Stuart M. Clarke

The crystalline structures of the even alkanes hexane, octane, decane, dodecane and tetradecane monolayers adsorbed on graphite at submonolayer coverages and from the liquid

Neutron and X-ray diffraction have been used to structurally characterise the crystalline monolayer structures of hexane, octane, decane, dodecane and tetradecane adsorbed on graphite at sub-monolayer coverages and when coexisting with liquid alkane. The structures of all the molecules investigated at both coverages and low temperatures are isomorphous with rectangular unit cells of plane group pgg containing two molecules per cell. In both high- and low-coverage structures the molecules have their extended axes parallel to the surface. The plane of the carbon skeleton is found to be parallel to the graphite surface. The monolayers at sub-monolayer coverages are interpreted as uniaxially commensurate while those monolayers coexisting with the liquid are fully commensurate. Dodecane and tetradecane are exceptional: dodecane forms additional phases at high temperatures just prior to melting, at both low and high coverages. In these structures the molecules in the unit cell are parallel to each other with plane group cmm. Tetradecane only forms a single phase at high coverages in which the molecules appear to be parallel and upright, similar to the dodecane high temperature, high coverage phase.

Orientational order in concentrated dispersions of plate-like kaolinite particles under shear

The orientational order in concentrated dispersions of plate-like, kaolinite particles subjected to shear has been studied using neutron diffraction. The relationship between the direction of orientation and the extent of alignment, calculated as an order parameter, is similar to that predicted for dilute dispersions. The plate-like particles align with their normals in the compressional direction. As the shear rate is increased, the degree of order increases and the particles align with normals closer to the gradient direction. However, the degree of order observed at a particular shear rate was found to vary with concentration in a complex manner. At intermediate concentrations, the extent of order is enhanced by the particle interactions. In contrast, at high concentrations the extent of order is reduced. An explanation for this effect is suggested in terms of an effective temperature arising from particle interactions and collisions.

The crystalline structures of the odd alkanes pentane, heptane, nonane, undecane, tridecane and pentadecane monolayers adsorbed on graphite at submonolayer coverages and from the liquid

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.

Soft elasticity and mechanical damping in liquid crystalline elastomers

The dynamic soft response of polydomain liquid crystalline elastomers to simple shear is reported. Significantly, these materials also show extremely large loss behavior with tan δ exceeding 1 or even 1.5 over very wide temperature ranges, with clear implications for damping applications. By comparing materials that exhibit different types of liquid crystalline phases, we identify the nematic state as a better damping phase than that in materials with smectic phases. Additionally, we provide experimental evidence for directions which should be explored for further improvements in the damping behavior of liquid crystalline elastomers.

Neutron diffraction and incoherent neutron scattering from adsorbed layers

Neutron techniques are now able to probe the detailed structure and dynamics of molecularly thin physisorbed layers even in the presence of a much larger excess of bulk phases. As a result there have been important developments in areas such as the study of adsorbed multicomponent mixtures and their kinetics of phase separation. The information now available provides opportunities for detailed comparison with computer simulation models.

Hexadecylamine Adsorption at the Iron Oxide–Oil Interface

The adsorption behavior of a model additive, hexadecylamine, onto an iron surface from hexadecane oil has been characterized using polarized neutron reflectometry, sum-frequency generation spectroscopy, solution depletion isotherm, and X-ray photoelectron spectroscopy (XPS). The amine showed a strong affinity for the metal surface, forming a dense monolayer at relatively low concentrations; a layer thickness of 16 (±3) Å at low concentrations, increasing to 20 (±3) Å at greater amine concentrations, was determined from the neutron data. These thicknesses suggest that the molecules in the layer are tilted. Adsorption was also indicated by sum-frequency generation spectroscopy and XPS, the latter indicating that the most dominant amine–surface interaction was via electron donation from the nitrogen lone pair to the positively charged iron ions. Sum-frequency generation spectroscopy was used to determine the alkyl chain conformation order and orientation on the surface.

