Quasielastic Neutron Scattering Study Research Articles

QENS study on local segmental dynamics of polyelectrolytes in complex coacervates

The segmental dynamics of complex coacervates were investigated as a function of salt concentration and a charge type using quasi-elastic neutron scattering (QENS). The coacervates were prepared by mixing two oppositely charged polyether-based polyelectrolyte solutions functionalized for cationic ammonium and guanidinium groups or anionic sulfonate groups. For the QENS time scale, the intrinsic ion pairs were considered effective cross-links, and therefore the dynamics of chain segments between the intrinsic junctions were probed. We observed that as salt concentration increases, both the number of mobile chain segments and their segmental dynamics increase for ammonium-based coacervates where the electrostatic interaction primarily governs the phase behavior. However, the guanidinium-based coacervates show slow and nearly identical segmental relaxation independent of the salt concentration, which is potentially attributed to the non-electrostatic interactions. The combination of the chemical structure of ionic moieties, salt concentration, and temperature plays a significant role to determine polymer chain dynamics mainly affected by the strength of ionic bonding.

Nanoscale Structure and Dynamics of Model Membrane Lipid Raft Systems, Studied by Neutron Scattering Methods

Quasi-elastic neutron scattering (QENS) and small angle neutron scattering (SANS), in combination with isotopic contrast variation, have been used to determine the structure and dynamics of three-component lipid membranes, in the form of vesicles, comprising an unsaturated [palmitoyl-oleoyl-phosphatidylcholine (POPC) or dioleoyl-phosphatidylcholine (DOPC)], a saturated phospholipid (dipalmitoyl-phosphatidylcholine (DPPC)), and cholesterol, as a function temperature and composition. SANS studies showed vesicle membranes composed of a 1:1:1 molar ratio of DPPC:DOPC:cholesterol and a 2:2:1 molar ratio of DPPC:POPC:cholesterol phase separated, forming lipid rafts of ∼18 and ∼7 nm diameter respectively, when decreasing temperature from 308 to 297 K. Phase separation was reversible upon increasing temperature. The larger rafts observed in systems containing DOPC are attributed to the greater mis-match in lipid alkyl chains between DOPC and DPPC, than for POPC and DPPC. QENS studies, over the temperature range 283–323K, showed that the resulting data were best modelled by two Lorentzian functions: a narrow component, describing the “in-plane” lipid diffusion, and a broader component, describing the lipid alkyl chain segmental relaxation. The overall “in-plane” diffusion was found to show a significant reduction upon increasing temperature due to the vesicle membranes transitioning from one containing rafts to one where the component lipids are homogeneously mixed. The use of different isotopic combinations allowed the measured overall reduction of in-plane diffusion to be understood in terms of an increase in diffusion of the saturated DPPC lipid and a corresponding decrease in diffusion of the unsaturated DOPC/POPC lipid. As the rafts are considered to be composed principally of saturated lipid and cholesterol, the breakdown of rafts decreases the exposure of the DPPC to cholesterol whilst increasing the exposure of cholesterol to unsaturated lipid. These results show the sensitivity of lipid diffusion to local cholesterol concentration, and the importance of considering the local, rather that the global composition of a membrane when understanding the diffusion processes of lipids within the membrane. The novel combination of SANS and QENS allows a non-intrusive approach to characterize the structure and dynamics occurring in phase-separated model membranes which are designed to mimic the lateral heterogeneity of lipids seen in cellular membranes–a heterogeneity that can have pathological consequences.

Quasi Elastic Neutron Scattering Study on Water Dynamics in Nafion Membrane

Quasi Elastic Neutron Scattering Study on Water Dynamics in Nafion Membrane

The Effect of Si/Al Ratio on Local and Nanoscale Water Diffusion in H-Zsm-5: A Quasielastic Neutron Scattering and Molecular Dynamics Simulation Study

The Effect of Si/Al Ratio on Local and Nanoscale Water Diffusion in H-Zsm-5: A Quasielastic Neutron Scattering and Molecular Dynamics Simulation Study

The charge transport mechanism in Brønsted-acidic protic ionic liquid/water systems – An NMR and QENS study

