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

doi:10.1063/1.3212095We present evidence from neutron diffraction measurements and molecular dynamics (MD) simulations of three different monolayer phases of the intermediate-length alkanes tetracosane (n-C24H50 denoted as C24) and dotriacontane (n-C32H66 denoted as C32) 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 (RC) 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 [110] direction such that the distance between molecular rows in a lamella is 4.26?� = mathag, where ag = 2.46?� 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 ~10% 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.This work was supported by U.S. National Science Foundation under Grant Nos. DMR-0411748 and DMR-0705974

Neutron diffraction and quasielastic neutron scattering studies of films of N-alkanes and a branched alkane adsorbed on graphite

The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file.Title from title screen of research.pdf file (viewed on March 11, 2008)Thesis (Ph. D.) University of Missouri-Columbia 2007.[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Neutron diffraction and quasielastic neutron scattering have been used to study the structure and dynamics of films of n-alkanes and of a branched alkane adsorbed on graphite. The structures of the intermediate-length alkane n-tetracosane and of the branched alkane squalane adsorbed on graphite were studied below room temperature using neutron diffraction. Comparison of the n-tetracosane and the squalane monolayers reveals that both undergo a phase transition from a more-ordered crystalline phase to a less-ordered "smectic" phase at low temperature, [approximately]210 K for n-tetracosane (n-C[subscript 24]D[subscript 50]) and [approximately]215 K for squalane (C[subscript 30]H[subscript 62]). However, the squalane monolayer is less ordered translationally than the n-tetracosane monolayer at every temperature studied as indicated by the smaller coherence length Lb of the squalane film. This is presumably due to the side branches along the squalane molecules that prevented them from translationally ordering translationally as well as the linear tetracosane molecules. Quasielastic neutron scattering studies of monolayer, bilayer, and trilayer nheptane (n-C[subscript 7]D[subscript 16]) above a crystalline n-dotriacontane monolayer adsorbed on graphite investigated the dynamics within these heptane layers. These studies found that the heptane layers exhibited both a slow motion dominating at low wave vector transfer Q and a faster motion dominating at higher wave vector transfer Q. The slow motion within the heptane layers at all coverages is similar to that measured for the n-dotriacontane monolayer alone. The faster motion measured for all coverages of heptane was comparable to the motions measured for bulk heptane. Two coverages of n-tetracosane monolayers, 1.00 and 1.16 layers based on the lattice parameters of n-tetracosane in its "smectic" phase, were studied by high-energy resolution quasielastic neutron scattering. Uniaxial rotational motion of the nearly alltrans n-tetracosane molecule begins in the monolayer crystalline phase at [approximate sign]160 K, independent of coverage. Uniaxial rotational motion continues up to a temperature of [approximately]230 K for the low-coverage sample and up to a temperature of [approximately]260 K for the highcoverage sample, as indicated by the Q-independence of the quasielastic scattering. At [approximately]260 K and above, the width of the quasielastic scattering of the low-coverage n-tetracosane sample begins to increase with increasing Q, indicating a more complex combination of translational and rotational diffusive motion in the "smectic" phase of the sample. The high-coverage sample continues to exhibit uniaxial rotational motion. The decrease in the quasielastic intensity and molecular dynamics (MD) simulations indicate that many of these motions become too fast to lie within the instrumental energy window above 285 K. However, the observed quasielastic intensity increases again at 330 K. MD simulations indicate that intramolecular and rotational diffusion of more globularly shaped C24 molecules, which contain many gauche defects, is being observed.Includes bibliographical reference

Slow Diffusive Motions in a Monolayer of Tetracosane Molecules Adsorbed on Graphite

http://link.aip.org/link/?APCPCS/708/201/1Monolayers of intermediate-length alkane molecules such as tetracosane (n-C24H50 or C24) serve as prototypes for studying the interfacial dynamics of more complex polymers, including bilayer lipid membranes. Using high-resolution quasielastic neutron scattering (QNS) and exfoliated graphite substrates, we have investigated the relatively slow diffusive motion in C24 monolayers on an energy/time scale of ~1-36 µeV (~0.1-4 ns). Upon heating, we first observe QNS in the crystalline phase at ~160 K. From the crystalline-to-smectic phase transition at ~215 K to a temperature of ~230 K, we observe the QNS energy width to be dispersionless, consistent with molecular dynamics simulations showing rotational motion of the molecules about their long axis. At 260 K, the QNS energy width begins to increase with wave vector transfer, suggesting onset of nonuniaxial rotational motion and bounded translational motion. We continue to observe QNS up to the monolayer melting temperature at ~340 K where our simulations indicate that the only motion slow enough to be visible within our energy window results from the creation of gauche defects in the moleculesThis work was supported by the National Science Foundation (NSF) under Grant Nos. DMR-9802476 and DMR-0109057, by the Chilean government under FONDECYT Grant No. 1010548, and by the U. S. Department of Energy through grant DE-FG02-01ER45912. The neutron scattering facilities used in this work are supported in part by the NSF under Agreement No. DMR-0086210