Quasielastic Neutron Scattering Spectra Research Articles

X-ray diffraction and quasielastic neutron scattering studies of the structure and dynamic properties of water confined in ordered microporous carbon pores

The structure and dynamic properties of capillary-condensed water confined in ordered microporous carbon (OMC) of pores diameter 18.7 Å are investigated over a temperature range of 200–300 K by adsorption/desorption isotherms, X-ray diffraction (XRD), and quasielastic neutron scattering (QENS). The nitrogen adsorption/desorption isotherm of OMC pores at 77 K exhibits an I-type pattern. The water adsorption/desorption isotherm of OMC pores at 298 K is of type V. The XRD data on water confined in OMC reveal that the tetrahedral network structure of water is perturbed from the bulk water structure, but not to such an extent as found for water confined in MCM-41 and Ph-PMO previously reported. With decreasing the temperature, the ∼2.8 Å peak shifts to shorter distances, while the second-neighbor H2O– H2O interaction distances gradually increase from 3.95 Å and 4.49 Å to 4.43 Å and 4.84 Å, indicating a tendency to form a more tetrahedral-like hydrogen-bonded water structure at subzero temperatures. Below 230 K, the hexagonal ice Ih was partially formed in OMC pores. The QENS spectra were analyzed using a jump-diffusion model to translate water molecules. The translational diffusion of water molecules in OMC pores is comparable to that of bulk water at 300 K and higher than that in MCM-41, and the mobility slows as the temperature decreases. The elastic incoherent structure factor (EISF) analysis found evidence for immobile and mobile fractions of confined water. The mobile fraction exhibited jump diffusion, with a jump length consistent with a sphere of confinement radius of 4–8 Å. The results obtained in the present work are compared with those reported for MCM-41 with a hydrophilic interface and Ph-PMO with an amphiphilic interface, and the influence of interface effect on local structure and dynamic properties of confined water is discussed.

Rotation of complex ions with ninefold hydrogen coordination studied by quasielastic neutron scattering and first-principles molecular dynamics calculations

Quasielastic neutron scattering (QENS) and neutron powder diffraction of the complex transition metal hydrides ${\mathrm{Li}}_{5}{\mathrm{MoH}}_{11}$ and ${\mathrm{Li}}_{6}{\mathrm{NbH}}_{11}$ were measured in a temperature range of 10--300 K to study their structures and dynamics, especially the dynamics of the hydrogen atoms. These hydrides contain unusual ninefold H-coordinated complex ions (${\mathrm{MoH}}_{9}^{3\ensuremath{-}}$ or ${\mathrm{NbH}}_{9}^{4\ensuremath{-}}$) and hydride ions $({\mathrm{H}}^{\ensuremath{-}})$. A QENS signal appeared >150 K due to the relaxation of H atoms. The intermediate scattering functions derived from the QENS spectra are well fitted by a stretched exponential function called the Kohlrausch-Williams-Watts functions with a small stretching exponent \ensuremath{\beta} \ensuremath{\approx} 0.3--0.4, suggesting a wide relaxation time distribution. The $Q$ dependence of the elastic incoherent structure factor is reproduced by the rotational diffusion of $M{\mathrm{H}}_{9}$ ($M=\mathrm{Mo}$ or Nb) anions. The results are well supported by a van Hove analysis for the motion of H atoms obtained using first-principles molecular dynamics calculations. We conclude that the wide relaxation time distribution of the $M{\mathrm{H}}_{9}$ rotation is due to the positional disorder of the surrounding Li ions and a unique rotation with $M{\mathrm{H}}_{9}$ anion deformation (pseudorotation).