In this study, a protic ionic liquid (PIL), 2-Sulfoethylmethylammonium triflate [2-Sema][TfO] is considered as a potential new proton conducting electrolyte for future polymer membrane fuel cells capable of ambient air operation above 100 °C. The proton dynamics of the PIL with residual water are examined as a function of the hydration level on different time scales using pulsed field gradient nuclear magnetic resonance (PFG-NMR) and quasi-elastic neutron scattering (QENS). The separation of the different contributing relaxation processes enables a quantification of the proton fractions for the underlying hopping or vehicular motions. The hopping motion of the water in the time scale of picosecond and the vehicular motion in the time scale of nanosecond are detected by means of QENS. Such dynamic processes can be well described by the Chudley-Elliot jump model. This emphasised the presence of fixed jump lenghts. In the timescale of millisecond, the cooperative transport of the active protons of the acidic SO3H group and of the H2O molecules, as well as the vehicular transport of the PIL cations are detected by NMR. The different diffusion coefficients obtained by the NMR and QENS techniques are discussed in detail.

Quasielastic Neutron Scattering Study on Polymorphism of Glycerol Deuterated Triacylglycerols: Comparison with Saturated, Trans-unsaturated and Cis-unsaturated Triacylglycerols

The relationship between the polymorphism and dynamic properties of acyl chains in triacylglycerols (TAGs) was studied using incoherent quasielastic neutron scattering (IQENS). The influence of cis...

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.

Quasi-Elastic Neutron Scattering Studies on Hydration Water in Phospholipid Membranes

The dynamic behavior of hydration water in phospholipid membranes has been investigated to understand the relationship between water and biological molecules using various experimental techniques. Quasi-elastic neutron scattering (QENS) is an effective method for this purpose because the dynamic behaviors of both water and lipid molecules could be identified by using selective deuteration. In addition, the measurable ranges from the 10−12 to 10−9 s time scale and the 10−11 to 10−8 m length scale are suitable to investigate the slowing down of water molecules due to their interaction with lipid membranes. In this mini-review, QENS experiments on the dynamic behavior of hydration water molecules in neighboring phospholipid membranes are summarized.

Investigation of Na Ion Dynamics in P2-Type Na2/3[Ni1/3Ti2/3]O2: A Combination of Quasi-Elastic Neutron Scattering and First-Principles Molecular Dynamics Study

While the lithium-ion batteries are dominating the portable electrical energy storage market owing to the high energy density, their implementation in large-scale energy storage is likely to be constrained by the price and availability of lithium resources. With overwhelming advantages of huge redundancy and low cost, the sodium ion batteries provide a potential solution to the increasing needs of the large-scale static energy storage for the intermittent renewable energy resources such as wind and solar energy. Thus, the materials for sodium-ion batteries have been attracting increasing research interests in recent years. In the search for the suitable electrode materials for sodium-ion batteries, the layered sodium transition metal (TM) oxide materials (NaxTMO2) are commonly studied following the successfully-commercialized example of their Li analogs (LixTMO2). Among these layered Na insertion materials, the P2-Type Na2/3[Ni1/3Ti2/3]O2 have been found to exhibit good electrochemical properties as either positive or negative electrode in batteries1. Such bi-functionality is due to the coexistence of the high redox potential couple Ni2+/ Ni3+ and the low redox potential couple Ti4+/ Ti3+, bringing further advantages of lower manufacture cost and simpler design. The dynamics of ionic charge carriers in electrodes, i.e., the sodium ions, are critical to the performance of the battery. In this study, we investigated the dynamics of Na ions in P2-Type Na2/3[Ni1/3Ti2/3]O2 with a combination of the experimental and computational tools in the similar time and length scale to complement each other. The quasi-elastic neutron scattering (QENS) experiments were conducted in the temperature range of 400 to 700K. The Na self-diffusion properties were derived from the broadening of the elastic peak (quasi-elastic features) in the small Q range of 0.2 to 2 Å-1. The experimental results provided the verification for the first-principles molecular dynamics (MD) simulations based on density functional theory (DFT). The intermediate scattering function in the time domain was calculated based on the MD trajectory to compare with the experimental results. The diffusion behavior of the Na ions can be described by the Singwi-Sjolander jump model2, in which the Na ion jumps, with an average jump distance d, take much less time than the residence time τ when the ions vibrate in certain sites. Such integrated experimental and computational study provides a better understanding of Na diffusion mechanism in the material as well as insights in designing electrode materials with better performance.

