Dynamics of surface water in ZrO2 studied by quasielastic neutron scattering

A quasielastic neutron scattering experiment has revealed the dynamics of surface water in a high surface area zirconium oxide in the temperature range of 300-360 K. The characteristic times of the rotational (picoseconds) and translational (tens of picoseconds) components of diffusion motion are well separated. The rotational correlation time shows an Arrhenius-type behavior with an activation energy of 4.48 kJ/mol, which is lower compared to bulk water. The rotational diffusion at room temperature is slower by about a factor of 2 compared to bulk water, whereas the translational diffusion slows down by a factor of 40. In contrast to bulk water, the translational correlation time exhibits an Arrhenius-type temperature dependence with an activation energy of 11.38 kJ/mol. Comparison of different models for jump diffusion processes suggests that water molecules perform two-dimensional jumps at a well-defined, almost temperature-independent distance of 4.21-4.32 A. Such a large jump distance indicates a low molecular density of the layer of diffusing molecules. We argue that undissociated water molecules on an average form two hydrations layers on top of the surface layer of hydroxyl groups, and all the layers have similar molecular density. Quasielastic neutron scattering experiment assesses the dynamics of the outermost hydration layer, whereas slower motion of the water molecules in the inner hydration layer contributes to the elastic signal.     

Structural dynamics of supercooled water from quasielastic neutron scattering and molecular simulations

One of the outstanding challenges presented by liquid water is to understand how molecules can move on a picosecond time scale despite being incorporated in a three-dimensional network of relatively strong H-bonds. This challenge is exacerbated in the supercooled state, where the dramatic slowing down of structural dynamics is reminiscent of the, equally poorly understood, generic behavior of liquids near the glass transition temperature. By probing single-molecule dynamics on a wide range of time and length scales, quasielastic neutron scattering (QENS) can potentially reveal the mechanistic details of water's structural dynamics, but because of interpretational ambiguities this potential has not been fully realized. To resolve these issues, we present here an extensive set of high-quality QENS data from water in the range 253-293 K and a corresponding set of molecular dynamics (MD) simulations to facilitate and validate the interpretation. Using a model-free approach, we analyze the QENS data in terms of two motional components. Based on the dynamical clustering observed in MD trajectories, we identify these components with two distinct types of structural dynamics: picosecond local (L) structural fluctuations within dynamical basins and slower interbasin jumps (J). The Q-dependence of the dominant QENS component, associated with J dynamics, can be quantitatively rationalized with a continuous-time random walk (CTRW) model with an apparent jump length that depends on low-order moments of the jump length and waiting time distributions. Using a simple coarse-graining algorithm to quantitatively identify dynamical basins, we map the newtonian MD trajectory on a CTRW trajectory, from which the jump length and waiting time distributions are computed. The jump length distribution is gaussian and the rms jump length increases from 1.5 to 1.9 A? as the temperature increases from 253 to 293 K. The rms basin radius increases from 0.71 to 0.75 A? over the same range. The waiting time distribution is exponential at all investigated temperatures, ruling out significant dynamical heterogeneity. However, a simulation at 238 K reveals a small but significant dynamical heterogeneity. The macroscopic diffusion coefficient deduced from the QENS data agrees quantitatively with NMR and tracer results. We compare our QENS analysis with existing approaches, arguing that the apparent dynamical heterogeneity implied by stretched exponential fitting functions results from the failure to distinguish intrabasin (L) from interbasin (J) structural dynamics. We propose that the apparent dynamical singularity at ～220 K corresponds to freezing out of J dynamics, while the calorimetric glass transition corresponds to freezing out of L dynamics.     

