Jason Crain

Molecular segregation observed in a concentrated alcohol-water solution

When a simple alcohol such as methanol or ethanol is mixed with water, the entropy of the system increases far less than expected for an ideal solution of randomly mixed molecules. This well-known effect has been attributed to hydrophobic headgroups creating ice-like or clathrate-like structures in the surrounding water, although experimental support for this hypothesis is scarce. In fact, an increasing amount of experimental and theoretical work suggests that the hydrophobic headgroups of alcohol molecules in aqueous solution cluster together. However, a consistent description of the details of this self-association is lacking. Here we use neutron diffraction with isotope substitution to probe the molecular-scale structure of a concentrated alcohol–water mixture (7:3 molar ratio). Our data indicate that most of the water molecules exist as small hydrogen-bonded strings and clusters in a ‘fluid’ of close-packed methyl groups, with water clusters bridging neighbouring methanol hydroxyl groups through hydrogen bonding. This behaviour suggests that the anomalous thermodynamics of water–alcohol systems arises from incomplete mixing at the molecular level and from retention of remnants of the three-dimensional hydrogen-bonded network structure of bulk water.

Structure and elasticity of MgO at high pressure

Abstract The structural and elastic properties of MgO periclase were studied up to 150 GPa with the first-principles pseudopotential method within the local density approximation. The calculated lattice constant of the B1 phase over the pressure range studied is within 1% of experimental values. The observed B1 phase of MgO was found to be stable up to 450 GPa, precluding the B1-B2 phase transition within the lower mantle. The calculated transition pressure is less than one-half of the previous pseudopotential prediction but is very close to the linearized augmented plane-wave result. All three independent elastic constants, c11, c12, and c44, for the B1 phase are calculated from direct computation of stresses generated by small strains. The calculated zero-pressure values of the elastic moduli and wave velocities and their initial pressure dependence are in excellent agreement with experiments. MgO was found to be highly anisotropic in its elastic properties, with the magnitude of the anisotropy first decreasing between 0 and 15 GPa and then increasing from 15 to 150 GPa. Longitudinal and shear-wave velocities were found to vary by 23 and 59%, respectively, with propagation direction at 150 GPa. The character of the anisotropy changes qualitatively with pressure. At zero pressure longitudinal and shear-wave propagations are fastest along [111] and [100], respectively, whereas above 15 GPa, the corresponding fast directions are [100] and [110]. The Cauchy condition was found to be strongly violated in MgO, reflecting the importance of noncentral many-body forces.

Elastic instabilities in crystals from ab initio stress - strain relations

Pressure-induced elastic instabilities are investigated in the prototypic ionic and covalent solids (MgO, CaO, and Si) using generalized elastic stability criteria based on the elastic stiffness coefficients which are determined directly from stress - strain relations. From first-principles computer simulations of the instabilities, we demonstrate the validity and importance of the generalized criteria relative to the conventional criteria in describing the crystal stability under hydrostatic pressure in relation to the real structural transformations. We examine systems for which the two phases can be related by a simple deformation, and in all cases we show that the generalized elastic stiffness coefficient associated with that deformation softens toward the transition. The shear stability criterion bounds the first-order B1 - B2 phase transition pressure from above and below in MgO and CaO, suggesting a wide pressure regime of metastability, whereas the tetragonal shear stability criterion predicts precisely the second-order rutile-to- transition in . The high-pressure elastic behaviour of diamond structure Si is studied in detail. A tetragonal shear instability corresponding to its transformation to the -Sn structure should occur in diamond structure Si at a pressure of 101 GPa, compared to the experimental value of 9 to 13 GPa for the transition pressure.

Methanol-water solutions: A bi-percolating liquid mixture

An extensive series of neutron diffraction experiments and molecular dynamics simulations has shown that mixtures of methanol and water exhibit extended structures in solution despite the components being fully miscible in all proportions. Of particular interest is a concentration region (methanol mole fraction between 0.27 and 0.54) where both methanol and water appear to form separate, percolating networks. This is the concentration range where many transport properties and thermodynamic excess functions reach extremal values. The observed concentration dependence of several of these material properties of the solution may therefore have a structural origin.

