John C. Dore

Diamond nanoparticles to carbon onions transformation: X-ray diffraction studies

Abstract Carbon onions prepared by high temperature annealing of ultradispersed diamond nanoparticles of about 5 nm in average diameter have been studied by X-ray diffraction using synchrotron radiation. The X-ray diffraction patterns show transformation of the diamond nanoparticles with sp 3 bonds into spherical carbon onions containing remaining diamond-like core and then into polyhedral onions with facets on their outer part and pure sp 2 graphitic bonds. The prepared onions form concentric-shell particles which comprise of about ten shells with an intershell distance of 0.35–0.36 nm. The large intershell distance suggests a considerable reduction in intershell interaction when compared to perfect graphite. The X-ray data are related to the previously performed studies by electron energy-loss spectroscopy and electron spin resonance.

Structural studies of water in confined geometry by neutron diffraction

Abstract Water in confined geometry shows significant modification in its structural characteristics from water in the bulk phase. Neutron diffraction studies may be used to investigate the modifications to the local molecular environment. It is found that the hydrogen bonding is enhanced in the confined state and the nucleation temperature is depressed. The diffraction studies show that there is a systematic variation in the structure factor, which is similar to that of the bulk phase but shifted in temperature. The behaviour at low temperatures is due to the formation of an extended hydrogen-bonded network similar to that of amorphous ice. The confined liquid eventually nucleates with the formation of a defective form of cubic ice rather than hexagonal ice formed in the bulk phase. The studies have been made primarily for mesoporous silicas but some preliminary results are also given for water in a lamellar liquid crystal. The results have relevance to the behaviour of interfacial water in membranes and biological systems.

Small-angle neutron-scattering study of microemulsions stabilised by aerosol-OT. Part 1.—Solvent and concentration variation

Small-angle neutron-scattering (SANS) measurements have been made for a series of aerosol-OT (AOT)-stabilised water-in-oil microemulsions. The intensity pattern has been used to extract a value for the radius of the water core, rw, using D2O to provide the required contrast profile. In heptane the radii are found to follow an approximately linear relationship with respect to the [D2O]/[AOT] concentration ratio, R. At 20 °C, and R= 20, the structure of the water-droplet system is dependent on the hydrocarbon chain length of the oil medium. The experimental SANS patterns show increasing discrepancies with a fitted function based on monodisperse spheres as the length of the alkane chain is increased from n-heptane to n-dodecane. This effect is attributed to polydispersity and indicates that the droplet-size distribution within these microemulsion systems is much larger than had previously been thought.

The structure factor for liquid nitrogen and liquid oxygen from neutron scattering studies

The structure factor, S(Q), has been measured for liquid nitrogen at 77 K and liquid oxygen at 84 K by neutron diffraction over a range of momentum transfer, Q, from 0·3 to 7·3 A-1. In the case of oxygen a correction for magnetic scattering has been included. The liquid structure factor for nitrogen compares well with that obtained from X-ray diffraction measurements but there are significant discrepancies in the case of oxygen. These may be partially attributed to uncertainty in the form factor for paramagnetic scattering. The data have been analysed in terms of several different models for the orientational correlation between neighbouring molecules. The results are particularly sensitive to uncertainties in the absolute normalization of the data, the inter-nuclear distance for the molecular structure and Placzek corrections; the effect of these errors is considered in detail. It is shown that some form of orientation correlation must exist but its precise nature could not be obtained from the present data.

Structural studies of liquid D-glycerol by neutron diffraction

Neutron diffraction measurements have been made for D-glycerol at temperatures of 193 and 296 K. The data show that the molecular conformation is similar to that of the solid phase and in good agreement with X-ray studies of the liquid in a higher temperature regime. The information on intermolecular correlations indicates a well-defined hydrogen-bond at 1·8 A and there are significant oscillations in the pair distribution function extending to 20 A. The results are consistent with the formation of a strongly hydrogenbonded network structure. The effect of temperature variation does not produce a large change in the structural relationships other than expected from the density variation. The general behaviour is similar to that shown by other hydrogen-bonded liquids.