Slow Stress Relaxation in Randomly Disordered Nematic Elastomers and Gels

Randomly disordered liquid crystalline elastomers align under stress. We study the dynamics of stress relaxation before, during, and after the polydomain-monodomain transition. The results for different materials show a universal logarithmic behavior, especially pronounced in the region of the transition. The data are approximated very well by an equation sstd 2s eq , 1ys1 1a ln td. We propose a theoretical model based on the cooperative mechanical resistance for the reorientation of each domain, attempting to follow the soft-deformation pathway. [S0031-9007(98)07718-7]

Crystalline Structures of Alkylamide Monolayers Adsorbed on the Surface of Graphite

Synchrotron X-ray and neutron diffraction have been used to determine the two-dimensional crystalline structures of alkylamides adsorbed on graphite at submonolayer coverage. The calculated structures show that the plane of the carbon backbone of the amide molecules is parallel to the graphite substrate. The molecules form hydrogen-bonded dimers, and adjacent dimers form additional hydrogen bonds yielding extended chains. By presenting data from a number of members of the homologous series, we have identified that these chains pack in different arrangements depending on the number of carbons in the amide molecule. The amide monolayers are found to be very stable relative to other closely related alkyl species, a feature which is attributed to the extensive hydrogen bonding present in these systems. The characteristics of the hydrogen bonds have been determined and are found to be in close agreement with those present in the bulk materials.

Light scattering studies of dispersions under shear

Abstract The use of light scattering to study the structure of colloidal dispersions under shear is described. An apparatus has been constructed which can collect two-dimensional scattering data from a range of sample environments, including sheared samples. Shear cells, made of fused quartz, of both the disc/disc and Couette type can be used to collect scattering data from all three principal axes of a flowing dispersion. The shear cells can be driven continuously or in oscillatory motion. Results of scattering experiments on sterically stabilised polymethyl methacrylate latices in organic media will be described. These systems can be adjusted so that the refractive index difference between the particles and the dispersion medium is small (less than 0.01). This permits measurements on concentrated systems up to 50% volume fraction. Data showing the onset and extent of ordering under shear are presented. These results are discussed in terms of models of ‘hard-sphere’ fluids.

To Mix or Not To Mix: 2D Crystallization and Mixing Behavior of Saturated and Unsaturated Aliphatic Primary Amides

Physisorbed monolayers based on relatively weak noncovalent interactions can serve as excellent model systems for understanding crystallization of materials in reduced dimensionality. Here we employ a powerful combination of scanning tunneling microscopy (STM), differential scanning calorimetry (DSC), and computational modeling to reveal two-dimensional (2D) crystallization and mixing behavior of saturated and unsaturated (cis as well as trans) aliphatic primary amides. The foundation of the present work is laid by DSC measurements, which reveal characteristic adsorption and mixing behavior of aliphatic amides. These results are further supported by STM visualization of the adlayers. STM reveals, at submolecular resolution, the adsorption as well as the two-component 2D phase behavior of these molecules at the liquid-solid interface. The saturated and trans-unsaturated amides exhibit random mixing in view of their size and shape complementarity. Binary mixtures of saturated and cis-unsaturated amides, on the other hand, display unprecedented mixing behavior. The linear saturated and bent cis-unsaturated amide molecules are found to mix surprisingly better at the liquid-solid interface than might have been expected on account of the dissimilarity in their shapes. Strong, directional intermolecular hydrogen-bonding interactions as well as the relative stabilization energies of the adlayers are responsible for such unusual mixing behavior. Computational modeling provides additional insight into all the possible interactions in 2D assemblies and their impact on stabilization energies of the supramolecular networks. This study provides a model for understanding the effect of nanoscale cocrystallization on the thin film structure at interfaces and demonstrates the importance of molecular geometry and hydrogen bonding in determining the coadsorption behavior.