Quasielastic and inelastic neutron scattering on hydrated calcium silicate pastes

Using the inverse geometry spectrometer QENS at the Intense Pulsed Neutron Source of the Argonne National Laboratory, we collected quasielastic and inelastic neutron scattering spectra of hydrated tricalcium and dicalcium silicate, the main components of ordinary Portland cement. Data were obtained at different curing time, from a few hours to several months. Both the quasielastic and inelastic spectra have been analyzed at the same time according to the relaxing cage model, which is a model developed to describe the dynamics of water at supercooled temperatures. Short-time and long-time dynamics of hydration water in hydrated cement pastes as a function of the curing time have been simultaneously obtained. The results confirm the findings reported in previous experiments showing that it is possible to fit consistently the quasielastic and inelastic spectra giving insights on the effect of the curing time on the short-time vibrational dynamics of hydration water.     

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.     

Spatial regimes in the dynamics of polyolefins: collective motion

Molecular simulation is used to characterize the spatial dependence of collective motion in four saturated hydrocarbon polymers. The observable is the distinct intermediate scattering function, as measured in coherent quasielastic neutron scattering experiments. Ranges of 0.01-1000 ps in time and 2-14 A in spatial scale are covered. In this time range, a two-step relaxation, consisting of a fast exponential decay and a slower stretched decay, is observed for all spatial scales. The relaxation times for the fast process are very similar to those obtained by following self motion, with a small modulation of relaxation times near the peak in the static structure factor which is well described by the narrowing picture suggested by de Gennes. For the slow process, self and collective relaxation times have larger numerical differences and follow different scaling with spatial scale. The modulation of slow relaxation times is larger than that observed for the fast process, but is overestimated by the de Gennes prediction, which only works qualitatively.     

Spin-polarized image-potential-state electrons as ultrafast magnetic sensors in front of ferromagnetic surfaces

We report on a spin-, time-, angle- and energy-resolved two-photon photoemission experiment with unprecedented resolution and adequate sensitivity, which allows us to study spin-dependent electron dynamics.Image-potential-state electrons on iron and cobalt thin films serve as well-defined model systems. The observed exchange splitting of these states reflects the exchange-split boundaries of the bulk-band gap. The temperature dependence of the spin polarization demonstrates that image-potential states are true sensors of the near surface magnetization.We have gained insight into quasielastic, i.e. resonant intra- and interband scattering processes and their inelastic counterparts. Lifetimes of minority and majority image-potential states differ primarily due to the spin-dependent density of states. In the minority channel of iron thin films quasielastic scattering processes become significant and are interpreted in terms of interband scattering between spin-up and spin-down image-potential-state bands. The latter process involves a spin flip on a sub-hundred femtosecond timescale and hints at quasielastic electron–magnon scattering.     

Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. II. Dynamics

The dynamics of monolayer films of the n-alkane tetracosane (n-C24H52) and the branched alkane squalane (C30H62) adsorbed on graphite have been studied by quasielastic and inelastic neutron scattering and molecular dynamics (MD) simulations. Both molecules have 24 carbon atoms along their carbon backbone, and squalane has an additional six methyl side groups symmetrically placed along its length. The authors' principal objective has been to determine the influence of the side groups on the dynamics of the squalane monolayer and thereby assess its potential as a nanoscale lubricant. To investigate the dynamics of these monolayers they used both the disk chopper spectrometer (DCS) and the high flux backscattering spectrometer (HFBS) at the National Institute of Standards and Technology. These instruments made it possible to study dynamical processes such as molecular diffusive motions and vibrations on very different time scales: 1-40 ps (DCS) and 0.1-4 ns (HFBS). The MD simulations were done on corresponding time scales and were used to interpret the neutron spectra. The authors found that the dynamics of the two monolayers are qualitatively similar on the respective time scales and that there are only small quantitative differences that can be understood in terms of the different masses and moments of inertia of the two molecules. In the course of this study, the authors developed a procedure to separate out the low-frequency vibrational modes in the spectra, thereby facilitating an analysis of the quasielastic scattering. They conclude that there are no major differences in the monolayer dynamics caused by intramolecular branching. It remains to be seen whether this similarity in monolayer dynamics also holds for the lubricating properties of these molecules in confined geometries.     

