Long-range Translational Diffusion Research Articles

Methanol diffusion and dynamics in zeolite H-ZSM-5 probed by quasi-elastic neutron scattering and classical molecular dynamics simulations.

Zeolite ZSM-5 is a key catalyst in commercially relevant processes including the widely studied methanol to hydrocarbon reaction, and molecular diffusion in zeolite pores is known to be a crucial factor in controlling catalytic reactions. Here, we present critical analyses of recent quasi-elastic neutron scattering (QENS) data and complementary molecular dynamics (MD) simulations. The QENS experiments show that the nature of methanol diffusion dynamics in ZSM-5 pores is dependent both on the Si/Al ratio (11, 25, 36, 40 and 140), which determines the Brønsted acid site density of the zeolite, and that the nature of the type of motion observed may vary qualitatively over a relatively small temperature range. At 373 K, on increasing the ratio from 11 to 140, the observed mobile methanol fraction increases and the nature of methanol dynamics changes from rotational (in ZSM-5 with Si/Al of 11) to translational diffusion. The latter is either confined localized diffusion within a pore in zeolites with ratios up to 40 or non-localized, longer-range diffusion in zeolite samples with the ratio of 140. The complementary MD simulations conducted over long time scales (1 ns), which are longer than those measured in the present study by QENS (≈1-440 ps), at 373 K predict the occurrence of long-range translational diffusion of methanol in ZSM-5, independent of the Si/Al ratios (15, 47, 95, 191 and siliceous MFI). The rate of diffusion increases slightly by increasing the ratio from 15 to 95 and thereafter does not depend on zeolite composition. Discrepancies in the observed mobile methanol fraction between the MD simulations (100% methanol mobility in ZSM-5 pores across all Si/Al ratios) and QENS experiments (for example, ≈80% immobile methanol in ZSM-5 with Si/Al of 11) are attributed to the differences in time resolutions of the techniques. This perspective provides comprehensive information on the effect of acid site density on methanol dynamics in ZSM-5 pores and highlights the complementarity of QENS and MD, and their advantages and limitations. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Toward Understanding of the Li-Ion Migration Pathways in the Lithium Aluminum Sulfides Li3AlS3 and Li4.3AlS3.3Cl0.7 via 6,7Li Solid-State Nuclear Magnetic Resonance Spectroscopy.

Li-containing materials providing fast ion transport pathways are fundamental in Li solid electrolytes and the future of all-solid-state batteries. Understanding these pathways, which usually benefit from structural disorder and cation/anion substitution, is paramount for further developments in next-generation Li solid electrolytes. Here, we exploit a range of variable temperature 6Li and 7Li nuclear magnetic resonance approaches to determine Li-ion mobility pathways, quantify Li-ion jump rates, and subsequently identify the limiting factors for Li-ion diffusion in Li3AlS3 and chlorine-doped analogue Li4.3AlS3.3Cl0.7. Static 7Li NMR line narrowing spectra of Li3AlS3 show the existence of both mobile and immobile Li ions, with the latter limiting long-range translational ion diffusion, while in Li4.3AlS3.3Cl0.7, a single type of fast-moving ion is present and responsible for the higher conductivity of this phase. 6Li-6Li exchange spectroscopy spectra of Li3AlS3 reveal that the slower moving ions hop between non-equivalent Li positions in different structural layers. The absence of the immobile ions in Li4.3AlS3.3Cl0.7, as revealed from 7Li line narrowing experiments, suggests an increased rate of ion exchange between the layers in this phase compared with Li3AlS3. Detailed analysis of spin-lattice relaxation data allows extraction of Li-ion jump rates that are significantly increased for the doped material and identify Li mobility pathways in both materials to be three-dimensional. The identification of factors limiting long-range translational Li diffusion and understanding the effects of structural modification (such as anion substitution) on Li-ion mobility provide a framework for the further development of more highly conductive Li solid electrolytes.