Dynamics of fresh and freeze-dried strawberry and red onion by quasielastic neutron scattering

The microscopic behavior of fresh and freeze-dried strawberry and red onion at different water contents (45 and 20 wt % water) has been investigated by quasielastic neutron scattering (QENS). To distinguish between the dynamics of the water and the biological material isotopic (H/D) substitution was used. The results show that all samples exhibit an onset of anharmonic motions on the experimental time scale (3-100 ps) at about 230-240 K. Above 250 K the dynamics is mainly of translational character and strongly dependent on the hydration level. The diffusion constant increases rapidly with increasing water content and at 280 K it is approximately 20% higher for the hydration water in freeze-dried strawberry than in freeze-dried red onion and around 2 orders of magnitude faster for the hydration water than for the biological material. Moreover, the diffusion constant of the biological part is about 50% faster in freeze-dried strawberry than in freeze-dried red onion. It was also found that the average relaxation time is slightly faster in fresh strawberry than in freeze-dried strawberry. From the results we can conclude that the water dynamics is not only promoting motions in the biological material, it is also affected by the structure (and possibly also the dynamics) of the biological material. Thus, the microscopic properties of the biological materials are interrelated with the properties of their hydration water.     

Water motion in reverse micelles studied by quasielastic neutron scattering and molecular dynamics simulations

Motion of water molecules in Aerosol OT [sodium bis(2-ethylhexyl) sulfosuccinate, AOT] reverse micelles with water content w(0) ranging from 1 to 5 has been explored both experimentally through quasielastic neutron scattering (QENS) and with molecular dynamics (MD) simulations. The experiments were performed at the energy resolution of 85 microeV over the momentum transfer (Q) range of 0.36-2.53 A(-1) on samples in which the nonpolar phase (isooctane) and the AOT alkyl chains were deuterated, thereby suppressing their contribution to the QENS signal. QENS results were analyzed via a jump-diffusion/isotropic rotation model, which fits the results reasonably well despite the fact that confinement effects are not explicitly taken into account. This analysis indicates that in reverse micelles with low-water content (w(0)=1 and 2.5) translational diffusion rate is too slow to be detected, while for w(0)=5 the diffusion coefficient is much smaller than for bulk water. Rotational diffusion coefficients obtained from this analysis increase with w(0) and are smaller than for bulk water, but rotational mobility is less drastically reduced than translational mobility. Using the Faeder/Ladanyi model [J. Phys. Chem. B 104, 1033 (2000)] of reverse micelle interior, MD simulations were performed to calculate the self-intermediate scattering function F(S)(Q,t) for water hydrogens. Comparison of the time Fourier transform of this F(S)(Q,t) with the QENS dynamic structure factor S(Q,omega), shows good agreement between the model and experiment. Separate intermediate scattering functions F(S) (R)(Q,t) and F(S) (CM)(Q,t) were determined for rotational and translational motion. Consistent with the decoupling approximation used in the analysis of QENS data, the product of F(S) (R)(Q,t) and F(S) (CM)(Q,t) is a good approximation to the total F(S)(Q,t). We find that the decay of F(S) (CM)(Q,t) is nonexponential and our analysis of the MD data indicates that this behavior is due to lower water mobility close to the interface and to confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay. However, rotational mobility of O-H bond vectors in the interfacial region remains fairly high due to the lower density of water-water hydrogen bonds in the vicinity of the interface.     

Coherent dynamics of meta-toluidine investigated by quasielastic neutron scattering

The coherent dynamics of a typical fragile glass former, meta-toluidine, was investigated at the molecular level using quasielastic neutron scattering, with time-of-flight and neutron spin echo spectrometers. It is well known that the static structure factor of meta-toluidine shows a prepeak originating from clustering of the molecules through hydrogen bonding between the amine groups. The dynamics of meta-toluidine was measured for several values of the wavevector transfer Q, which is equivalent to an inverse length scale, in a range encompassing the prepeak and the structure factor peak. Data were collected in the temperature range corresponding to the liquid and supercooled states, down to the glass transition. At least two dynamical processes were identified. This paper focuses on the slowest relaxation process in the system, the α-relaxation, which was found to scale with the macroscopic shear viscosity at all the investigated Q values. No evidence of "de Gennes" narrowing associated with the prepeak was observed, in contrast with what happens at the Q value corresponding to the interparticle distance. Moreover, using partially deuterated samples, the dynamics of the clusters was found to be correlated to the single-particle dynamics of the meta-toluidine molecules.     