A Quasielastic Neutron Scattering Study of Water Diffusion in Model Anion Exchange Membranes over Localized and Extended Volume Increments

Anion exchange membranes provide the basis for potentially more cost-effective fuel cells, electrochemical energy conversion, and energy storage. The membranes consist of a polymer domain with tethered cationic side groups and a pervading aqueous domain that conducts water and anions. Water diffusion is investigated through temperature-dependent quasielastic neutron scattering (QENS) at application relevant water contents, employing the BASIS spectrometer at ORNL. The spectrometer resolves two distinct quasielastic components corresponding to localized water diffusion (on length scales lower than the water domain thickness) and to extended water diffusion (on length scales approaching the water domain thickness). Water diffusion coefficients in membranes are time and length scale dependent and water exchanges between diffusion on a localized and extended volume increment. An exchange model for QENS is developed to describe the exchange of water between the two volume increments. The populations and lifeti...

Hydration-dependent dynamics of water in calcium-silicate-hydrate: A QENS study by global model.

In a saturated cement paste, there are three different types of water: the structural water chemically reacted with cement, the constrained water absorbed to the surface of the pores, and the free water in the center of the pores. Each type has different physicochemical state and unique relation to cement porosity. The different water types have different dynamics which can be detected using quasi-elastic neutron scattering (QENS). Since the porosity of a hardened cement paste is impacted strongly by the water to cement ratio (w/c), it should be possible to extract the hydration dependence of the pores by exploiting the dynamical parameters of the confined water. Three C-S-H samples with different water levels, 8%, 17% and 30% were measured using QENS. The measurements were carried out in the scattering vector, Q, range from 0.5 Å-1 to 1.3 Å-1, and in the temperature interval from 230 K to 280 K. The data were analyzed using a novel global model developed for cement QENS spectra. The results show that while increasing the water content, the structural water index (SWI) decreases and the confining radius, a, increases. Both SWI and a have a linear relationship with the water content. The Arrhenius plot of the translational relaxation time shows that the constrained water dominates the non-structural water at water contents lower than 17%. The rotational activation energy is smaller for lower water content. The analysis demonstrated that our newly proposed global model is practical and useful for analyzing cement QENS data.

Effect of magnetic fields on the methyl rotation in a paramagnetic cobalt(ii) complex. Quasielastic neutron scattering studies.

Molecular dynamics is a fundamental property of metal complexes. These dynamic processes, especially for paramagnetic complexes under external magnetic fields, are in general not well understood. Quasielastic neutron scattering (QENS) in 0-4 T magnetic fields has been used to study the dynamics of Co(acac)2(D2O)2 (1-d4, acac = acetylacetonate). At 80-100 K, rotation of the methyl groups on the acac ligands is the dominant dynamical process. This rotation is slowed down by the magnetic field increase. Rotation times at 80 K are 5.6(3) × 10-10 s at 0 T and 2.04(10) × 10-9 s at 4 T. The QENS studies suggest that methyl groups in these paramagnetic Co(ii) molecules do not behave as isolated units, which is consistent with results from earlier magnetic susceptibility studies indicating the presence of intermolecular interactions. DFT calculations show that unpaired electron spin density in 1 is dispersed to the atoms of both acac and H2O ligands. Methyl torsions in 1-d4 have also been observed at 5-100 K in inelastic neutron spectroscopy (INS). The QENS and INS results here help understand the dynamics of the compound in the solid state.

Dynamics of Silica Aerogel’s Hydrophobic Groups: A Quasielastic Neutron Scattering Study

Silica aerogels can be hydrophobized with a variety of functional groups, resulting in different material properties. Here we compare the mobility of hydrophobic groups using neutron spectroscopy. Data were collected between 1.5 and 300 K on the MARS backscattering spectrometer at SINQ (PSI) for three different silica aerogels and octakis(trimethylsiloxy)silsesquioxane (Q8M8) as a reference. The mobility persists to below 10 K for silica aerogels whose surfaces are decorated with trimethylsilyl (TMS) and ethoxy groups. The high low-temperature mobility of the hydrogen in these surface functional groups originates from the large rotational freedom in the aerogel’s open mesopores. In contrast, the mobility freezes in below 30 K for Q8M8, consistent with a reduced rotational freedom within the dense Q8M8 crystal structure. At room temperature, both the rotations of hydrogen within the methyl groups and the rotations of the entire TMS groups contribute to the mean square displacements for both the silylated s...