Microscopic diffusion in cationic vesicles across different phases

Understanding the phase behavior and microscopic diffusion mechanism of cationic vesicles is crucial for controlled-release kinetics in gene/DNA transfection. Here, we report our findings on the structure, phase behavior, and microscopic dynamics of a vesicle based on a cationic double-chained surfactant, dihexadecyldimethylammonium bromide (DHDAB), which is a promising candidate for a nonviral gene/DNA carrier. Small-angle neutron-scattering measurements reveal that DHDAB aggregates are in the form of unilamellar vesicles. Calorimetric studies show a strong thermal hysteresis in the heating and cooling cycles following different pathways. Two first-order transitions are observed at 303 and 318 K in the heating cycle. Fourier transform infrared spectroscopy reveals that the first transition (at 303 K) is associated with a polymorphic (solid-solid) transition from coagel to an intermediate crystalline (IC) phase, and subsequently the second transition (at 318 K) is an order-disorder transition from IC to the fluid phase. Interestingly, it is found that in the cooling cycle, the fluid phase directly transforms back into the coagel phase without passing through the IC phase. This is also confirmed by an elastic fixed window scan (EFWS) using incoherent neutron scattering. The phase behavior of DHDAB is in strong contrast to that of its longer-chain counterpart, dioctadecyldimethylammonium bromide (DODAB), where the intermediate gel phase was observed during the cooling cycle. Moreover, no polymorphic transition was observed in the heating cycle of DODAB, in which the coagel phase directly converts to the fluid phase. These sharply contrasting phase behaviors for DHDAB and DODAB suggest the strong dependence of membrane phase behavior to the alkyl chain length. EFWS measurements reveal that the phase transitions observed in the DHDAB membrane are associated with strong changes in the DHDAB dynamics. The changes in conformational entropy of DHDAB molecules estimated from EFWS confirm that the coagel-fluid and IC-fluid phase transitions are accompanied by strong changes in the conformations of the alkyl chains. The microscopic dynamics of DHDAB molecules in each of these phases is investigated by employing quasielastic neutron-scattering (QENS). In the ordered phases (coagel and IC), only localized internal dynamics of the DHDAB is observed in the QENS spectra. However, in the fluid phase, the presence of a long-range lateral diffusion is detected in addition to the localized internal dynamics of the alkyl chain. The lateral motion is found to follow Fickian diffusion. The internal dynamics of DHDAB depends on the phase state of the bilayer. In the ordered phase, the internal dynamics is described by a fractional uniaxial rotational diffusion model. However, in the fluid phase, where alkyl tails are disordered with significant gauche defects, the internal dynamics is described using localized translation within confined spheres. This study highlights the rich phase behavior of the DHDAB membrane and the surfactant dynamics across these phases.

Study of the water dynamics near hydrophilic, nanostructured CuO surfaces by quasielastic and inelastic neutron scattering

We have used quasielastic and inelastic neutron scattering to investigate the structure, dynamics, and phase transitions of water interacting with superhydrophilic CuO surfaces that not only possess a strong affinity for water but also a “grass-like” topography that is rough on both micro- and nanoscales. Here, we report quasielastic neutron scattering (QENS) measurements on two samples differing in water content at five temperatures below 280 K. The QENS spectra show water undergoing two different types of diffusive motion near the CuO surfaces: a “slow” translational diffusion occurring on a nanosecond time scale and a faster rotational motion. Further from the surfaces, there is “fast” translational diffusion comparable in rate to that of bulk supercooled water and the rotational motion occurring in the interfacial water. Analysis of the QENS spectra supports wetting of water to the CuO blades as seen in electron microscopy images. In addition, we observe an anomalous temperature dependence of the QENS spectra on cooling from 270 to 230 K with features consistent with a liquid–liquid phase transition. We suggest that the solvent-like properties of the coexisting bulk-like water in our CuO samples are a significant factor in determining the temperature dependence of the interfacial water’s dynamics. Our results are compared with those obtained from two well-studied substrate classes: (1) silicas that contain ordered cylindrical nanopores but have weaker hydrophilicity and (2) nanoparticles of other transition-metal oxides, such as TiO2, which share the strong hydrophilicity of our samples but lack their porosity.

Ligand Dynamics in Nanocrystal Solids Studied with Quasi-Elastic Neutron Scattering.