Quasielastic Neutron Scattering and Molecular Dynamics Simulation Study on the Molecular Behaviour of Catechol in Zeolite Beta

The dynamics of catechol in zeolite Beta was studied using quesielastic neutron scattering (QENS) experiments and molecular dynamics simulations at 393 K, to understand the behaviour of phenolic monomers relevant in the catalytic conversion of lignin via metal nanoparticles supported on zeolites. Compared to previous work studying phenol, both methods observe that the presence of the second OH group in catechol can hinder mobility significantly, as explained by stronger hydrogen-bonding interactions between catechol and the Brønsted sites of the zeolite. The instrumental timescale of the QENS experiment allows us to probe rotational motion, and the catechol motions are best fit to an isotropic rotation model with a D^{rot} of 2.9 × 10^{10} s^{-1}. While this D^{rot} is within error of that measured for phenol, the fraction of molecules immobile on the instrumental timescale is found to be significantly higher for catechol. The MD simulations also exhibit this increased in ‘immobility’, showing that the long-range translational diffusion coefficients of catechol are lower than phenol by a factor of 7 in acidic zeolite Beta, and a factor of sim3 in the siliceous material, further illustrating the significance of Brønsted site H-bonding. Upon reproducing QENS observables from our simulations to probe rotational motions, a combination of two isotropic rotations was found to fit the MD-calculated EISF; one corresponds to the free rotation of catechol in the pore system of the zeolite, while the second rotation is used to approximate a restricted and rapid “rattling”, consistent with molecules anchored to the acid sites through their OH groups, the motion of which is too rapid to be observed by experiment.

Simple analytical model for fitting QENS data from liquids

The advent of new generation of neutron spectrometers for quasielastic scattering measurements has demonstrated the need for multi-component (at least two-component) fitting of data collected from liquids. The narrowest component is associated with long-range translational diffusion, whereas the broader component originates from localized center-of-mass motions in the transient cage of the nearest neighbors. Intra-particle dynamics (e.g., of molecular side groups) may give rise to additional components besides the center-of-mass motions. We present a simple analytical model for two-component fits of quasielastic neutron scattering data collected from liquids. The use of such models is optional (albeit helpful) in the traditional approach, where fits are performed sequentially and independently for the data collected at separate Q values, but imperative in the global fitting, where the data are fitted simultaneously at all Q values, which may be necessary to rationalize the data from increasingly complex systems.

Dynamics in the Plastic Crystalline Phases of Cyclohexanol and Cyclooctanol Studied by Quasielastic Neutron Scattering.

Plastic crystals are a promising candidate for solid state ionic conductors. In this work, quasielastic neutron scattering is employed to investigate the center of mass diffusive motions in two types of plastic crystalline cyclic alcohols: cyclohexanol and cyclooctanol. Two separate motions are observed which are attributed to long-range translational diffusion (α-process) and cage rattling (fast β-process). Residence times and diffusion coefficients are calculated for both processes, along with the confinement distances for the cage rattling. In addition, a binary mixture of these two materials is measured to understand how the dynamics change when a second type of molecule is added to the matrix. It is observed that, upon the addition of the larger cyclooctanol molecules into the cyclohexanol solution, the cage size decreases, which causes a decrease in the observed diffusion rates for both the α- and fast β-processes.

Influence of cellulose gel matrix on BMIMCl ionic liquid dynamics and conductivity

Fast field-recycling magnetic resonance relaxometry (FFC NMR) was applied to measure the spin-lattice relaxation time, T 1, of protons in pure ionic liquid (IL) 1-butyl-3-methylimidazolium chloride (BMIMCl) and when confined in cellulose (Cell) ion gel (Cell/BMIMCl) at different temperatures and different Larmor frequencies. The rotational and translational contributions were taken to interpret the relaxation data of neat BMIMCl and were described by Woessner’s and Torrey’s theoretical models, respectively. The ionic liquid–cellulose matrix interaction detected in 10 wt% polymer-ion gel was interpreted on the basis of a dynamical process called reorientation mediated by translational displacements (RMTD), which allow explanation of the significant slowing of the dynamics of IL cations at cellulose surfaces. Two types of cation diffusion were identified in this gel: a long-range translational diffusion within large cavities of the Cell matrix and diffusion occurring at the polymer surface. The correlational time constants and self-diffusion coefficients of the BMIMCl ionic liquid in bulk-like state and interaction with the cellulose matrix surface were determined. The conductivity measurements performed for pure IL and that confined in the Cell/BMIMCl ion gel show that the gelation only results in a small decrease of the ionic conductivity.

Differential Microscopic Mobility of Components within a Deep Eutectic Solvent.

From macroscopic measurements of deep eutectic solvents such as glyceline (1:2 molar ratio of choline chloride to glycerol), the long-range translational diffusion of the larger cation (choline) is known to be slower compared to that of the smaller hydrogen bond donor (glycerol). However, when the diffusion dynamics are analyzed on the subnanometer length scale, we find that the displacements associated with the localized diffusive motions are actually larger for choline. This counterintuitive diffusive behavior can be understood as follows. The localized diffusive motions confined in the transient cage of neighbor particles, which precede the cage-breaking long-range diffusion jumps, are more spatially constrained for glycerol than for choline because of the stronger hydrogen bonds the former makes with chloride anions. The implications of such differential localized mobility of the constituents should be especially important for applications where deep eutectic solvents are confined on the nanometer length scale and their long-range translational diffusion is strongly inhibited (e.g., within microporous media).