Dynamics from picoseconds to nanoseconds of trehalose in aqueous solutions as seen by quasielastic neutron scattering

We present a study of the dynamical behavior of trehalose, a cryoprotecting agent, in concentrated aqueous solutions. Dynamics in a wide time range from picoseconds to nanoseconds has been observed using both neutron time of flight and neutron spin-echo techniques. Fast dynamics has been described using a simple diffusion model, while dynamical processes at longer times show a more complex behavior, described by a stretched exponential decay. Obtained relaxation times show a good agreement with data from viscosity measurements on aqueous trehalose solutions by Magazu et al. [Branca, Magazu, Maisano et al., J. Phys.: Condens. Matter 11, 3823 (1999)]. Experimental data provide us with some insight into the cryoprotecting properties and processes of trehalose. We conclude that an increase of the solvent viscosity in embedded biological material due to the production or the presence of trehalose might prevent biomolecules from damage.     

Incoherent quasielastic neutron scattering study of molecular dynamics of 4-n-octyl-4'-cyanobiphenyl

We report incoherent quasielastic neutron scattering experiments on the thermotropic liquid crystal 4-n-octyl-4'-cyanobiphenyl. The combination of time-of-flight and backscattering data allows analysis of the intermediate scattering function over about three decades of relaxation times. Translational diffusion and uniaxial molecular rotations are clearly identified as the major relaxation processes in, respectively, the nanosecond and picosecond time scales.     

Self-diffusion in molecular liquids: medium-chain n-alkanes and coenzyme Q10 studied by quasielastic neutron scattering

A systematic time-of-flight quasielastic neutron scattering (TOF-QENS) study on diffusion of n-alkanes in a melt is presented for the first time. As another example of a medium-chain molecule, coenzyme Q(10) is investigated in the same way. The data were evaluated both in the frequency and in the time domain. TOF-QENS data can be satisfactorily described by different models, and it turned out that the determined diffusion coefficients are largely independent of the applied model. The derived diffusion coefficients are compared with values measured by pulsed-field gradient nuclear magnetic resonance (PFG-NMR). With increasing chain length, an increasing difference between the TOF-QENS diffusion coefficient and the PFG-NMR diffusion coefficient is observed. This discrepancy in the diffusion coefficients is most likely due to a change of the diffusion mechanism on a nanometer length scale for molecules of medium-chain length.     

Diffusive solvent dynamics in a polymer gel electrolyte studied by quasielastic neutron scattering

A quasielastic neutron scattering study has been performed on a polymer gel electrolyte consisting of lithium perchlorate dissolved in ethylene carbonate/propylene carbonate and stabilized with poly(methyl methacrylate). The dynamics of the solvent, which is crucial for the ion conduction in this system, was probed using the hydrogen/deuterium contrast variation method with nondeuterated solvent and a deuterated polymer matrix. Two relaxation processes of the solvent were studied in the 10-400 microeV range at different temperatures. From analysis of the momentum transfer dependence of the processes we conclude that the faster process ( approximately 100 microeV) is related to rotational diffusion of the solvent and the slower process ( approximately 10 microeV) to translational diffusion of the solvent. The translational diffusion is found to be similar to the diffusion in the corresponding liquid electrolyte at short distances, but geometrically constrained by the polymer matrix at distances beyond approximately 5 A. The study indicates that the hindered diffusion of the solvent on a length scale of the polymer network interchain distance ( approximately 5-20 A) is sufficient to explain the reduced macroscopic diffusivity and ion conductivity of the gel electrolyte compared to the liquid electrolyte.     

Comparison between neutron and quasielastic light scattering by polyacrylamide gels

The authors describe small-angle neutron scattering measurements of the screening length ζ in polyacrylamide-water gels. Although these are inhomogeneous systems, the screening length is clearly observable and is in good numerical agreement with the relation E = 3kT/4πζ³, where E is the longitudinal elastic modulus of the gel obtained from measurements of the intensity of qu-asielastically scattered light. Static light scattering observations reveal a larger-scale (ca. 30 nm) superstructure in the gel.     

