Chris J. Benmore

Benchmark oxygen-oxygen pair-distribution function of ambient water from x-ray diffraction measurements with a wide Q-range

Four recent x-ray diffraction measurements of ambient liquid water are reviewed here. Each of these measurements represents a significant development of the x-ray diffraction technique applied to the study of liquid water. Sources of uncertainty from statistical noise, Q-range, Compton scattering, and self-scattering are discussed. The oxygen-hydrogen contribution to the measured x-ray scattering pattern was subtracted using literature data to yield an experimental determination, with error bars, of the oxygen-oxygen pair-distribution function, g(OO)(r), which essentially describes the distribution of molecular centers. The extended Q-range and low statistical noise of these measurements has significantly reduced truncation effects and related errors in the g(OO)(r) functions obtained. From these measurements and error analysis, the position and height of the nearest neighbor maximum in g(OO)(r) were found to be 2.80(1) Å and 2.57(5) respectively. Numerical data for the coherent differential x-ray scattering cross-section I(X)(Q), the oxygen-oxygen structure factor S(OO)(Q), and the derived g(OO)(r) are provided as benchmarks for calibrating force-fields for water.

The structure of liquid ethanol: A neutron diffraction and molecular dynamics study

A comprehensive neutron scattering study has been performed using selective hydrogen–deuterium substitution on the hydroxyl, methyl, and methylene hydrogen atoms in pure ethanol at room temperature. In this experiment we have directly measured 10 structure factors of the possible 21 partial structure factors that exist for liquid ethanol. The data has been used to obtain details on the conformation of the liquid ethanol molecule in the liquid state. The intermolecular structure were compared to molecular dynamics simulations using the Jorgensen potential for four- and nine-site liquid ethanol molecules. All ten neutron structure factors are in good agreement with the simulation results, although there are some small differences in the structural detail. The neutron structure factors show that hydrogen bonding dominates the structure of liquid ethanol at room temperature, which is consistent with the existence of winding chains predicted by the simulation.

The structure of subcritical and supercritical methanol by neutron diffraction, empirical potential structure refinement, and spherical harmonic analysis

Pulsed neutron diffraction with isotope substitution on the hydroxyl hydrogens (H) is used to study the structure of methanol in two supercritical conditions (253 °C, 117.7 MPa, 0.700 g cm−3 and 253 °C, 14.3 MPa, 0.453 g cm−3) as well as a subcritical state (202 °C, 73.7 MPa, 0.700 g cm−3) (Tc=240 °C, Pc=8.1 MPa, ρc=0.272 g cm−3 for methanol). From three experiments on CD3OD, CD3OH, and a CD3O(H0.28+D0.72) mixture at the three thermodynamic states, the composite partial structure factors and pair correlation functions, XX, XH, and HH, are derived, where X represents a weighted sum of correlations from carbon (C), oxygen (O), and methyl hydrogen (M) atoms on the methanol molecule. The data are used in an empirical potential structure refinement (EPSR) computer simulation of methanol at the three thermodynamics states. Model distributions of molecules consistent with these data are used to estimate the individual site-site radial distribution functions, the coefficients of the spherical harmonic expansion o...

Isotopic quantum effects in water structure measured with high energy photon diffraction

High energy electromagnetic radiation scattering techniques have been used to measure the structural differences between light and heavy water: we have studied both intra- and intermolecular effects. These methods and our data analysis are described in detail. We have observed a maximum isotopic effect of 1.6% relative to the magnitude of the x-ray structure factor. Our uncertainties are an order of magnitude smaller than those of previous -ray measurements (Root J H, Egelstaff P A and Hime A 1986 Chem. Phys. 109 5164) and this has permitted us to test accurately the available quantum simulation results on water. The SPC and TIP4P potentials reproduce the measured results in r -space moderately well for intermolecular effects at distances greater than 2.5 ?. These results show that H2 O is a slightly more disordered liquid than D2 O at the same temperature.

A neutron diffraction study of yttrium- and lanthanum-aluminate glasses

Abstract Neutron diffraction data for three aluminate glasses have been collected for a range of scattering vector from 0.3 to 25 A −1 . A single phase glass, representing the high-density glass polymorph of yttrium-aluminate (AY25) composition has a structure resembling that of levitated YAG liquid. This glass has a first-neighbor Al–O distance of 1.82 A and a first-shell coordination number of 4.16±0.21. The Y–O distance is represented by a single Gaussian peak centered at 2.28 A which yields a first-neighbor coordination number of 6.64±0.33. These values indicate an increase in volume on melting of YAG solid solution. An equivalent lanthanum-aluminate glass (La25), also produced from the stable high-temperature aluminate liquid, has an Al–O distance of 1.79 A and a coordination number of 4.55±0.23. The La–O distance is increased due to an increase in ionic radius for La(III). The magnitude of this La–O peak is lower than expected and there is evidence for a second La–O distance at 2.79 A. The T ( r ) for the lanthanum-aluminate glass shows stronger features than the yttrium-aluminate glass at radial distances greater than 3 A; these suggest a higher degree of order in the lanthanum-aluminate liquid. A two-phase yttrium-aluminate (AY20) glass, which represents a partly complete transition between a high-density liquid and a lower density polymorph, has an Al–O distance of 1.81 A and a mean Al–O coordination number of 4.38±0.22. There is an increase in the Y–O radial distance on transition and also additional mid-range ordering. This mid-structure is similar to that seen in the La25 glass. The data show that rare earth aluminate liquids are dominated by a tetrahedral aluminate framework and that the liquid–liquid (transition) in the yttrium-aluminate liquids results from differences in packing among AlO 4 and Y–O polyhedra and not from a change in the mean Al–O coordination number.