Rotational motion in LiBH4/LiI solid solutions

We investigated the localized rotational diffusion of the (BH(4))(-) anions in LiBH(4)/LiI solid solutions by means of quasielastic and inelastic neutron scattering. The (BH(4))(-) motions are thermally activated and characterized by activation energies in the order of 40 meV. Typical dwell times between jumps are in the picosecond range at temperatures of about 200 K. The motion is dominated by 90° reorientations around the 4-fold symmetry axis of the tetrahedraly shaped (BH(4))(-) ions. As compared to the pure system, the presence of iodide markedly reduces activation energies and increases the rotational frequencies by more than a factor of 100. The addition of iodide lowers the transition temperature, stabilizing the disordered high temperature phase well below room temperature.     

Single particle and collective hydration dynamics for hydrophobic and hydrophilic peptides

We have conducted extensive molecular dynamics simulations to study the single particle and collective dynamics of water in solutions of N-acetyl-glycine-methylamide, a model hydrophilic protein backbone, and N-acetyl-leucine-methylamide, a model (amphiphilic) hydrophobic peptide, as a function of peptide concentration. Various analytical models commonly used in the analysis of incoherent quasielastic neutron scattering (QENS), are tested against the translational and rotational intermediate scattering function, the mean square displacement of the water molecule center of mass, and fits to the second-order rotational correlation function of water evaluated directly from the simulation data. We find that while the agreement between the model-free analysis and analytical QENS models is quantitatively poor, the qualitative feature of dynamical heterogeneity due to caging is captured well by all approaches. The center of mass collective and single particle intermediate scattering functions of water calculated for these peptide solutions show that the crossover from collective to single particle-dominated motions occurs at a higher value of Q for high concentration solutions relative to low concentration because of the greater restriction in movement of water molecules due to confinement. Finally, we have shown that at the same level of confinement of the two peptides, the aqueous amphiphilic amino acid solution shows the strongest deviation between single particle and collective dynamics relative to the hydrophilic amino acid, indicating that chemical heterogeneity induces even greater spatial heterogeneity in the water dynamics.     

Molecular reorientation in the nematic and rotatory phases of 4,4'-di-n-pentyloxyazoxybenzene

Results of dielectric relaxation, quasielastic neutron scattering, calorimetric D.S.C. and preliminary X-ray measurements on the fifth member, POAOB, of the 4,4'-di-n-alkoxyazoxybenzene homologous series are presented. It has been found that POAOB exhibits two mesophases: a nematic (N) and an intermediate crystalline phase (CI) just below it. From comparison of the dielectric relaxation and quasielastic neutron scattering studies we can conclude that in the nematic phase the molecule as a whole performs rotational diffusion around the long axis (ω ~ 150ps) and at the same time the two moieties perform faster independent reorientations around the N-δ bonds (ω denotes a benzene ring) with δ ~ 5 ps. The intermediate crystal phase is identified as a solid uniaxial rotational phase in which fast molecular reorientations exist. It seems that the fast reorientations observed in the nematic phase to some extent survive to the crystal I phase. A model of molecular arrangements in the crystal I phase is proposed, and it explains the reduction of the dielectric increment observed on passing from the nematic phase to this phase.     

Molecular reorientation in the nematic and rotatory phases of 4,4'-di-n-pentyloxyazoxybenzene

Abstract

Results of dielectric relaxation, quasielastic neutron scattering, calorimetric D.S.C. and preliminary X-ray measurements on the fifth member, POAOB, of the 4,4'-di-n-alkoxyazoxybenzene homologous series are presented. It has been found that POAOB exhibits two mesophases: a nematic (N) and an intermediate crystalline phase (CI) just below it. From comparison of the dielectric relaxation and quasielastic neutron scattering studies we can conclude that in the nematic phase the molecule as a whole performs rotational diffusion around the long axis (ω ~ 150ps) and at the same time the two moieties perform faster independent reorientations around the N-δ bonds (ω denotes a benzene ring) with δ ~ 5 ps. The intermediate crystal phase is identified as a solid uniaxial rotational phase in which fast molecular reorientations exist. It seems that the fast reorientations observed in the nematic phase to some extent survive to the crystal I phase. A model of molecular arrangements in the crystal I phase is proposed, and it explains the reduction of the dielectric increment observed on passing from the nematic phase to this phase.     