Ab initio elasticity of three high‐pressure polymorphs of silica

Full elastic constant tensors of three high- pressure polymorphs of silica: stishovite, CaCl2-type and columbite-type (-PbO2 structure); are determined at lower mantle pressures from rst-principles using the plane wave pseudopotential method within the local density approxi- mation. The calculated zero pressure athermal elastic mod- uli are within a few percent of the experiments. We nd that the elastic properties of silica are strongly pressure dependent. The shear wave velocity decreases rapidly (by 60 %) and the anisotropy increases rapidly (by a factor of ve) between 40 and 47 GPa prior to the transition from stishovite to the CaCl2 structure at 47 GPa. At this phase transition, the isotropically averaged shear wave velocity changes discontinuously by 60 %, while the S-wave polar- ization anisotropy decreases by a factor of two. The trans- formation of the CaCl2 phase to the columbite phase at 98 GPa is accompanied by a discontinuous change of 1-2 % in elastic wave velocity and decrease by a factor of two in anisotropy. We suggest that even a small amount of silica in the lower mantle may contribute signicantly to observed seismic anisotropy, and may provide an explanation of ob- served seismic reflectivity near 1000 km.

Exposure of Epitaxial Graphene on SiC(0001) to Atomic Hydrogen

Graphene films on SiC exhibit coherent transport properties that suggest the potential for novel carbon-based nanoelectronics applications. Recent studies suggest that the role of the interface between single layer graphene and silicon-terminated SiC can strongly influence the electronic properties of the graphene overlayer. In this study, we have exposed the graphitized SiC to atomic hydrogen in an effort to passivate dangling bonds at the interface, while investigating the results utilizing room temperature scanning tunneling microscopy.

High-speed holographic optical tweezers using a ferroelectric liquid crystal microdisplay

We demonstrate the advantages of a ferroelectric liquid crystal spatial light modulator for optical tweezer array applications. The fast switching speeds of the ferroelectric device (compared to conventional nematic systems) is shown to enable very rapid reconfiguration of trap geometries, controlled, high speed particle movement, and tweezer array multiplexing.

Cellular solid behaviour of liquid crystal colloids - 1. Phase separation and morphology

Abstract:We study the phase ordering colloids suspended in a thermotropic nematic liquid crystal below the clearing point and the resulting aggregated structure. Small () PMMA particles are dispersed in a classical liquid crystal matrix, 5CB or MBBA. With the help of confocal microscopy we show that small colloid particles densely aggregate on thin interfaces surrounding large volumes of clean nematic liquid, thus forming an open cellular structure, with the characteristic size of inversely proportional to the colloid concentration. A simple theoretical model, based on the Landau mean-field treatment, is developed to describe the continuous phase separation and the mechanism of cellular structure formation.

Nanoscale imaging reveals laterally expanding antimicrobial pores in lipid bilayers

Antimicrobial peptides are postulated to disrupt microbial phospholipid membranes. The prevailing molecular model is based on the formation of stable or transient pores although the direct observation of the fundamental processes is lacking. By combining rational peptide design with topographical (atomic force microscopy) and chemical (nanoscale secondary ion mass spectrometry) imaging on the same samples, we show that pores formed by antimicrobial peptides in supported lipid bilayers are not necessarily limited to a particular diameter, nor they are transient, but can expand laterally at the nano-to-micrometer scale to the point of complete membrane disintegration. The results offer a mechanistic basis for membrane poration as a generic physicochemical process of cooperative and continuous peptide recruitment in the available phospholipid matrix.

Hydration of methanol in aqueous solutions: a Raman spectroscopic study

High-resolution Raman spectroscopy was used to study methanol-water mixtures over the whole concentration range. We report a highly non-linear dependence of the carbon-oxygen and carbon-hydrogen stretching frequencies with composition. The difference between the polarized and depolarized frequencies of the carbon-oxygen stretching mode (non-coincidence effect) was also measured. Taken together, the data suggest a global picture of the progressive hydration of methanol: water first breaks up the molecular chains which exist in pure methanol, and then completely hydrates the hydroxyl groups before solvating the hydrophobic methyl groups.