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.     

Molecular dynamics of glucose in solution: a quasielastic neutron scattering study

The molecular dynamics of glucose dissolved in heavy water have been investigated at 280 K by the technique of quasielastic neutron scattering. The scattering was described by a dynamic structure factor that accounts for decoupled diffusive jumps and free rotational motions of the glucose molecules. With increasing glucose concentration, the diffusion constant decreases by a factor five and the time between jumps increases considerably. Our observations validate theoretical predictions concerning the impact of concentration on the environment of a glucose molecule and the formation of cages made by neighboring glucose molecules at higher concentrations.     

A quasielastic neutron scattering study of molecular reorientation in the nematic phases of PAP and POAB

We present the results of Q.N.S. studies for two members of the homologous series of alkoxyazoxybenzenes, C_nH_2n+1O-ϕ-N₂O-ϕ-OC_nH_2n+1; PAP (n = 2) and POAB (n = 3). The Q.N.S. measurements were performed on the non-deuteriated (d₀-PAP and d₀-POAB) and the chain deuteriated samples, d₁₀-PAP and d₁₄-POAB. Three models were fitted to the experimental data: (1) uniaxial rotational diffusion of the molecule around the axis with the smallest moment of inertia, (2) uniaxial rotational diffusion of the two moieties of the molecule around the N-ϕ bonds, (3) 180° instantaneous jumps of the two moieties of the molecule around N-ϕ bonds. We have assumed the molecule to exist in the trans conformation. The translational diffusion of the molecules and the methyl groups' reorientation were neglected. It turned out that model (3) does not describe the experimental data well. Models (1) and (2) describe the experimental data equally well, giving no preference for the axis of rotation. However, comparison of our results with those obtained from dielectric relaxation suggests the choice of model (2) as responsible for the Q.N.S. data. The correlation times determined by fitting to both rotational diffusion models are of the order of several picoseconds. However, the correlation times determined for d₁₀-PAP and d₁₄-POAB are two or three times longer than for d₀-PAP and d₀-POAB, respectively, which indicates the existence of additional motion of the end chains.     

A quasielastic neutron scattering study of molecular reorientation in the nematic phases of PAP and POAB

Abstract

We present the results of Q.N.S. studies for two members of the homologous series of alkoxyazoxybenzenes, C_nH_2n+1O-?-N₂O-?-OC_nH_2n+1; PAP (n = 2) and POAB (n = 3). The Q.N.S. measurements were performed on the non-deuteriated (d ₀-PAP and d ₀-POAB) and the chain deuteriated samples, d ₁₀-PAP and d ₁₄-POAB. Three models were fitted to the experimental data: (1) uniaxial rotational diffusion of the molecule around the axis with the smallest moment of inertia, (2) uniaxial rotational diffusion of the two moieties of the molecule around the N-? bonds, (3) 180° instantaneous jumps of the two moieties of the molecule around N-? bonds. We have assumed the molecule to exist in the trans conformation. The translational diffusion of the molecules and the methyl groups' reorientation were neglected. It turned out that model (3) does not describe the experimental data well. Models (1) and (2) describe the experimental data equally well, giving no preference for the axis of rotation. However, comparison of our results with those obtained from dielectric relaxation suggests the choice of model (2) as responsible for the Q.N.S. data. The correlation times determined by fitting to both rotational diffusion models are of the order of several picoseconds. However, the correlation times determined for d ₁₀-PAP and d ₁₄-POAB are two or three times longer than for d ₀-PAP and d ₀-POAB, respectively, which indicates the existence of additional motion of the end chains.     

Correlated atomic motions in liquid deuterium fluoride studied by coherent quasielastic neutron scattering

The collective dynamics of liquid deuterium fluoride are studied by means of high-resolution quasielastic and inelastic neutron scattering over a range of four decades in energy transfer. The spectra show a low-energy coherent quasielastic component which arises from correlated stochastic motions as well as a broad inelastic feature originating from overdamped density oscillations. While these results are at variance with previous works which report on the presence of propagating collective modes, they are fully consistent with neutron diffraction, nuclear magnetic resonance, and infrared/Raman experiments on this prototypical hydrogen-bonded fluid.     

