Vibrational dynamics of a glass forming liquid in nanoscopic confinement as probed by inelastic neutron scattering

In this article we present incoherent inelastic neutron scattering results, as a function of temperature, on the vibrational dynamics of a glass-forming liquid, namely propylene glycol, confined to the 26 Å pores of a controlled porous glass. The aim is to elucidate the effects induced by surface interactions (chemical traps) and geometrical restrictions (physical traps) on the fast microscopic dynamics of hydrogen-bonded liquids. The most prominent effect is the appearance of the ‘boson peak’ in the vibrational spectra. It is ascribed to an excess density of vibrational states due to quasilocalized collective atomic vibrations induced by confinement. A destructuring effect on the transient aggregates with the highest degree of connectivity, promoted by PG in the bulk phase, is hypothesized under confinement as a consequence of interactions, via hydrogen bond, between the hydroxyl groups (OH) of the PG molecule and the active silanol groups (Si–OH) on the surface of the porous glass.

Interfacial and/or finite-size effects are also found to give rise to a destructuring effect, under confinement, of the disordered Longitudinal Acoustic Mode, together with a broadening of the highest frequency torsional vibration and a stabilization, vs. T, of the internal CCO bending vibration.     

Examination of some binary mixtures by inelastic neutron scattering with a radioisotope neutron source

Magnesium, almuinium and iron have been determined in sand at levels of >1% by inelastic neutron scattering. Neutrons were obtained from a 1 Ci polonium-beryllium neutron source and analytical determination was based on measurement of the characteristic γ-lines emitted during nuclear de-excitation.     

Elemental analysis by gamma-ray detection following inelastic neutron scattering

A method of elemental analysis based on the detection of prompt γ-rays produced by the inelastic scattering of fast monoenergetic neutrons has been investigated. Time-of-flight discrimination techniques and the use of Ge(Li) detectors provide significant advances over previous studies. This method is multielemental, non-destructive, and essentially free of interferences and matrix effects. Sample analysis is rapid and selective excitation of various elements is possible. Many elements have been examined by this technique and minimum detection limits have been established. This method has been applied to the analysis of metal alloys.     

The determination of tungsten in steels by neutron inelastic scattering

A simple, rapid and non-destructive method is described for determining tungsten in steels over the concentration range 0.08 to about 3% by mass. The method uses the low energy gamma-rays induced in¹⁸³W by neutrons from the 3 MeV bombardment of beryllium with deuterons. The precision is normally better than 10%.     

Lattice dynamics of tetrahedrally bonded boron nitride probed by inelastic X-ray scattering

We report experimental results on the lattice dynamics of zincblende and wurtzite boron nitride obtained by inelastic X-ray scattering (IXS) on polycrystalline samples. The generalized vibrational density of states and the orientation averaged longitudinal acoustic phonon dispersion are determined, and the longitudinal and shear sound speeds as well as the Debye temperature are derived. Our results are compared to ab initio lattice dynamics calculations and available elastic and thermodynamic data.     

Inverse freezing in α-cyclodextrin solutions probed by quasi elastic neutron scattering

Solutions of α-cyclodextrin, 4-methylpyridine and water undergo a reversible liquid–solid phase transition on heating (“inverse freezing”). In this paper quasi elastic neutron scattering (QENS) measurements are reported, from which the diffusive dynamics of different components in the liquid and solid phases is determined. Results imply that, in solution, α-cyclodextrin is contained in a solvation shell of 4-methylpyridine molecules, while in the crystal phase, the majority of the cell contents are immobilized on the nanosecond timescale. This information will be important in evaluating the entropy of the system in its different phases and in understanding the unusual phase transition.     

The inelastic neutron scattering spectrum of chloroform in solution

The incoherent inelastic neutron scattering (INS) spectrum of CHCl₃ in liquid SO₂ (ambient temperature) in the region 0–3600 cm⁻¹ is presented. For the first time it has been demonstrated that good quality INS data on solutes can be obtained over this energy transfer range. A discussion of suitable solvents for further work is included.     