Nanocrystal surfaces are commonly populated by organic ligands, which play a determining role in the optical, electronic, thermal, and catalytic properties of the individual nanocrystals and their assemblies. Understanding the bonding of ligands to nanocrystal surfaces and their dynamics is therefore important for the optimization of nanocrystals for different applications. In this study, we use temperature-dependent, quasi-elastic neutron scattering (QENS) to investigate the dynamics of different surface bound alkanethiols in lead sulfide nanocrystal solids. We select alkanethiols with mono- and dithiol terminations, as well as different backbone types and lengths. QENS spectra are collected both on a time-of-flight spectrometer and on a backscattering spectrometer, allowing us to investigate ligand dynamics in a time range from a few picoseconds to nanoseconds. Through model-based analysis of the QENS data, we find that ligands can either (1) precess around a central axis, while simultaneously rotating around their own molecular axis, or (2) only undergo uniaxial rotation with no precession. We establish the percentage of ligands undergoing each type of motion, the average relaxation times, and activation energies for these motions. We determine, for example, that dithiols which link facets of neighboring nanocrystals only exhibit uniaxial rotation and that longer ligands have higher activation energies and show smaller opening angles of precession due to stronger ligand-ligand interactions. Generally, this work provides insight into the arrangement and dynamics of ligands in nanocrystal solids, which is key to understanding their mechanical and thermal properties, and, more generally, highlights the potential of QENS for studying ligand behavior.

Experimental demonstration of the novel \u201cvan-Hove integral method (vHI)\u201d for measuring diffusive dynamics by elastic neutron scattering

Quasi-elastic neutron scattering (QENS)—based on the seminal work of Nobel Laureate Brockhouse—has been one of the major methods for studying pico-second to nano-second diffusive dynamics over the past 70 years. This is regarded as an “inelastic” method for dynamics. In contrast, we recently proposed a new neutron-scattering method for dynamics, which uses the elastic line of the scattering to access system dynamics directly in the time domain (Benedetto and Kearley in Sci Rep 9:11284, 2019). This new method has been denoted “vHI” that stands for “van Hove Integral”. The reason is that, under certain conditions, the measured elastic intensity corresponds to the running-time integral of the intermediate scattering function, Ileft( {Q,t} right), up to a time that is inversely proportional to the energy band-width incident on the sample. As a result, Ileft( {Q,t} right) is accessed from the time derivative of the measured vHI profile. vHI has been supported by numerical and Monte-Carlo simulations, but has been difficult to validate experimentally due to the lack of a suitable instrument. Here we show that vHI works in practice, which we achieved by using a simple modification to the standard QENS backscattering spectrometer methodology. Basically, we varied the neutron-energy band-widths incident at the sample via a step-wise variation of the frequency of the monochromator Doppler-drive. This provides a measurement of the vHI profile at the detectors. The same instrument and sample were also used in standard QENS mode for comparison. The intermediate scattering functions, Ileft( {Q,t} right), obtained by the two methods—vHI and QENS—are strikingly similar providing a direct experimental validation of the vHI method. Perhaps surprisingly, the counting statistics of the two methods are comparable even though the instrument used was expressly designed for QENS. This shows that the methodology modification adopted here can be used in practice to access vHI profiles at many of the backscattering spectrometers worldwide. We also show that partial integrations of the measured QENS spectrum cannot provide the vHI profile, which clarifies a common misconception. At the same time, we show a novel approach which does access Ileft( {Q,t} right) from QENS spectra.

Analysis of the dynamics of water confined in hydrated calcium silica(C-S-H) based on the quasi-elastic neutron scattering spectra

