Haskell Taub

Phase Transitions in Surface Films 2

Monolayers.- Phase Transitions of Monolayer Films Adsorbed on Graphite.- Computer Simulations of Two-Dimensional Systems.- Modulated Structures of Adsorbed Rare Gas Monolayers.- Signatures and Consequences of the Substrate Corrugation.- Epitaxial Rotation and Rotational Phase Transitions.- Structures and Phase Transitions in Alkali Metal Overlayers on Transition Metal Surfaces.- Original Properties of Thin Adsorbed Films on an Ionic Surface of Square Symmetry and High Surface Homogeneity: MgO(100).- Neutron Scattering Studies of Quantum Films.- A Molecular Dynamics Study of the Effect of Steric Properties on the Melting of Quasi Two-Dimensional Systems.- Probing Film Phase Transitions Through Measurements of Sliding Friction.- Phase Transitions in Lipid Monolayers on Water: New Light on an Old Problem.- Surface Reconstruction, Roughening, and Melting.- Phenomenology of Surface Reconstruction.- The Roughening Transition on Surfaces.- Interplay Between Surface Roughening, Preroughening, and Reconstruction.- Thermal Dynamics of Nonvicinal Metal Surfaces.- Surface-induced Melting of Solids.- Surface Melting and Diffusion.- On Surface Melting.- Frost Heave and the Surface Melting of Ice.- Wetting Phenomena, Multilayer Structures, and Film Growth.- Multilayer Physisorbed Films on Graphite.- Fluid Interfaces: Wetting, Critical Adsorption, van der Waals Tails, and the Concept of the Effective Interface Potential.- Adsorption Studies with a Graphite Fiber Microbalance.- The Methane/Graphite Phase Diagram.- Kinetics of Overlayer Growth.- Summary.- Conference Summary Where From? Where To?.- Participants.

Quasielastic neutron scattering and molecular dynamics simulation studies of the melting transition in butane and hexane monolayers adsorbed on graphite

~Received 14 May 1997; accepted 24 June 1997! Quasielastic neutron scattering experiments and molecular dynamics ~MD! simulations have been used to investigate molecular diffusive motion near the melting transition of monolayers of flexible rod-shaped molecules. The experiments were conducted on butane and hexane monolayers adsorbed on an exfoliated graphite substrate. For butane, quasielastic scattering broader than the experimental energy resolution width of 70 meV appears abruptly at the monolayer melting point of Tm 5116 K, whereas, for the hexane monolayer, it appears 20 K below the melting transition ( Tm 5170 K). To facilitate comparison with experiment, quasielastic spectra calculated from the MD simulations were analyzed using the same models and fitting algorithms as for the neutron spectra. This combination of techniques gives a microscopic picture of the melting process in these two monolayers which is consistent with earlier neutron diffraction experiments. Butane melts abruptly to a liquid phase where the molecules in the trans conformation translationally diffuse while rotating about their center of mass. In the case of the hexane monolayer, the MD simulations show that the appearance of quasielastic scattering below Tm coincides with transformation of some molecules from trans to gauche conformations. Furthermore, if gauche molecules are prevented from forming in the simulation, the calculated incoherent scattering function contains no quasielastic component below Tm . Modeling of both the neutron and simulated hexane monolayer spectra below Tm favors a plastic phase in which there is nearly isotropic rotational diffusion of the gauche molecules about their center of mass, but no translational diffusion. The elastic scattering observed above Tm is consistent with the coexistence of solid monolayer clusters with a fluid phase, as predicted by the simulations. For T/Tm>1.3, the elastic scattering vanishes from the neutron spectra where the simulation indicates the presence of a fluid phase alone. The qualitative similarities between the observed and simulated quasielastic spectra lend support to a previously proposed ‘‘footprint reduction’’ mechanism of melting in monolayers of flexible, rod-shaped molecules.

Fabry-Pérot cavity sensors for multipoint on-column micro gas chromatography detection

We developed and characterized a Fabry-Pérot (FP) sensor module based micro gas chromatography (microGC) detector for multipoint on-column detection. The FP sensor was fabricated by depositing a thin layer of metal and a layer of gas-sensitive polymer consecutively on the endface of an optical fiber, which formed the FP cavity. Light partially reflected from the metal layer and the polymer-air interface generated an interference spectrum, which shifted as the polymer layer absorbed the gas analyte. The FP sensor module was then assembled by inserting the FP sensor into a hole drilled in the wall of a fused-silica capillary, which can be easily connected to the conventional gas chromatography (GC) column through a universal quick seal column connector, thus enabling on-column real-time detection. We characterized the FP sensor module based microGC detector. Sensitive detection of various gas analytes was achieved with subnanogram detection limits. The rapid separation capability of the FP sensor module assembled with both single- and tandem-column systems was demonstrated, in which gas analytes having a wide range of polarities and volatilities were well-resolved. The tandem-column system obtained increased sensitivity and selectivity by employing two FP sensor modules coated with different polymers, showing great system versatility.

A LEED and neutron diffraction study of hexane adsorbed on graphite in the monolayer range: uniaxial commensurate-incommensurate transition

Abstract The structure of a n-hexane molecule (C6H14) adsorbed on graphite has been studied by LEED and neutron diffraction in the range 100 to 177 K. Below 151 K the monolayer has an ordered herringbone structure. The rectangular unit mesh is commensurate 4 × √3 in only one direction. Along the other direction, the lattice constant is 5.3 ± 0.05 A . With increasing coverage, a continuous uniaxial incommensurate to commensurate transition to a 2 × 4√3 phase occurs. At 151 K the ordered structure is replaced by a fluid-like phase, as indicated by a diffuse ring in the LEED pattern. This melting transition appears to be first order. The isosteric heat at monolayer condensation is measured to be 16 kcal/mol via LEED graphite spot intensity isotherms from 160 to 177 K.