Study of low-frequency molecular motions in polydimethylsiloxane polymers by neutron inelastic scattering

The intrachain and interchain vibrations below 900 cm^?1 of polydimethylsiloxane (PDMS) have been studied by slow neutron inelastic scattering. A composite motion observed at +25°C for the methyl groups corresponds to nearly free rotation about the threefold axis of symmetry together with a large-amplitude rotation of the entire methyl group. At ?123°C, rotation about the threefold axis evolves to a torsional oscillation. The large-amplitude rotation evolves to the skeletal vibrations of a helical conformation. Vestiges of the cooperative skeletal vibrations of the conformation at ?123°C persist into the 25°C spectrum. The results indicate the presence of interrupted helical conformations at 25°C, which result from thermal disordering of the low temperature helices. The effects of crosslinking, low molecular-weight oils, and silica filler on the freedom of the methyl group motions and on skeletal vibrations have been determined. The effects of different crosslinking agents and different relative amounts of filler and oil on both the macroscopic physical properties and the observed molecular motions of PDMS can also be interpreted in terms of an interrupted helix.     

The analytical potential of neutron inelastic scattering reactions

The analytical application of neutrons produced by the⁹Be(d, n)¹⁰B reaction using 3-MeV deuterons is presented. The principal reaction for such neutrons is that of inelastic scattering. Fifteen elements were chosen for study (Se, Br, St, Y, Ag, Cd, In, Sn, Er, Lu, Hf, W, Ir, Au and Hg). Gamma-ray spectra of the metastable isomers produced were obtained with a Ge(Li) detector. The minimum weight of each element detectable (L_D) was then determined from the spectra. Assuming a sample weight of 5g, several elements (Se, Br, Sr, Y, Hf and Au) had L_D values of less than 100 ppm.     

Vibrational spectroscopy of adsorbed species on nickel by neutron inelastic scattering

In a test of the utility of neutron inelastic spectroscopy (NIS) for studying molecular vibrations in surface reactions, we have examined the adsorption, co-adsorption, and reaction of hydrogen and carbon monoxide on Raney nickel catalysts.     

Dynamics of water studied by coherent and incoherent inelastic neutron scattering

This paper reviews the more recent results obtained on the dynamics of water by neutron scattering and shows that some information can be obtained by this technique at the microscopic level of the hydrogen bond. It also accounts for some very recent results obtained with the hydrated protein C-phycocyanin.

Incoherent quasi-elastic and inelastic neutron scattering by water has been performed in a temperature range extending to the supercooled state. The analysis of the quasi-elastic spectrum separates two main components and gives two characteristic times, one of them being related to the hydrogen-bond lifetime. The inelastic spectra extend until 600 meV, i.e. covering the intramolecular vibration region, showing for the first time the stretching band.

Collective excitations propagating at 3310 m s⁻¹ have been observed by coherent inelastic neutron scattering. This result was predicted by previous computer molecular dynamics simulations of water. The data are interpreted as a manifestation of short wavelength collective modes propagating within patches of highly bonded water molecules, and distinct from the ordinary sound wave.     

Solvent entrainment in and flocculation of asphaltenic aggregates probed by small-angle neutron scattering

While small-angle neutron scattering (SANS) has proven to be very useful for deducing the sizes and masses of asphaltenic aggregates in solution, care must be taken to account for solvation effects within the aggregates so as to not err in the characterization of these important systems. SANS measurements were performed on solutions of asphaltenes dispersed in deuterated solvents in which a broad spectrum of solute and solvent chemical compositions was represented. Fits to the scattering intensity curves were performed using the Guinier approximation, the Ornstein-Zernike (or Zimm) model, a mass-fractal model, and a polydisperse cylinder model. The mass-fractal model provided apparent fractal dimensions (2.2-3) for the aggregates that generally decreased with increasing aggregate size, indicating increased surface roughness for larger aggregates. The polydisperse cylinder model provided typical values of the particle thicknesses from 5 to 32 angstroms, the average particle radius from 25 to 125 angstroms, and approximately 30% radius polydispersity. Subsequent calculation of average aggregate molar masses suggested a range of solvent entrainment from 30 to 50% (v/v) within the aggregates that were consistent with previous viscosity measurements. Additional calculations were performed to estimate the proportion of microparticle to nanoparticle aggregates in the solutions. The results indicate that the inclusion of solvation effects is essential for the accurate determination of aggregate molecular weights and fractal dimensions.     