A study of out-of-plane cation dynamics in a bis-thiourea pyridinium chloride inclusion compound

The out-of-plane motion of the pyridinium cation in the bis-thiourea pyridinium chloride inclusion compound has been studied in a wide temperature range using (1)H NMR, dielectric spectroscopy and quasielastic neutron scattering. The geometry of this motion is obtained from the Q-dependence of the elastic incoherent structure factor determined from the quasielastic neutron scattering measurements. We find that the pyridinium cation performs out-of-plane reorientations around the axis passing through two opposite atoms of the ring. The correlation times as a function of temperature were measured in the three known crystallographic phases, finding a good agreement between the three techniques employed. The activation energy for this motion changes from 5 ± 1 kJ mol(-1) in the low-temperature phase to 1.2 ± 0.2 kJ mol(-1) in the intermediate and high-temperature phases.     

Hierarchical structure and dynamics of an ionic liquid 1-octyl-3-methylimidazolium chloride

We have performed the heat capacity, neutron diffraction, and neutron quasielastic scattering measurements of an ionic liquid 1-octyl-3-methylimidazolium chloride (C8mimCl). The heat capacity data revealed that C8mimCl exhibits a glass transition with a large heat capacity jump at T(g) = 214 K, which is lower than T(g) of C4mimCl with a shorter alkyl-chain. In the neutron diffraction measurement for a deuterated analogue, d-C8mimCl, the peaks associated with the inter-domain, inter-ionic, and inter-alkyl-chain correlations appeared at (3, 11, and 14) nm(-1), respectively. The temperature dependence of these peaks indicates that the packing of the alkyl-chains becomes more compact and the domains become more vivid and larger as decreasing temperature. The quasielastic neutron scattering measurements using neutron spin echo and time-of-flight type instruments demonstrated that C8mimCl has faster relaxations probably owing to the alkyl-group and a slower relaxation owing to the ions. The latter relaxation, which is related to the glass transition, is of non-exponential as in the α relaxation of glass-forming molecular liquids. The relaxation of domains could not be observed in the present experiment but should have relaxation times longer than 100 ns. This is the first report to clarify temperature dependence of the hierarchical structure and relaxations simultaneously for a typical ionic liquid.     

Collective dynamics in molten potassium: an inelastic x-ray scattering study

The high-frequency collective dynamics of molten potassium has been investigated by inelastic x-ray scattering, disclosing an energy/momentum transfer region unreachable by previous inelastic neutron scattering (INS) experiments. We find that a two-step relaxation scenario, similar to that found in other liquid metals, applies to liquid potassium. In particular, we show how the sound velocity determined by INS experiments, exceeding the hydrodynamic value by approximately 30%, is the higher limit of a speedup, located in the momentum region 1 < Q < 3 nm(-1), which marks the departure from the isothermal value. We point out how this phenomenology is the consequence of a microscopic relaxation process that, in turn, can be traced back to the presence of "instantaneous" disorder, rather than to the crossover from a liquid to solidlike response.     

Recent progress on polymer dynamics by neutron scattering: From simple polymers to complex materials

The recent (from 2010 onward) contributions of quasielastic neutron scattering techniques (time of flight, backscattering, and neutron spin echo) to the characterization and understanding of dynamical processes in soft materials based on polymers are analyzed. The selectivity provided by the combination of neutron scattering and isotopic—in particular, proton/deuterium—labeling allows the isolated study of chosen molecular groups and/or components in a system. This opportunity, together with the capability of neutrons to provide space/time resolution at the relevant length scales in soft matter, allows unraveling the nature of the large variety of molecular motions taking place in materials of increasing complexity. As a result, recent relevant works can be found dealing with dynamical process which associated characteristic lengths and nature are as diverse as, for example, phenyl motions in a glassy linear homopolymer like polystyrene and the chain dynamics of a polymer adsorbed on dispersed clay platelets. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Anisotropy on the collective dynamics of water confined in swelling clay minerals