Brownian motion of single glycerol molecules in an aqueous solution as studied by dynamic light scattering.

Aqueous solutions of glycerol are investigated by dynamic light scattering (DLS) over the whole concentration range (10-98 wt.% water) and in the temperature range 283-303 K. The measurements reveal one slow relaxation process in the geometry of polarized light scattering. This process is present in the whole concentration range, although it is very weak at the highest and lowest water concentrations and is considerably slower than the structural α relaxation, which is too fast to be observed on the experimental time scale in the measured temperature range. The relaxation time of the observed process exhibits a 1/q2 dependence, proving that it is due to long-range translational diffusion. The Stokes-Einstein relation is used to estimate the hydrodynamic radius of the diffusing particles and from these calculations it is evident that the observed relaxation process is due to the Brownian motion of single or a few glycerol molecules. The fact that it is possible to study the self-diffusion of such small molecules may stimulate a broadening of the research field used to be covered by the DLS technique.

On the mechanism of proton conductivity in H3OSbTeO6

Pyrochlore-type H3OSbTeO6 is a remarkable proton conductor exhibiting an outstanding electrical conductivity even at ambient temperature. It consists of a three-dimensional interconnected (Sb,Te)O6 framework, built from randomly distributed corner-shared SbO6 and TeO6 octahedra, forming large cages in which the H3O+ ions are located. The dynamics of the caged species has been investigated by temperature-dependent neutron diffraction, quasielastic neutron scattering, and NMR experiments. Three types of motion may be discerned, namely, stochastic rotations of the H3O group around its 3-fold axis, jumps between four equivalent positions within the cage, and long-range inter-cage translational diffusion. The onset of ionic conductivity is clearly reflected by structural changes. Details of the complex diffusion mechanism are given.

Influence of amyloid-β peptides with different lengths and amino acid sequences on the lateral diffusion of lipids in model membranes

We have investigated the influence of two neurotoxic Alzheimer's disease peptide analogues with different lengths and amino acid compositions, amyloid-β(25–35) and amyloid-β(22–40), on the dynamics of phospholipid membranes by means of quasi-elastic neutron scattering in the picosecond time domain. Samples of pure phospholipids (DMPC/DMPS) and samples with embedded amyloid-β peptides have been compared. The peptide concentration was 3 mol% for both amyloid-β peptides. The sample temperature was set to 320 K, where the samples are in the liquid crystalline phase, which is the physiologically relevant phase of biological membranes. The data have been analyzed with a purely phenomenological model, which combines two different motions with different time constants. These two motions have been assigned to a long-range translational diffusion and a spatially restricted (localized) diffusion. The different lengths of the peptides leads to different positions and orientations inside the membrane bilayers. Thus, different influences on the dynamics are also expected. Both amyloid-β peptides significantly affected the ps-dynamics of oriented lipid membranes. Mainly, they accelerated the long-range translational diffusion even though the peptide concentration was quite low. This finding is of relevance for all kinds of protein–protein interactions, which are strongly influenced by the lateral diffusion, such as signal transduction cascades and energy transfer. This influence might be involved in the pathology of Alzheimer's disease as well as being a clue for early diagnosis and in therapy.

Microscopic Diffusion Dynamics of Silver Complex‐Based Room‐Temperature Ionic Liquids Probed by Quasielastic Neutron Scattering

Quasielastic neutron scattering is used to probe the microscopic diffusion dynamics of the hydrogen-bearing cations of two different silver complex-derived room-temperature ionic liquids, [Ag(propylamine)(2)(+)][Tf(2)N(-)] (Tf=trifluoromethanesulfonyl) and [Ag(1-pentene)(+)][Tf(2)N(-)]. In the temperature range from 300 to 340 K, analysis of the scattering momentum transfer dependence of the data provides evidence for three distinct diffusion components. The slowest component describes the long-range cationic translational diffusion. A possible link between the microscopic diffusion parameters and the structural features of the cations comprising these two ionic liquids is discussed.