Biomolecular and amphiphilic films probed by surface sensitive X-ray and neutron scattering

In this review article we discuss the thin film analytical techniques of interface sensitive X-ray and neutron scattering applied to aligned stacks of amphiphilic bilayers, in particular phospholipid membranes in the fluid L phase. We briefly discuss how the structure, composition, fluctuations and interactions in lipid or synthetic membranes can be studied by modern surface sensitive scattering techniques, using X-rays or neutrons as a probe. These techniques offer an in-situ approach to study lipid bilayer systems in different environments over length scales extending from micrometer to nanometer, both with and without additional membrane-active molecules such as amphiphilic peptides or membrane proteins.     

Analysis of some binary and ternary oxide mixtures by inelastic neutron scattering

Aluminium, iron and silicon have been determined in a number of binary and ternary oxide mixtures at levels>1% by measuring the gamma-lines emitted as a result of inelastic neutron scattering.     

Intense quasielastic neutron scattering of 4,4'-diethyoxyazoxybenzene and its connection with the inelastic neutron scattering spectrum

This is a continuation of a previous study of the odd-even effect for compounds of the series of 4,4'-di-n-alkyloxyazoxybenzenes. The reduction of the elastic component of the neutron scattering spectra (the elasticity depression), studied earlier at constant ΔT from the clearing point, has now been studied at constant temperature. The ethyoxy member of the series shows an exceptionally large elasticity depression; this is interpreted as evidence of a small steric hindrance for molecular motions due to the high nematic order parameter. This statement has been corroborated by the inelastic neutron scattering measurements for solid compounds of the series.     

Interaction of blockcopolymer micelles as probed by small-angle neutron and by small-angle X-ray scattering

 The analysis of the interaction of micelles formed by a blockcopolymer is given by means of small-angle X-ray (SAXS) and small-angle neutron scattering (SANS). The blockcopolymer consists of poly(styrene) and poly(ethylene oxide) (molecular weight of each block: 1000 g/mol) and forms well-defined micelles (weight-association number: 400, weight-average diameter: 15.4 nm) in water. The internal structure has been studied previously (Macromolecules 29:4006 (1996)) by SAXS. There it has been shown that the micelles are spherical objects. The structure factor S(q) as a function of the scattering vector q (q=(4π/λ) sin (θ/2); λ: wavelength of the radiation in the medium; θ: scattering angle) can be extracted from both sets of small-angle scattering data (SANS: q≤0.4 nm^-1; SAXS: q≤0.6 nm^-1). It is shown that particle interaction in the present system can be described by assuming soft interaction which is modeled by a square-step potential. Received: 12 May 1997 Accepted: 9 July 1997     

Characterization of H2 binding sites in prototypical metal-organic frameworks by inelastic neutron scattering

The hindered rotor transitions of H(2) adsorbed in the chemically related and prototypical porous metal-organic frameworks IRMOF-1, IRMOF-8, IRMOF-11, and MOF-177 were studied by inelastic neutron scattering to gain information on the specifics of H(2) binding in this class of adsorbents. Remarkably sharp and complex spectra of these materials signify a diversity of well-defined binding sites. Similarities in the spectral features as a function of H(2) loading and correlations with recent crystallographic studies were used to assign transitions ranging in rotational barrier from <0.04 to 0.6 kcal/mol as corresponding to localized adsorption sites on the organic and inorganic components of these frameworks. We find that binding of H(2) at the inorganic cluster sites is affected by the nature of the organic link and is strongest in IRMOF-11 in accord with our adsorption isotherm data. The sites on the organic link have lower binding energies, but a much greater capacity for increases in H(2) loading, which demonstrates their importance for hydrogen uptake by these materials.     

Density of phonon states in solid parahydrogen from inelastic neutron scattering

We have measured the inelastic neutron scattering spectrum of solid parahydrogen (at low pressure and T=13.3 K) using the thermal original spectrometer with cylindrical analyzers spectrometer at the ISIS pulsed neutron source (UK). From the experimental spectrum we have obtained the parahydrogen density of phonon states which has been compared with the estimates available in the literature. The present determination improves substantially the previous experimental scenario from the point of view of both statistics and accuracy. The comparison with the most recent estimate obtained from a quantum mechanical simulation of the molecular dynamics calls for an improvement of the computational methods..