Neutron diffraction studies of water in porous silica. II: Temperature variation in the super-cooled regime

Neutron diffraction measurements have been made on D2O water dispersed in the porous silicas Spherisorb (pore diameter = 90 A) and Gasil (pore diameter = 20 A) in the liquid and supercooled liquid region (20°C to -22°C). The structural features of water in Spherisorb are found to be essentially identical to those of bulk water and the temperature variation behaves in an analogous manner until nucleation occurs at -13°C. The diffraction pattern at -15°C indicated the presence of cubic ice (I c) together with some unfrozen water. Results for water in Gasil silica show evidence for an enhanced hydrogen-bonding compared with that of bulk water. These enhanced interactions were maintained into the supercooled region which extended to -22°C.

The structure factor for liquid bromine by neutron diffraction

Thermal neutrons from steady-state (reactor) and pulsed (linac) neutron facilities have been used to study the structure factor for liquid bromine at 20°C, covering a wide range of values (0·6-35 A-1) of the momentum transfer (ħQ). The diffraction pattern at high Q-values (>10 A-1) gives information on the structural properties of the individual molecules but detailed interpretation is complicated by the vibrational motion which causes a systematic variation in the periodic oscillations of the molecular form-factor. An internuclear distance (bond length) of 2·28 ± 0·01 A is found to be suitable for data at lower Q-values and has been used in the analysis of the liquid structure. The results show that some form of orientational correlation between molecules must be present and the nuclear (atom) pair correlation function is split into two peaks for the coordination shell corresponding to nearest-neighbour molecules. The results are compared with other studies of liquid bromine and similar neutron experimen...

Neutron diffraction studies of water in meso- and micro-pores

Neutron diffraction studies have been made for D2O water in two high surface area silica samples, Spherisorb and Gasil. The results show that the structural arrangement is similar to that of bulk water and that the modification due to the interface is restricted to a range of the order of 10 A or less. Measurements also indicate that the dispersed water undergoes significant structural changes with temperature variation but the behaviour is similar to that of the bulk liquid. The differences observed in the diffraction patterns can be partially described in terms of diffraction broadening due to the small volume of the dispersed water, but there remains a significant variation which appears to be related to the hydrogen bonding interactions.

Structural studies of liquid hydrogen fluoride by neutron diffraction: I. Liquid DF at 293 K

Neutron diffraction measurements have been made on liquid DF at 293 K using 0·7 A neutrons. The intramolecular distance r DF was found to be 0·95 A in satisfactory agreement with accepted values. The intermolecular contribution has been transformed to give the composite pair correlation function d L(r) and exhibits a well-defined hydrogen-bond at a distance of 1·62 A. The results are compared with four predictions based on computer simulation with different models for the intermolecular interaction potential. The experimental results discriminate clearly between the four potentials. They favour the HF3 potential [14] but the very simple HFC potential [16] is also satisfactory. Comparison is also made with diffraction data for the solid and vapour phase measurements on HF and DF. The possible formation of cyclic hexamers in the liquid phase is discussed.

A RISM analysis of structural data for tetrahedral molecular systems

A systematic study has been made of available X-ray and neutron diffraction data for liquids and amorphous solids composed of tetrahedral molecules. The intermolecular function, D M(Q), has been compared with predictions computed by the reference interaction site model (RISM) in order to determine the effective hard core radii which characterize the molecular contours. It is found that the RISM formulation gives a satisfactory first-order representation of the data in most cases but there are significant discrepancies which prevent the model being used to give a detailed quantitative description. Specific features are noted in the case of P4 and As4 systems which suggest that a central auxiliary site is required to give agreement with the experimental results. The geometrical packing is considered in relation to orientational correlations between adjacent molecules and the systematic behaviour examined for a range of materials.