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

Using high-resolution ellipsometry and stray light intensity measurements, we have investigated during successive heating-cooling cycles the optical thickness and surface roughness of thin dotriacontane (n-C32H66) films adsorbed from a heptane (n-C7H16) solution onto SiO2-coated Si(100) single-crystal substrates. Our results suggest a model of a solid dotriacontane film that has a phase closest to the SiO2 surface in which the long-axis of the molecules is oriented parallel to the interface. Above this “parallel film” phase, a solid monolayer adsorbs in which the molecules are oriented perpendicular to the interface. At still higher coverages and at temperatures below the bulk melting point at Tb=341 K, solid bulk particles coexist on top of the “perpendicular film.” For higher temperatures in the range Tb Ts, a uniformly thick fluid film wets to the parallel film phase. This struc...

Molecular dynamics studies of the melting of butane and hexane monolayers adsorbed on the basal‐plane surface of graphite

The effect of molecular steric properties on the melting of quasi‐two‐dimensional solids is investigated by comparing results of molecular dynamics simulations of the melting of butane and hexane monolayers adsorbed on the basal‐plane surface of graphite. These molecules differ only in their length, being members of the n‐alkane series [CH3(CH2)n−2CH3] where n=4 for butane and n=6 for hexane. The simulations employ a skeletal model, which does not include the hydrogen atoms explicitly, to represent the intermolecular and molecule–substrate interactions. Nearest‐neighbor intramolecular bonds are fixed in length, but the molecular flexibility is preserved by allowing the bend and dihedral torsion angles to vary. The simulations show a qualitatively different melting behavior for the butane and hexane monolayers consistent with neutron and x‐ray scattering experiments. The melting of the low‐temperature herringbone (HB) phase of the butane monolayer is abrupt and characterized by a simultaneous breakdown of ...

Determination of orientational order in submonolayer butane films adsorbed on graphite by elastic neutron diffraction

Abstract Elastic neutron diffraction has been used to determine the molecular orientation and unit cell of a butane submonolayer adsorbed on exfoliated graphite. The molecular orientation provides a sensitive test of empirical potentials being applied to the calculation of the structure and dynamics of hydrocarbons adsorbed on graphite.

A novel growth mode of alkane films on a SiO2 surface

Abstract Synchrotron X-ray specular scattering measurements confirm microscopically a structural model recently inferred by very-high-resolution ellipsometry of a solid dotriacontane ( n -C 32 H 66 or C32) film formed by adsorption from solution onto a SiO 2 surface. Sequentially, one or two layers adsorb on the SiO 2 surface with the long-axis of the C32 molecules oriented parallel to the interface followed by a C32 monolayer with the long-axis perpendicular to it. Finally, preferentially oriented bulk particles nucleate having two different crystal structures. This growth model differs from that found previously for shorter alkanes deposited from the vapor phase onto solid surfaces.

Diffusion of water and selected atoms in DMPC lipid bilayer membranes

Molecular dynamics simulations have been used to determine the diffusion of water molecules as a function of their position in a fully hydrated freestanding 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) bilayer membrane at 303 K and 1 atm. The diffusion rate of water in a ∼10 Å thick layer just outside the membrane surface is reduced on average by a factor of ∼2 relative to bulk. For water molecules penetrating deeper into the membrane, there is an increasing reduction in the average diffusion rate with up to one order of magnitude decrease for those deepest in the membrane. A comparison with the diffusion rate of selected atoms in the lipid molecules shows that ∼6 water molecules per lipid molecule move on the same time scale as the lipids and may therefore be considered to be tightly bound to them. The quasielastic neutron scattering functions for water and selected atoms in the lipid molecule have been simulated and compared to observed quasielastic neutron scattering spectra from single-supported bilayer DMPC membranes.

Atomic force microscopy measurements of topography and friction on dotriacontane films adsorbed on a SiO2 surface.

We report comprehensive atomic force microscopy (AFM) measurements at room temperature of the nanoscale topography and lateral friction on the surface of thin solid films of an intermediate-length normal alkane, dotriacontane (n-C32H66), adsorbed onto a SiO2 surface. Our topographic and frictional images, recorded simultaneously in the contact mode, reveal a multilayer structure in which one to two layers of molecules adsorb adjacent to the SiO2 surface oriented with their long axis parallel to the interface followed by partial layers of molecules oriented perpendicular to the surface. The thicknesses of the parallel and perpendicular layers that we measured with the AFM agree with those inferred from previous x-ray specular reflectivity measurements on similarly prepared samples. We also observe bulk dotriacontane particles and, in contrast with our previous measurements, are able to determine their location. Above a minimum size, the bulk particles are separated from islands of perpendicularly oriented molecules by regions of exposed parallel layers that most likely extend underneath the particles. We find that the lateral friction is sensitive to the molecular orientation in the underlying crystalline film and can be used effectively with topographic measurements to resolve uncertainties in the film structure. We measure the same lateral friction on top of the bulk particles as on the perpendicular layers, a value that is about 2.5 times smaller than on a parallel layer. Scans on top of parallel layers indicate a constant height but reveal domains having different sublevels of friction. We explain this by the domains having different azimuthal orientations of the molecules.