Alzheimer's disease amyloid- β peptide analogue alters the ps-dynamics of phospholipid membranes

We have investigated the influence of the neurotoxic Alzheimer's disease peptide amyloid- β (25–35) on the dynamics of phospholipid membranes by means of quasi-elastic neutron scattering in the picosecond time-scale. Samples of pure phospholipids (DMPC/DMPS) and samples with amyloid- β (25–35) peptide included have been compared. With two different orientations of the samples the directional dependence of the dynamics was probed. The sample temperature was varied between 290 K and 320 K to cover both the gel phase and the liquid-crystalline phase of the lipid membranes. The model for describing the dynamics combines a long-range translational diffusion of the lipid molecules and a spatially restricted diffusive motion. Amyloid- β (25–35) peptide affects significantly the ps-dynamics of oriented lipid membranes in different ways. It accelerates the lateral diffusion especially in the liquid-crystalline phase. This is very important for all kinds of protein–protein interactions which are enabled and strongly influenced by the lateral diffusion such as signal and energy transducing cascades. Amyloid- β (25–35) peptide also increases the local lipid mobility as probed by variations of the vibrational motions with a larger effect in the out-of-plane direction. Thus, the insertion of amyloid- β (25–35) peptide changes not only the structure of phospholipid membranes as previously demonstrated by us employing neutron diffraction (disordering effect on the mosaicity of the lipid bilayer system) but also the dynamics inside the membranes. The amyloid- β (25–35) peptide induced membrane alteration even at only 3 mol% might be involved in the pathology of Alzheimer's disease as well as be a clue in early diagnosis and therapy.

Rotational Diffusion in NaBH4

Hydrogen dynamics in complex hydrides comprise long-range translational diffusion as well as localized motions like vibrations, librations, or rotations. All different motions are characterized by their specific length scale and time scales. Here we present a combined experimental and theoretical study on the rotational diffusion of the (BH4)− ions in crystalline NaBH4. The motion is thermally activated and characterized by an activation energy of 117 meV and a prefactor of 11 fs. Thereby the motion is dominated by 90° reorientations of the (BH4)− ion around the 4-fold symmetry axis of the cubic crystal. The experimental results are discussed on the basis of DFT calculations, revealing the potential energy landscape of a (BH4)− subunit in the crystalline matrix.

Dynamics of hydration water in deuterated purple membranes explored by neutron scattering

The function and dynamics of proteins depend on their direct environment, and much evidence has pointed to a strong coupling between water and protein motions. Recently however, neutron scattering measurements on deuterated and natural-abundance purple membrane (PM), hydrated in H2O and D2O, respectively, revealed that membrane and water motions on the ns–ps time scale are not directly coupled below 260 K (Wood et al. in Proc Natl Acad Sci USA 104:18049–18054, 2007). In the initial study, samples with a high level of hydration were measured. Here, we have measured the dynamics of PM and water separately, at a low-hydration level corresponding to the first layer of hydration water only. As in the case of the higher hydration samples previously studied, the dynamics of PM and water display different temperature dependencies, with a transition in the hydration water at 200 K not triggering a transition in the membrane at the same temperature. Furthermore, neutron diffraction experiments were carried out to monitor the lamellar spacing of a flash-cooled deuterated PM stack hydrated in H2O as a function of temperature. At 200 K, a sudden decrease in lamellar spacing indicated the onset of long-range translational water diffusion in the second hydration layer as has already been observed on flash-cooled natural-abundance PM stacks hydrated in D2O (Weik et al. in J Mol Biol 275:632–634, 2005), excluding thus a notable isotope effect. Our results reinforce the notion that membrane-protein dynamics may be less strongly coupled to hydration water motions than the dynamics of soluble proteins.

Solute−Solvent Contact by Intermolecular Cross-Relaxation. 2. The Water−Micelle Interface and the Micellar Interior

The intermolecular dipole-dipole cross-relaxation is measured between 19F nuclei of sodium perfluorooctanoate in micelles and 1H nuclei of the water solvent. The cross-relaxation rates for fluorines in the different moieties along the surfactant vary strongly by the resonance frequency in the investigated range of 188-470 MHz. This frequency dependence indicates that the cross-relaxation between water and amphiphilic aggregates is not controlled solely by the fast local water dynamics but significantly contributed to by the long-range translational diffusion of water. The cross-relaxation rates, analyzed in the framework of a model (Nordstierna, L.; Yushmanov, P. V.; Furó, I. J. Chem. Phys. 2006, 125, 074704), provide information about the dynamic retardation of water molecules by the micellar headgroup region and the location of the various moieties along the hydrophobic tail with respect to the water-micelle interface. Both intermolecular cross-relaxation and aggregation-induced 19F chemical shift changes indicate no direct water contact to fluorines except for those closest to the head group.