Collective excitations of water confined in the interlayer space of swelling clay minerals were studied by means of inelastic neutron scattering. The effect of bidimensional confinement on the dynamics of the interlayer water was investigated by using a synthetic Na-saponite sample with a general formula of Si(7.3)Al(0.7)Mg(6)O(20)(OH)(4)Na(0.7) in a bilayer hydration state. Experimental results reveal two inelastic signals, different from those described for bulk water with a clear anisotropy on the low-energy excitation of the collective dynamics of interlayer water, this difference being stronger in the perpendicular direction. Results obtained for the parallel direction follow the same trend as bulk water, and the effect of the confinement is mainly manifested from the fact that clay interlayer water is more structured than bulk water. Data obtained in the perpendicular direction display a nondispersive behavior below a cutoff wavenumber value, Q(c), indicating a nonpropagative excitation below that value. Molecular dynamics simulations results agree qualitatively with the experimental results.     

Time and depth dependent fluxes due to 14 MeV neutrons impinging soil

A spectroscopy system measuring inelastic neutron scattering and thermal neutron capture induced gamma-rays is being developed for in-situ soil analysis. Because a pulsed fast D-T neutron source is utilized, fast neutron induced inelastic neutron scattering reactions are separated in time from thermal neutron capture events. Models utilizing the MCNPX code have been developed to study the time and energy variations of the neutrons in the soil matrix.     

Protein boson peak originated from hydration-related multiple minima energy landscape

The boson peak is a broad peak found in the low-frequency region of inelastic neutron and Raman scattering spectra in many glassy materials, including biopolymers below approximately 200 K. Here, we give a novel insight into the origins of the protein boson peak, which may also be valid for materials other than proteins. Molecular simulation reveals that the structured water molecules around a protein molecule increase the number of local minima in the protein energy landscape, which plays a key role in the origin of the boson peak. The peak appears when the protein dynamics are trapped within a local energy minimum at cryogenic temperatures. This trapping causes very low frequency collective motions to shift to higher frequencies. We demonstrate that the characteristic frequency of such systems shifts higher as the temperature decreases also in model one-dimensional energy surfaces with multiple minima.     

Solid-state ligand dynamics in interpenetrating Mn[N(CN)(2)](2)(pyrazine): a neutron spectroscopy study

We have used quasielastic neutron scattering to probe the solid-state ligand dynamics in the coordination polymer Mn[N(CN)(2)](2)(pyz) [pyz = pyrazine] which has double-interpenetrating 3D lattices. A reversible structural phase transition occurs at 410 K as shown by neutron spectroscopy and differential scanning calorimetry. The origin of this transition is linked to rotational dynamics associated with the bridging pyz ligands. At 425 K, the pyrazine ring motion can be solely regarded as a 180 degrees reorientational jump about the axis defined by the Mn-N coordinative bonds, occurring with a correlation time of approximately 70 ps. This model can be extended to the 200-410 K temperature region using high-resolution backscattering spectroscopy to measure an identical motion on the time scale of nanoseconds with an activation energy of 24 +/- 2 kJ mol(-1). In contrast, no quasielastic scattering is seen for the 2D layered variant beta-Cu[N(CN)(2)](2)(pyz), owing to its more compact layer packing motif. Importantly, this work represents the very first study of solid-state rotational dynamics in an interpenetrating lattice structure.     

Molecular dynamics simulation of liquid sulfur dioxide

A previously proposed model for molecular dynamics (MD) simulation of liquid sulfur dioxide, SO(2), has been reviewed. Thermodynamic, structural, and dynamical properties were calculated for a large range of thermodynamic states. Predicted (P,V,T) of simulated system agrees with an elaborated equation of state recently proposed for liquid SO(2). Calculated heat capacity, expansion coefficient, and isothermal compressibility are also in good agreement with experimental data. Calculated equilibrium structure agrees with X-ray and neutron scattering measurements on liquid SO(2). The model also predicts the same (SO(2))(2) dimer structure as previously determined by ab initio calculations. Detailed analysis of equilibrium structure of liquid SO(2) is provided, indicating that, despite the rather large dipole moment of the SO(2) molecule, the structure is mainly determined by the Lennard-Jones interactions. Both single-particle and collective dynamics are investigated. Temperature dependency of dynamical properties is given. The MD results are compared with previous findings obtained from the analysis of inelastic neutron scattering spectra of liquid SO(2), including wave-vector dependent structural relaxation, tau(k), and viscosity, eta(k).