Cement is a widely used construction material in the world. Calcium-silicate-hydrate (C-S-H) is the main component of aged cement (concrete). The quality and durability of concrete are strongly affected by the dynamics of water confined in it. Dynamics of the confined water can be studied experimentally by the quasi-elastic neutron scattering (QENS). In this paper, the jump-diffusion and rotation-diffusion model (JRM) is used to fit the QENS spectra of C-S-H paste samples at different measured temperatures for the whole scattering vector Q-range from 0.3 to 1.7 -1. Five important parameters are extracted to describe the dynamics of water confined in C-S-H samples: the index of immobile water C, the rotational diffusion constant Dr, the mean square displacement (MSD) u2 , the self-diffusion coefficient Dt, and the average residence time between jumps 0. Here, all the C-S-H samples, each with a 30% weight ratio of water to cement, are measured at temperatures ranging from 230 to 280 K. The fitted parameters can quantitatively describe the dynamics of water confined on different scales of C-S-H gel pores. The results show that the QENS spectra are fitted very well not only for small Q (Q 1 -1) but also for large Q (Q 1 -1). The obtained rotational diffusion constant is Q dependent. Thus the contribution of the water molecule rotation to a QENS spectrum increases with the value of Q increasing. The mean square displacement (MSD) u2 decreases with the increase of Q, which can be used to distinguish the confined water, ultra-confined water, and bound water contained in C-S-H samples. When Q is small, the fitted Dt and 0 vary with the measured temperature. Dt has a maximum value at 230 K and 0 has a peak at 240 K. These indicate that the dynamics of the confined water shows different behaviors at 230 K and 240 K. There are crossover or critical phenomena for water confined in C-S-H sample at low temperature.

Spectral analysis of water dynamics in cement paste by quasi-elastic neutron scattering

Quasi-elastic neutron scattering (QENS) is a novel experimental technique for studying the translational dynamics of water in cement paste. In our work, the improved empirical diffusion model has been used to the nonlinear least square fit of the QENS experimental data of cement samples cured for 7, 14 and 30 days, where an energy resolution function R (Q, E) of four Gaussian terms instead of one Gaussian term is utilized during the fitting process. Parameters of the translational dynamics of water in cement paste have been deduced: the number density of immobile water A, the free water index FWI=B1/(A+B1+B2), the full width half maximum of Lorentzian function Γ, the average residence time τ0 between jumps and the self-diffusion coefficient Dt of mobile water. Fitted QENS spectra are more accurate and the dynamics of water in cement paste can be quantitatively explained with these deduced parameters.All this provides a practical and useful method for spectral analysis of QENS on translational dynamics of water in cement.

Proton diffusivity in spark plasma sintered BaCe0.8Y0.2O3−δ: In-situ combination of quasi-elastic neutron scattering and impedance spectroscopy

Abstract The thermal activation of proton conductivity metal-oxide based ceramics is typically studied with electrochemical impedance spectroscopy (EIS), particularly in the context of solid electrolyte material research. To a lesser extent quasi-elastic neutron scattering (QENS) is applied for the same purpose. The latter probes the proton dynamics at the molecular scale. We have designed a simple in-situ cell which allows collecting EIS and QENS spectra on the same ceramic sample at high temperatures, and we present QENS plus EIS data collected in-situ and operando on the ceramic proton conductor BaCe 0.8 Y 0.2 O 3 − δ from 300 K to 800 K. The diffusion coefficient and average proton jumping length are derived from the temperature dependent measurement to study the activation processes and near neighborhood interactions for proton conductivity. The proton jump lengths coincide with the mean distance between the O 2 –O 2 and O 3 –O 3 oxygen ions and indicate proton inter-octahedra transfer.

High-temperature high pressure cell for neutron-scattering studies

In this study, a simple high-temperature high pressure cell concept has been developed to enable neutron diffraction and quasi-elastic and inelastic neutron-scattering studies to be conducted on large-volume powder samples of ceramic proton conductors at pressures of up to 1 GPa and at temperatures of up to 770 K. Details of the cell are provided, along with the first experimental neutron diffractograms (at 0.62 GPa and 300 K), quasi-elastic neutron-scattering spectra (at 0.58 GPa and 770 K) and inelastic neutron spectra (at 0.75 GPa and 530 K) determined for samples of BaCe0.8Y0.2O3.

Unified Effect of Hydrophobic Hydration on the Dynamics and the Structure of Water Molecules in Lower Alcohol Aqueous Solutions

After the analysis of the quasi-elastic neutron scattering (QENS) spectra of lower alcoholic aqueous solutions, we found that the parameter α, which is the number fraction of hydrophobic hydrating water molecules, has an important role in describing the dynamics and other properties of these solutions. The diffusion coefficient D was obtained from QENS spectra using the relaxing cage model, and D was found to have the same linear relation against α for ethyl alcohol, n -propyl alcohol, i -propyl alcohol, and t -butyl alcohol (with a small deviation) aqueous solutions. The excess molar volume and average hydration number of these solutions could also be expressed with common parameters, which are independent of the alcohol species.