Water Mobility in Chalk: A Quasielastic Neutron Scattering Study

Water mobility through porous rock has a role to play in many systems, such as contaminant remediation, CO2 storage, and oil recovery. We used inelastic and quasielastic neutron scattering to describe water dynamics in two different chalk samples that have similar pore volume (ranging from tens of micrometers to a few nanometers) but different water uptake properties. We observed distinct water populations, where the analysis of the quasielastic data shows that after the hydration process most of the water behaves as bulk water. However, the lack of quasielastic signal, together with the observation of a translational mode at 10 meV, imply that in chalk samples that take up less water confinement occurs mostly in the pore volume that is accessible with nitrogen adsorption measurements.

Aminopolymer Mobility and Support Interactions in Silica-PEI Composites for CO2 Capture Applications: A Quasielastic Neutron Scattering Study.

Composite gas sorbents, formed from an active polymer phase and a porous support, are promising materials for the separation of acid gases from a variety of gas streams. Significant changes in sorption performance (capacity, rate, stability etc.) can be achieved by tuning the properties of the polymer and the nature of interactions between polymer and support. Here we utilize quasielastic neutron scattering (QENS) and coarse-grained molecular dynamics (MD) simulations to characterize the dynamic behavior of the most commonly reported polymer in such materials, poly(ethylenimine) (PEI), both in bulk form and when supported in a mesoporous silica framework. The polymer chain dynamics (rotational and translational diffusion) are characterized using two neutron backscattering spectrometers that have overlapping time scales, ranging from picoseconds to a few nanoseconds. Two modes of motion are detected for the PEI molecule in QENS. At low energy transfers, a "slow process" on the time scale of ∼200 ps is found and attributed to jump-mediated, center-of-mass diffusion. A second, "fast process" at ∼20 ps scale is also found and is attributed to a locally confined, jump-diffusion. Characteristic data (time scale and spectral weight) of these processes are compared to those characterized by MD, and reasonable agreement is found. For the nanopore-confined PEI, we observe a significant reduction in the time scale of polymer motion as compared to the bulk. The impacts of silica surface functionalization and of polymer fill fraction in the silica pores (controlling the portion of polymer molecules in contact with the pore walls), are both studied in detail. Hydrophobic functionalization of the silica leads to an increase of the PEI mobility above that in native silanol-terminated silica, but the dynamics are still slower than those in bulk PEI. Sorbents with faster PEI dynamics are also found to be more efficient for CO2 capture, possibly because sorption sites are more accessible than those in systems with slower PEI dynamics. Thus, this work supports the existence of a link between the affinity of the support for PEI and the accessibility of active sorbent functional groups.

Quasi-Elastic Neutron Scattering Study of Hydration Water in Synthetic Cement: An Improved Analysis Method Based on a New Global Model

Quasi-elastic neutron scattering (QENS) measuring the energy transfer (E) and momentum exchange (Q) between a neutron and a sample target is often used to study hydration water in cement. A global model introducing a Q-independent elastic component, a Q-dependent elastic component, and a quasi-elastic component with explicit translational and rotational intermediate scattering functions has been developed to analyze the quasi-elastic neutron scattering (QENS) spectra from water confined in cement pastes. To demonstrate the advantage of this model, it has been used to analyze four sets of cement data: calcium–silicate–hydrate (CSH), pure magnesium–silicate–hydrate (MSH), MSH with aluminum–silicate nanotubes (ASN), and MSH with carboxyl-functionalized ASN (ASN-COOH), previously studied with a much simpler model. It is shown that, with this new model, several important parameters that describe the structure of the cementitious materials can be extracted from the QENS data. Particularly, the structural water ...

