The crystalline structures of the odd alkanes pentane, heptane, nonane, undecane, tridecane and pentadecane monolayers adsorbed on graphite at submonolayer coverages and from the liquidElectronic supplementary information (ESI) available: Fractional coordinates of single repeat units of some alkanes at sub-monolayer coverage and of the monolayer coexisting with the liquid. See http://www.suppdata/cp/b2/b201988b/

Abstract

A combination of neutron and X-ray diffraction has been used to structurally characterise the crystalline monolayer structures of all the alkanes with odd number of carbon atoms in their alkyl chains from pentane to pentadecane adsorbed on graphite. The structures of all the molecules investigated at submonolayer coverages are isomorphous with centred rectangular unit cells containing two molecules per cell in a parallel arrangement. This is a completely different structure from the ' herringbone ' arrangement of molecules found for the shorter ' even ' alkanes, such as hexane, octane and decane. The monolayers at sub-monolayer coverages are interpreted as uniaxial commensurate with the underlying graphite while those monolayers coexisting with the liquid, while structurally similar, are fully commensurate. The difference between the two structures is a uniaxial compression in the b-direction with the monolayers coexisting with the liquids significantly more dense than at submonolayer coverages. In the low coverage structures the ' odd ' molecules have an all trans conformation with their extended axes parallel to the surface with the plane of the carbon skeleton also parallel to the graphite surface. At high coverages the carbon skeleton is no longer parallel to the graphite surface but significantly tilted. The longest alkanes, tridecane and pentadecane also show evidence of positional and/or rotational disorder at high coverages.

Different melting behavior in pentane and heptane monolayers on graphite: Molecular dynamics simulations

“…In each panel, the corresponding symbol is: for n=2 (black squares), n=4 (blue triangles), n=6 (green diamonds), and n=8 (red circles). The monolayers with cell parameters given by Matthies 6 are on the top two panels, and those with cell parameters given by Arnold et al 10 (for a fully commensurate monolayer) are in the bottom two panels.…”

“…Note that a (*) refers to a fully commensurate monolayer, in which cell parameters are taken from experimental work Table II. Melting transition temperatures (T m ) with respective uncertainty for each monolayer studied (including nonane) compared to experimental observations of melting temperatures 6,10 in each monolayer (if this is studied). Fully commensurate monolayers (with unit cell parameters) are indicated with an asterisk (* Figure 1.…”

“…From this data, the model chosen for both C5 and C7 monolayers is presented in table I, as well as the cell parameters utilized from [10] for C9 that involve a fully commensurate RC phase. proposes that one possible scenario for the solid crystalline phase observed involves a slight HB rotation, and after careful consideration of submonolayer cells proposed in [10] that are slightly extended in the b-axis direction of the unit cell (which could indicate a HB rotation), this work adopts a slight HB rotation for C5 and C7 monolayers that is reported in table I as well.…”

“…Unlike the short-chained even n-alkanes, the odd alkanes [5][6][7][8][9][10] show significantly different lowtemperature solid phase behavior. Where the even alkanes form a low-temperature HB phase, the odd alkanes form a low temperature rectangular-centered (RC) phase that is at least partially commensurate with the graphite substrate.…”

“…Due to its attractive properties and its popularity in previous experimental work, graphite is utilized for the substrate in this study. The importance of physisorbed n-alkanes on graphite is emphasized by the vast experimental effort in recent years [3][4][5][6][7][8][9][10] to study the solid and liquid-state behavior in such systems. These studies have found that for even-numbered short-chained n-alkanes [3][4][5] , with n<12, the monolayer arranges in a lowtemperature herringbone (HB) phase that is commensurate with the substrate (except for butane, which is not found to exhibit a HB phase).…”