Internal motions of actin characterized by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) experiments were carried out on powders of F-actin and G-actin hydrated with D(2)O to characterize the internal dynamics on the picosecond time scale and the Ångstrom length scale. To investigate the effects of hydration, the measurements were done on samples at hydration ratio (h) of 0.4 (mg D(2)O/mg protein), containing only the first layer of hydration water, and at h = 1.0, containing more layers of water. The QENS spectra, obtained from the measurements at two energy resolutions of 110 and 15 μeV, indicated that the internal motions of both F-actin and G-actin have distributions of motions with distinct correlation times and amplitudes. Increasing hydration changes relative populations of these distinct motions. The effects of hydration were shown to be different between F-actin and G-actin. Elastic incoherent neutron scattering measurements provided the concerted results. The observed effects were interpreted in terms of the dynamical heterogeneity of the actin molecule: in G-actin, more surface loops become flexible and undergo diffusive motions of large amplitudes, whereas in F-actin the molecular interactions that keep the polymerized state suppress the large motions of the surface loops involved with polymerization so that the population of atoms undergoing large motions can increase only to a lesser degree.

The Dynamic Response Function χ T(Q,t) of Confined Supercooled Water and its Relation to the Dynamic Crossover Phenomenon

Abstract We have made a series of Quasi-Elastic Neutron Scattering (QENS) studies of supercooled water confined in 3-D and 1-D geometries, specifically, interstitial water in aged cement paste (3-D) and water confined in MCM-41-S and Double Wall Nano Tube DWNT (1-D). In addition, we also include the cases of hydration water on protein surface and other biopolymer surfaces (pseudo 2-D). By analyzing the QENS spectra using Relaxing Cage Model (RCM), we are able to extract accurately the self-intermediate scattering function of hydrogen atoms FH(Q,t), at low-Q as a function of temperature T, showing an α-relaxation process at long time. We can then construct the Dynamic Response Function χT(Q,t) = -dFH(Q,t)/dT. χT(Q,t) as a function of t at constant Q shows a single peak at the characteristic α-relaxation time 〈τ〉, the amplitude of which grows as we approach the dynamic crossover temperature TL observed before in each of these geometries. However, the peak height of χT(Q,t) decreases after passing the crossover temperature TL. We make an argument to relate the occurrence of the extremum of the peak height in χT to the existence of the dynamic crossover temperature in each of these cases.

Surface Self-Diffusion and Mean Displacement of Hydrogen on Graphite and a PEM Fuel Cell Catalyst Support

Quasielastic neutron scattering (QENS) measurements together with equilibrium molecular dynamic (EMD) simulations have been performed to investigate the surface interaction between hydrogen molecules and a carbon material commonly used in polymer electrolyte membrane fuel cells (PEMFC), called XC-72. Half a monolayer of molecular hydrogen was adsorbed on to the carbon material at 2 K. QENS spectra were recorded at the time-of-flight spectrometer IN5 at 40, 45, 50, 60, 70, 80, and 90 K. Simultaneously the pressure was measured as a function of time to monitor the equilibrium surface coverage at each temperature. By using the Chudley and Elliott model for jump diffusion we found the diffusion coefficient at each temperature. At 350 K, a typical fuel cell temperature, the temperature function was extrapolated to a self-diffusion coefficient of 2.3 × 10−7 m2/s. We simulated graphite in contact with hydrogen molecules using EMD simulation. We simulated the system at different temperatures from 70 to 350 K in 2...