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.     

Reduced mobility of di-propylene glycol methylether in its aqueous mixtures by quasielastic neutron scattering

The hydrogen (H-) bonding interplay between water and other organic molecules is important both in nature and in a wide range of technological applications. Structural relaxation and, thus, diffusion in aqueous mixtures are generally dependent on both the strength and the structure of the H-bonds. To investigate diffusion in H-bonding mixtures, we present a quasielastic neutron scattering study of di-propylene glycol methylether (2PGME) mixed with H(2)O (or D(2)O) over the concentration range 0-90 wt.% water. We observe a nonmonotonic behavior of the dynamics with a maximum in average relaxation time for the mixture with 30 wt.% water, which is more than a factor 2 larger compared to that of either of the pure constituents. This is a result in qualitative agreement with previous calorimetric studies and the behavior of aqueous mixtures of simple mono-alcohols. More surprisingly, we notice that the dynamics of the 2PGME molecules in the mixture is slowed down by more than a factor 3 at 30 wt.% water but that the water dynamics indicates an almost monotonous behavior. Furthermore, in the low momentum transfer (Q) range of the 2PGME, where the intermediate scattering function I(Q,t) is considerably stretched in time (i.e., the stretching parameter β ? 1), it is evident for the 2PGME-D(2)O samples that the Q-dependence of the inverse average relaxation time, <τ>(-1), is greater than 2. This implies that the relaxation dynamics is partly homogenously stretched, i.e., the relaxation of each relaxing unit is somewhat intrinsically stretched in time.     

Solvent and lipid dynamics of hydrated lipid bilayers by incoherent quasielastic neutron scattering

The microscopic dynamics of the planar, multilamellar lipid bilayer system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) has been investigated using quasielastic neutron scattering. The DMPC was hydrated to a level corresponding to approximately nine water molecules per lipid molecule. Selective deuteration has been used to separately extract the dynamics of the water, the acyl chains, and the polar head groups from the strong incoherent scattering of the remaining hydrogen atoms. Furthermore, the motions parallel and perpendicular to the bilayers were probed by using two different sample orientations relative to the incident neutron beam. For both sample orientations, the results showed an onset of water motions at 260 K on the experimental time scale of about 100 ps. From lack of wave-vector dependence of the onset temperature for water motions, it is evident that the observed water dynamics is of mainly rotational character at such low temperatures. At 290 K, i.e., slightly below the gel-to-liquid transition around 295 K, the nature of the water dynamics had changed to a more translational character, well described by a jump-diffusion model. On the limited experimental time and length (about 10 A) scales, this jump-diffusion process was isotropic, despite the very anisotropic system. The acyl chains exhibited a weak onset of anharmonic motions already at 120 K, probably due to conformational changes (trans-gauche and/or syn-anti) in the plane of the lipid bilayers. Other anharmonic motions were not observed on the experimental time scale until temperature had been reached above the gel-to-liquid transition around 295 K, where the acyl chains start to show more substantial motions.     

Hydration of tricalcium and dicalcium silicate mixtures studied using quasielastic neutron scattering

Quasielastic neutron scattering was used to study the hydration reaction of tricalcium and dicalcium silicate mixtures by following the fixation of hydrogen into the reaction products, and by applying hydration models to the data. The reaction kinetics were well-described by an Avrami-derived model for the nucleation and growth regime during early hydration times and a diffusion-limited model for later periods. This study showed that the hydration reaction is not a simple linear combination of the reactions for the individual components. Compressive strength tests correlated with the neutron scattering data, suggesting that the details of the interaction affect the microstructure and therefore the strength of the product. Results suggest that favorable reaction mechanics provide optimal strength when an 80-95% tricalcium silicate and 20-5% dicalcium silicate mixture is used.