Molecular Dynamics of POPC Phospholipid Bilayers through the Gel to Fluid Phase Transition: An Incoherent Quasi-Elastic Neutron Scattering Study

The microscopic dynamics for the gel and liquid-crystalline phase of highly aligned D2O-hydrated bilayers of 1-palmitoyl-oleoyl-sn-glycero-phosphocholine (POPC) were investigated in the temperature range from 248 to 273 K by using incoherent quasi-elastic neutrons scattering (QENS). We develop a model for describing the molecular motions of the liquid phase occurring in the 0.3 to 350 ps time range. Accordingly, the complex dynamics of hydrogen are described in terms of simple dynamical processes involving different parts of the phospholipid chain. The analysis of the data evidences the existence of three different motions: the fast motion of hydrogen vibrating around the carbon atoms, the intermediate motion of carbon atoms in the acyl chains, and the slower translational motion of the entire phospholipid molecule. The influence of the temperature on these dynamical processes is investigated. In particular, by going from gel to liquid-crystalline phase, we reveal an increase of the segmental motion mainly affecting the terminal part of the acyl chains and a change of the diffusional dynamics from a localized rattling-like motion to a confined diffusion.

New Insights into the Molecular Dynamics of P3HT:PCBM Bulk Heterojunction: A Time-of-Flight Quasi-Elastic Neutron Scattering Study.

The molecular dynamics of organic semiconductor blend layers are likely to affect the optoelectronic properties and the performance of devices such as solar cells. We study the dynamics (5-50 ps) of the poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) blend by time-of-flight quasi-elastic neutron scattering, at temperatures in the range 250-360 K, thus spanning the glass transition temperature region of the polymer and the operation temperature of an OPV device. The behavior of the QENS signal provides evidence for the vitrification of P3HT upon blending, especially above the glass transition temperature, and the plasticization of PCBM by P3HT, both dynamics occurring on the picosecond time scale.

Solvent Dynamics in Solutions of PNIPAM in Water/Methanol Mixtures-A Quasi-Elastic Neutron Scattering Study.

The solvent dynamics of concentrated solutions of poly(N-isopropylacrylamide) (PNIPAM, 25 wt %) in water/methanol mixtures (85:15 v/v) are measured with the aim of shedding light onto the cononsolvency effect. Quasi-elastic neutron scattering (QENS) with contrast variation has been carried out at temperatures below and above the cloud point by using in the first set of experiments the mixture H2O:d-MeOD (d-MeOD denotes fully deuterated methanol) as a solvent and in the second set of experiments the mixture D2O:MeOH (MeOH denotes methanol). As a reference, bulk H2O, bulk MeOH and the mixtures H2O:d-MeOD and D2O:MeOH (both 85:15 v/v) have been investigated as well. In the PNIPAM solution in H2O:d-MeOD, two water populations are identified, namely strongly and less strongly arrested water. At the cloud point, the former is partially released from PNIPAM. The diffusion coefficient of the latter one is similar to the one in the water/methanol mixture, and its residence time decreases at the cloud point. The PNIPAM solution in D2O:MeOH reveals similar dynamics to the one in H2O:d-MeOD which may reflect that the dynamics of MeOH near the PNIPAM chain is similar to the one of H2O. The similarity may, however, partially be due to H/D exchange between D2O and MeOH. In both PNIPAM solutions, the mean-square displacement of the PNIPAM chain decreases gradually above the cloud point.

A Quasi-Elastic Neutron Scattering Study of the Dynamics of Electrically Constrained Water.

We have measured the quasi-elastic neutron scattering (QENS) of an electrohydrodynamic liquid bridge formed between two beakers of pure water when a high voltage is applied, a setup allowing to investigate water under high-voltage without high currents. From this experiment two proton populations were distinguished: one consisting of protons strongly bound to oxygen atoms (immobile population, elastic component) and a second one of quasi-free protons (mobile population, inelastic component) both detected by QENS. The diffusion coefficient of the quasi-free protons was found to be D = (26 ± 10) × 10(-5) cm(2) s(-1) with a jump length lav ∼ 3 Å and an average residence time of τ0 = 0.55 ± 0.08 ps. The associated proton mobility in the proton channel of the bridge is ∼9.34 × 10(-7) m(2) V(-1) s(-1), twice as fast as diffusion-based proton mobility in bulk water. It also matches the so-called electrohydrodynamic or "apparent" charge mobility, an experimental quantity which so far has lacked molecular interpretation. These results further corroborate the proton channel model for liquid water under high voltage and give new insights into the molecular mechanisms behind electrohydrodynamic charge transport phenomena and delocalization of protons in liquid water.