Adsorption and melting of hydrogen in potassium-intercalated graphite

Volumetric adsorption and quasielastic neutron scattering are used to study the diffusion and thermodynamics of sorbed ${\text{H}}_{2}$ in the graphite intercalation compound ${\text{KC}}_{24}$. A sorption enthalpy of 8.5 kJ/mol at zero coverage is determined from ${\text{H}}_{2}$ adsorption isotherms. From measurements of total elastic-neutron-scattering intensity as a function of temperature, a melting transition of the ${\text{H}}_{2}$ adsorbate is observed at 35 K for ${\text{KC}}_{24}{({\text{H}}_{2})}_{1}$. Quasielastic-neutron-scattering (QENS) spectra reveal distinct slow- and fast-${\text{H}}_{2}$-diffusion processes which exist simultaneously at temperatures above the transition point. The temperature dependence of the characteristic diffusion times follows an Arrhenius relation $\ensuremath{\tau}={\ensuremath{\tau}}_{0}\text{ }\text{exp}({E}_{a}/T)$, where ${\ensuremath{\tau}}_{0}^{\text{fast}}=1.0\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{ps}$, ${\ensuremath{\tau}}_{0}^{\text{slow}}=21\ifmmode\pm\else\textpm\fi{}2\text{ }\text{ps}$, ${E}_{a}^{\text{fast}}=156\ifmmode\pm\else\textpm\fi{}5\text{ }\text{K}$, and ${E}_{a}^{\text{slow}}=189\ifmmode\pm\else\textpm\fi{}5\text{ }\text{K}$. The fast-diffusion process is attributable to individual motions of ${\text{H}}_{2}$ molecules in a static potassium structure, and the slow-diffusion process could be attributable to fluctuations in ${\text{H}}_{2}$ particle density correlated with jumps of potassium atoms. The QENS spectra at low $Q$ are used to discuss the dimensionality of the diffusion process.

The structure, methyl rotation reflected in inelastic and quasielastic neutron scattering and vibrational spectra of 1,2,3,5-tetramethoxybenzene and its 2:1 complex with 1,2,4,5-tetracyanobenzene

X-ray diffraction studies show that molecules of the 1,2,3,5-tetramethoxybenzene (TMOB)(2) x 1,2,4,5-tetracyanobenzene complex form ...CCDCCDCC... columns with the short distances between molecular planes of C and D molecules equal to 3.186 A. The vibrational spectra recorded by using the inelastic neutron scattering, Raman, IR, and quasielastic neutron scattering (QENS) techniques aided by density functional theory calculations for the isolated molecules and the crystalline state enabled all four inequivalent librational modes, ascribed to the methoxy groups, to be analyzed. A rather good consistency was found between the experimental frequencies and those calculated for the crystal. The consistency was also achieved between the experimental structure of molecules and the theoretically reproduced one. A close similarity of the structures of the TMOB molecule isolated and in the complex is taken as a sign of dominating intramolecular interaction. The QENS spectra contain three Lorentzians of relative intensities of 1:1:2. Thus the two most strongly hindered of the four inequivalent methoxy groups in the crystalline lattice are characterized by rather similar barrier heights in good agreement with the packing analysis.

Methyl group dynamics in glassy, polycrystalline, and liquid coenzyme Q10 studied by quasielastic neutron scattering

The methyl group rotation of coenzyme Q(10) confined in nanosized droplets was studied using quasielastic neutron scattering (QENS). Q(10) as an oligoisoprene derivative with ten isoprene units can easily be supercooled in nanodroplets. Fixed window scans and QENS spectra at several temperatures of glassy Q(10) were recorded to study the methyl group rotation which can be described by a logarithmic Gaussian distribution of hopping rates for temperatures below the glass transition temperature (T(g) approximately 200 K). A mean activation energy of 4.8 kJ/mol with a distribution width of 2.1 kJ/mol was obtained from the evaluation of the QENS spectra. A corresponding analysis of a fixed window scan yielded an average activation energy of 5.1 kJ/mol with a distribution width of 1.8 kJ/mol. The results are compared and discussed with those of chain deuterated polyisoprene-d(5). For polycrystalline Q(10), the QENS spectra could be described by the same model yielding a similar average activation energy as found for glassy Q(10). However, no temperature dependence of the distribution width was observed. Based on the performed low-temperature measurements, the correlation times for the methyl group rotation were extrapolated to temperatures of liquid Q(10). The complex dynamics of liquid Q(10) could be described by a model yielding an apparent diffusion coefficient, the jump rate of the methyl groups, as well as an overall molecular rotational diffusion coefficient. The correlation times of the methyl group rotation in liquid Q(10) at a given temperature T(0) coincide with values determined in the glassy phase and extrapolated to T(0).

