High-resolution ellipsometric study of an <i>n</i>-alkane film, dotriacontane, adsorbed on a SiO2 surface

Volkmann, U.G.; Pino, Manuel del; Altamirano, Leopoldo; Taub, H.; Hansen, Flemming Yssing

doi:10.1063/1.1429645

Cited by 55 publications

(80 citation statements)

References 19 publications

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“…Instead, liquid droplets have been observed by optical reflection microscopy at the bulk melting point [6][7][8]. Complete wetting of intermediate-length alkane films a few degrees above the bulk melting point was inferred indirectly by X-ray specular reflectivity [6,9] and stray light intensity measurements [10].…”

Section: Copyright C Epla 2007mentioning

confidence: 99%

Nanoscale observation of delayering in alkane films

et al. 2007

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-Tapping-mode Atomic Force Microscopy and synchrotron X-ray scattering measurements on dotriacontane (n-C32H66 or C32) films adsorbed on SiO2-coated Si(100) wafers reveal a narrow temperature range near the bulk C32 melting point T b in which a monolayer phase of C32 molecules oriented perpendicular to surface is stable. This monolayer phase undergoes a delayering transition to a three-dimensional (3D) fluid phase on heating to just above T b and to a solid 3D phase on cooling below T b . An equilibrium phase diagram provides a useful framework for interpreting the unusual spreading and receding of the monolayer observed in transitions to and from the respective 3D phases.

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Section: Copyright C Epla 2007mentioning

confidence: 99%

Nanoscale observation of delayering in alkane films

et al. 2007

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“…3,24,25 Of these systems, probably the best characterized are monolayers of C32 and C24 on high-quality graphite basal-plane surfaces that have been investigated by scanning tunneling microscopy ͑STM͒, [10][11][12][13][14] x-ray diffraction, 15 and neutron scattering. [16][17][18][19][20][21] Graphite substrates also have the advantage that relatively reliable empirical atom-atom potentials are available for representing the molecule/graphite basal-plane interaction.…”

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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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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“…This must be the case for alkanes with fewer than 16 carbons at the vacuum interface. Interestingly, silica-coated Si (100) surfaces produce surface freezing in which the alkane molecules are oriented parallel to the surface [113], opposite to the order they present at the vacuum interface [1][2][3]. These results stress that surface freezing can be attained by solid surfaces that interact very weakly and strongly with the alkanes.…”

Section: Discussionmentioning

confidence: 53%

“…These results stress that surface freezing can be attained by solid surfaces that interact very weakly and strongly with the alkanes. Likewise, experiments and simulations indicate that crystalline surfaces that interact strongly with alkanes can induce heterogeneous nucleation with the chains ordered parallel to the surface [12,112,113]. This region of ∆G bind < 0 for highly interacting surfaces cannot be accessed with a fluid interface, as we observe that increase in interaction strength results in mixing instead of heterogeneous nucleation.…”

Section: Discussionmentioning

confidence: 67%

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Strength of Alkane–Fluid Attraction Determines the Interfacial Orientation of Liquid Alkanes and Their Crystallization through Heterogeneous or Homogeneous Mechanisms

Qiu

Molinero

2017

Crystals

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Abstract:Alkanes are important building blocks of organics, polymers and biomolecules. The conditions that lead to ordering of alkanes at interfaces, and whether interfacial ordering of the molecules leads to heterogeneous crystal nucleation of alkanes or surface freezing, have not yet been elucidated. Here we use molecular simulations with the united-atom OPLS and PYS alkane models and the mW water model to determine what properties of the surface control the interfacial orientation of alkane molecules, and under which conditions interfacial ordering results in homogeneous or heterogeneous nucleation of alkane crystals, or surface freezing above the melting point. We find that liquid alkanes present a preference towards being perpendicular to the alkane-vapor interface and more parallel to the alkane-water interface. The orientational order in the liquid is short-ranged, decaying over~1 nm of the surface, and can be reversed by tuning the strength of the attractions between alkane and the molecules in the other fluid. We show that the strength of the alkane-fluid interaction also controls the mechanism of crystallization and the face of the alkane crystal exposed to the fluid: fluids that interact weakly with alkanes promote heterogeneous crystallization and result in crystals in which the alkane molecules orient perpendicular to the interface, while crystallization of alkanes in the presence of fluids, such as water, that interact more strongly with alkanes is homogeneous and results in crystals with the molecules oriented parallel to the interface. We conclude that the orientation of the alkanes at the crystal interfaces mirrors that in the liquid, albeit more pronounced and long-ranged. We show that the sign of the binding free energy of the alkane crystal to the surface, ∆G bind , determines whether the crystal nucleation is homogeneous (∆G bind ≥ 0) or heterogeneous (∆G bind < 0). Our analysis indicates that water does not promote heterogeneous crystallization of the alkanes because water stabilizes more the liquid than the crystal phase of the alkane, resulting in ∆G bind > 0. While ∆G bind < 0 suffices to produce heterogeneous nucleation, the condition for surface freezing is more stringent, ∆G bind < −2 γ xl , where γ xl is the surface tension of the liquid-crystal interface of alkanes. Surface freezing of alkanes is favored by their small value of γ xl . Our findings are of relevance to understanding surface freezing in alkanes and to develop strategies for controlling the assembly of chain-like molecules at fluid interfaces.

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High-resolution ellipsometric study of an n-alkane film, dotriacontane, adsorbed on a SiO2 surface

Cited by 55 publications

References 19 publications

Nanoscale observation of delayering in alkane films

Nanoscale observation of delayering in alkane films

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

Strength of Alkane–Fluid Attraction Determines the Interfacial Orientation of Liquid Alkanes and Their Crystallization through Heterogeneous or Homogeneous Mechanisms

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