Combining flagelliform and dragline spider silk motifs to produce tunable synthetic biopolymer fibers.

The two Flag/MaSp 2 silk proteins produced recombinantly were based on the basic consensus repeat of the dragline silk spidroin 2 protein (MaSp 2) from the Nephila clavipes orb weaving spider. However, the proline‐containing pentapeptides juxtaposed to the polyalanine segments resembled those found in the flagelliform silk protein (Flag) composing the web spiral: (GPGGX1 GPGGX2)2 with X1/X2 = A/A or Y/S. Fibers were formed from protein films in aqueous solutions or extruded from resolubilized protein dopes in organic conditions when the Flag motif was (GPGGX1 GPGGX2)2 with X1/X2 = Y/S or A/A, respectively. Post‐fiber processing involved similar drawing ratios (2–2.5×) before or after water‐treatment. Structural (ssNMR and XRD) and morphological (SEM) changes in the fibers were compared to the mechanical properties of the fibers at each step. Nuclear magnetic resonance indicated that the fraction of β‐sheet nanocrystals in the polyalanine regions formed upon extrusion, increased during stretching, and was maximized after water‐treatment. X‐ray diffraction showed that nanocrystallite orientation parallel to the fiber axis increased the ultimate strength and initial stiffness of the fibers. Water furthered nanocrystal orientation and three‐dimensional growth while plasticizing the amorphous regions, thus producing tougher fibers due to increased extensibility. These fibers were highly hygroscopic and had similar internal network organization, thus similar range of mechanical properties that depended on their diameters. The overall structure of the consensus repeat of the silk‐like protein dictated the mechanical properties of the fibers while protein molecular weight limited these same properties. Subtle structural motif re‐design impacted protein self‐assembly mechanisms and requirements for fiber formation.

Evidence for a temperature-driven structural transformation in liquid bismuth

The thermodynamic properties of liquid bismuth have been explored from the melting point to 1100 °C by high-resolution measurements of the density, the heat capacity and the static structure factor. These physical properties display a number of anomalies. In particular, we have observed evidence for the presence of a temperature-driven liquid-liquid structural transformation that takes place at ambient pressure. The latter is characterized by a density discontinuity that occurs at 740 °C. Differential thermal analysis measurements revealed the endothermal nature of this transformation. A rearrangement of liquid bismuth structure was found by neutron diffraction measurements, supporting the existence of a liquid-liquid transformation far above the liquidus.

Ion transport regimes in chalcogenide and chalcohalide glasses: from the host to the cation-related network connectivity

Recent ionic conductivity and tracer diffusion measurements over a large range of the mobile ion content x, carried out for Ag + - and Cu + -conducting chalcogenide and chalcohalide glasses, show two distinctly different ion transport regimes above the percolation threshold at = 30 ppm M + : (i) a critical percolation regime at low x, and (ii) modifier-controlled ion transport at high x. Using a number of structural and spectroscopic techniques (high-resolution neutron diffraction, small-angle neutron scattering, high-energy X-ray diffraction, EXAFS, 129 I-Mossbauer spectroscopy), we will show that the two regimes have a clear structural basis. Transport properties in the critical percolation domain depend almost exclusively on the connectivity of the host matrix represented by the average coordination number : the nature of the mobile cations and chemical form of the dopant or of the host network do not play any important role. In contrast, the connectivity of the cation-related structural units MY z (Y = chalcogen or halide, z = 3 or 4), evidenced by the short M-M correlations (from 2.7 to 4.2 A) and reflected by the M-M coordination number, appears to be predominant in the modifier-controlled region. Highly connected edge- or corner-sharing (ES or CS) MY 2 units, which form at least 2D sheets or tunnels in the glass network, lead to the highest mobility of the M ions.

Acoustic levitation: recent developments and emerging opportunities in biomaterials research

Containerless sample environments (levitation) are useful for study of nucleation, supercooling, and vitrification and for synthesis of new materials, often with non-equilibrium structures. Elimination of extrinsic nucleation by container walls extends access to supercooled and supersaturated liquids under high-purity conditions. Acoustic levitation is well suited to the study of liquids including aqueous solutions, organics, soft materials, polymers, and pharmaceuticals at around room temperature. This article briefly reviews recent developments and applications of acoustic levitation in materials R&D. Examples of experiments yielding amorphous pharmaceutical materials are presented. The implementation and results of experiments on supercooled and supersaturated liquids using an acoustic levitator at a high-energy X-ray beamline are described.

Structure of the floating water bridge and water in an electric field

The floating water bridge phenomenon is a freestanding rope-shaped connection of pure liquid water, formed under the influence of a high potential difference (approximately 15 kV). Several recent spectroscopic, optical, and neutron scattering studies have suggested that the origin of the bridge is associated with the formation of anisotropic chains of water molecules in the liquid. In this work, high energy X-ray diffraction experiments have been performed on a series of floating water bridges as a function of applied voltage, bridge length, and position within the bridge. The two-dimensional X-ray scattering data showed no direction-dependence, indicating that the bulk water molecules do not exhibit any significant preferred orientation along the electric field. The only structural changes observed were those due to heating, and these effects were found to be the same as for bulk water. These X-ray scattering measurements are supported by molecular dynamics (MD) simulations which were performed under electric fields of 106 V/m and 109 V/m. Directional structure factor calculations were made from these simulations parallel and perpendicular to the E-field. The 106 V/m model showed no significant directional-dependence (anisotropy) in the structure factors. The 109 V/m model however, contained molecules aligned by the E-field, and had significant structural anisotropy.