Proton dynamics in the perchloric acid clathrate hydrate HClO4⋅5.5H2O

In the perchloric acid clathrate hydrate HClO4.5.5H2O, the perchlorate anions are contained inside an aqueous host crystalline matrix, positively charged because of the presence of delocalized acidic protons. Our experimental results demonstrate that the microscopic mechanisms of proton conductivity in this system are effective on a time scale ranging from nanosecond to picosecond. In the present paper, we discuss more specifically on the relaxation processes occurring on a nanosecond time scale by combining high-resolution quasielastic neutron scattering and 1H pulse-field-gradient nuclear magnetic resonance experiments. The combination of these two techniques allows us to probe proton dynamics in both space and time domains. The existence of two types of proton dynamical processes has been identified. The slowest one is associated to long-range translational diffusion of protons between crystallographic oxygen sites and has been precisely characterized with a self-diffusion coefficient of 3.5 x 10(-8) cm2/s at 220 K and an activation energy of 29.2+/-1.4 kJ/mol. The fastest dynamical process is due to water molecules' reorientations occurring every 0.7 ns at 220 K with an activation energy of 17.4+/-1.5 kJ/mol. This powerful multitechnique approach provides important information required to understand the microscopic origin of proton transport in an ionic clathrate hydrate.

Hierarchy of dynamic structure of super-ionic conducting glass CuI–Cu 2MoO 4

A composite glass, CuI–Cu 2MoO 4, has an ionic conductivity at room temperature, corresponding to ionic liquid conductivity. Pulsed neutron scattering and XAFS measurements show that the main framework of the sample is constructed from [MoO 4]-tetrahedra and [MoO 6]-octahedra. Cu + ions in the glass are bound to 2 O atoms or 4 I atoms. he atomic motion below 20 meV in the sample measured at room temperature is close to that of the α-CuI crystalline state near its melting point. In addition to the Boson peak, above 200 K, quasielastic neutron scattering due to the local diffusion of Cu + ions is found at energies 5 meV. The long-range translational diffusion of the Cu + ions in the sample is directly demonstrated by high resolution quasielastic neutron scattering results. The broadening is less than 100 μeV. The hierarchic static and dynamic structure of the sample is shown to control the properties of the super-ionic conducting glass, CuI–Cu 2MoO 4.

Hydrogen Motion In Lanthanum Hydrides By Means Of Quasielastic Neutron Scattering

ABSTRACTThe hydrogen dynamics in LaHx was studied for different concentrations x ≥ 2.5 at different temperatures by quasielastic neutron scattering. The neutron spectra show the existence of both long-range translational diffusion and localized motion. The prefactor of the jump rate for the first process exhibits a maximum around x= 2.75. The radius for the second motion is in agreement with neutron powder diffraction measurements which support a localized motion of H confined in an octahedral site.

Evidence for Molecular Translational Diffusion during the Crystallization of Amorphous Solid Water

Molecular diffusion in amorphous solid water is studied by measuring the extent of isotope exchange and mixing in vapor-deposited ultrathin films using temperature-programmed desorption. Our results indicate there is long-range translational diffusion that occurs concomitantly with the amorphous to crystalline ice phase transition. The apparent diffusivity estimated from these data is a factor of 106±1 greater than an Arrhenius extrapolation for diffusion in crystalline ice. This finding suggests that the amorphous material exhibits liquid-like translational diffusion prior to crystallization at temperatures near 155 K.

Light – microscopic analysis of the physical properties of cytoplasm in living cells

We have used a variety of optical techniques to explore the intracellular constraints on diffusion, cytoplasmic compartmentalization and biomechanics of living tissue culture cells. Fluorescence ratio imaging measurements of solvent viscosity in single cells indicate that the solvent viscosity of cytoplasm does not differ detectably from bulk water. Nevertheless, data obtained by fluorescence recovery after photobleaching (FRAP) of inert tracer molecules suggest that macromolecular crowding and molecular sieving retard the long-range translational diffusion of protein-sized molecules from 4 to 50-fold relative to their diffusion in dilute aqueous solution.Transient binding interactions with intracellular components may additionally retard the diffusion of proteins. Recent FRAP studies of fluorescent analogs of calmodulin suggest that as little as 5% of this protein is freely diffusing, even in unstimulated cells where intracellular [Ca2+] is below the threshold for activation of calmodulin binding to calcium-dependent targetsin vitro. Preliminary data show that binding is mitigated or abolished by charge reversal mutations in the central helix of calmodulin or by a point mutation in calcium binding loop 4 that greatly reduces the affinity of calmodulin for Ca2+.