13 publications

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“…3,38 Quasielastic neutron scattering measurements and MD simulations are consistent with both of these types of molecular diffusive motion occurring in the C24 monolayer smectic phase along with motions associated with conformational changes in the molecules. 4 The neutron diffraction and MD results that we have presented here indicate that these diffusive motions occur within a 2D lamellar structure characterized by a shorter coherence length than the low-temperature crystalline phase.…”

Section: Discussionmentioning

confidence: 54%

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Structure and phase transitions of monolayers of intermediate-length n-alkanes on graphite studied by neutron diffraction and molecular dynamics simulation

Diama

Matthies

Herwig

et al. 2009

The Journal of Chemical Physics

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We present evidence from neutron diffraction measurements and molecular dynamics ͑MD͒ simulations of three different monolayer phases of the intermediate-length alkanes tetracosane ͑n-C 24 H 50 denoted as C24͒ and dotriacontane ͑n-C 32 H 66 denoted as C32͒ adsorbed on a graphite basal-plane surface. Our measurements indicate that the two monolayer films differ principally in the transition temperatures between phases. At the lowest temperatures, both C24 and C32 form a crystalline monolayer phase with a rectangular-centered ͑RC͒ structure. The two sublattices of the RC structure each consists of parallel rows of molecules in their all-trans conformation aligned with their long axis parallel to the surface and forming so-called lamellas of width approximately equal to the all-trans length of the molecule. The RC structure is uniaxially commensurate with the graphite surface in its ͓110͔ direction such that the distance between molecular rows in a lamella is 4.26 Å = ͱ 3a g , where a g = 2.46 Å is the lattice constant of the graphite basal plane. Molecules in adjacent rows of a lamella alternate in orientation between the carbon skeletal plane being parallel and perpendicular to the graphite surface. Upon heating, the crystalline monolayers transform to a "smectic" phase in which the inter-row spacing within a lamella expands by ϳ10% and the molecules are predominantly oriented with the carbon skeletal plane parallel to the graphite surface. In the smectic phase, the MD simulations show evidence of broadening of the lamella boundaries as a result of molecules diffusing parallel to their long axis. At still higher temperatures, they indicate that the introduction of gauche defects into the alkane chains drives a melting transition to a monolayer fluid phase as reported previously.

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Section: Discussionmentioning

confidence: 54%

“…2 It may also help explain whether or not complete wetting of the solid film occurs at a higher coverage. 3 Knowledge of the monolayer structure is also essential for understanding its dynamical properties, including both vibratory 4 and diffusive 5,6 motion.…”

Section: Introductionmentioning

confidence: 99%

Structure and phase transitions of monolayers of intermediate-length n-alkanes on graphite studied by neutron diffraction and molecular dynamics simulation

Diama

Matthies

Herwig

et al. 2009

The Journal of Chemical Physics

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“…The hexadecane monolayer on graphite melts at 55 C [25]. Thus, the state of the confined material, not the branching, is the key condition dictating the squeeze out behavior and dynamics [26]. The degree of branching and temperature influence the specific state of the monolayer.…”

Section: Prl 100 076101 (2008) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

Squeeze-Out of Branched Alkanes on Graphite

2008

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We study squalane and heptamethylnonane (HMN) confined between a conducting atomic force microscope tip and a graphite surface. Solvation layering occurs for both liquids but marked differences in the squeeze out mechanics are observed for ordered or disordered monolayers. The squalane monolayer at 25 C is an ordered solid, as verified by direct imaging, and the squeeze out can be modeled using elastic continuum mechanics. HMN is in a disordered state at 25 C and cannot be modeled as a single elastic asperity even in solid-solid contact because HMN liquid is trapped in the contact zone.

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“…͑c͒ Are the melting mechanisms the same for the two films? In the following paper, 11 we use quasielastic neutron scattering and MD simulations to study the dynamics of such layers.…”

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confidence: 99%

Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. I. Structure

Enevoldsen

Hansen

Diama

et al. 2007

The Journal of Chemical Physics

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The structure of a monolayer film of the branched alkane squalane ͑C 30 H 62 ͒ adsorbed on graphite has been studied by neutron diffraction and molecular dynamics ͑MD͒ simulations and compared with a similar study of the n-alkane tetracosane ͑n-C 24 H 52 ͒. Both molecules have 24 carbon atoms along their backbone and squalane has, in addition, six methyl side groups. Upon adsorption, there are significant differences as well as similarities in the behavior of these molecular films. Both molecules form ordered structures at low temperatures; however, while the melting point of the two-dimensional ͑2D͒ tetracosane film is roughly the same as the bulk melting point, the surface strongly stabilizes the 2D squalane film such that its melting point is 91 K above its value in bulk. Therefore, squalane, like tetracosane, will be a poor lubricant in those nanoscale devices that require a fluid lubricant at room temperature. The neutron diffraction data show that the translational order in the squalane monolayer is significantly less than in the tetracosane monolayer. The authors' MD simulations suggest that this is caused by a distortion of the squalane molecules upon adsorption on the graphite surface. When the molecules are allowed to relax on the surface, they distort such that all six methyl groups point away from the surface. This results in a reduction in the monolayer's translational order characterized by a decrease in its coherence length and hence a broadening of the diffraction peaks. The MD simulations also show that the melting mechanism in the squalane monolayer is the same footprint reduction mechanism found in the tetracosane monolayer, where a chain melting drives the lattice melting.

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Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. II. Dynamics

Cited by 17 publications

References 13 publications

Structure and phase transitions of monolayers of intermediate-length n-alkanes on graphite studied by neutron diffraction and molecular dynamics simulation

Structure and phase transitions of monolayers of intermediate-length n-alkanes on graphite studied by neutron diffraction and molecular dynamics simulation

Squeeze-Out of Branched Alkanes on Graphite

Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. I. Structure

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