Quasi-elastic Neutron Scattering Investigation of the Hydrogen Surface Self-Diffusion on Polymer Electrolyte Membrane Fuel Cell Catalyst Support

Quasi-elastic neutron scattering (QENS) measurements have been performed to investigate the surface self-diffusion of hydrogen molecules. A monolayer of molecular hydrogen was adsorbed on a carbon material commonly used in polymer electrolyte membrane fuel cells, called XC-72. QENS spectra were recorded at the time-of-flight spectrometer IN5 at Institut Laue-Langevin (ILL) in Grenoble at 40, 50, 60, and 70 K. By using the Chudley & Elliott model for jump diffusion, we found the diffusion coefficient at each temperature. The logarithm of the diffusion coefficient was plotted versus the inverse of the temperature to give the coefficient in the Arrhenius equation. From this, we can estimate the diffusion at higher temperatures as well. Our observed diffusion follows the same trend as that found for hydrogen molecules on the surface of single-walled carbon nanotubes and Grafoil.

An Investigation of Water Dynamics in Binary Mixtures of Water and Dimethyl Sulfoxide

The motion of water molecules in mixtures of water and d6-dimethyl sulfoxide (DMSO) has been explored through molecular dynamics (MD) simulations using the SPC/E water model (J. Chem. Phys. 1987, 91, 6269) and the P2 DMSO model (J. Chem. Phys. 1993, 98, 8160). We evaluate the self-intermediate scattering functions, FS(Q,t), which are related by a Fourier transform to the incoherent structure factors, S(Q,omega), measured in quasielastic neutron scattering (QNS) experiments. We compare our results to recent QNS experiments on these mixtures reported by Bordallo et al. (J. Chem. Phys. 2004, 121, 12457). In addition to comparing the MD data to the experimental signals, which correspond to a convolution of S(Q,omega) with a resolution function, we examine the rotational and translational components of FS(Q,t) and investigate to what extent simulation results for the single-molecule dynamics follow the dynamical models that are used in the analysis of the experimental data. We find that the agreement between the experimental signal and the MD data is quite good and that the portion of FS(Q,t) due to translational dynamics is well represented by the jump-diffusion model. The model parameters and their composition dependence are in reasonable agreement with experiment, exhibiting similar trends in water mobility with composition. Specifically, we find that water motion is less hindered in water-rich and water-poor mixtures than it is near equimolar composition. We find that the extent of coupling between rotational and translational motion contributing to FS(Q,t) increases as the equimolar composition of the mixture is approached. Thus, the decoupling approximation, which is used to extract information on rotational relaxation from QNS spectra at higher momentum transfer (Q) values, becomes less accurate than that in water-rich or DMSO-rich mixtures. We also find that rotational relaxation deviates quite strongly from the isotropic rotational diffusion model. We explore this issue further by investigating the behavior of orientational time correlations for different unit vectors and corresponding to Legendre polynomials of orders 1-4. We find that the rotational time correlations of water molecules behave in a way that is more consistent with the extended jump rotation model recently proposed by Laage and Hynes (Science 2006, 311, 832).

Interlayer water molecules in vanadium pentoxide hydrate. IX. Anisotropic translational diffusion leading to anisotropic ac conductivity

The anisotropy of the dynamic properties of interlayer water molecules along the a and b axes of vanadium pentoxide hydrate, orthorhombic V2O5.nH2O, was studied using quasielastic neutron scattering (QENS) in relation to the anisotropy of the ac conductivity. The QENS spectra were analyzed using a stretched exponential function and a Lorentzian function. Both methods showed that the double-layer water molecules along the b axis are more mobile than those along the a axis. The difference in mobility between the two axes is more pronounced using a Lorentzian function analysis. These facts suggest that the diffusion coefficient of water molecules along the b axis is larger than that along the a axis, which is closely related to the ac conductivity originating from proton hopping. The anisotropy of the dynamic motion of water molecules can be attributed to the shorter b-axis length (b=3.60 A), with respect to the longer and less regular repetition of the atomic arrangements along the a axis (42.34 A).