129Xe Nuclear Magnetic Resonance Research Articles

Role of particle gradation of clay–sand mixtures on the interfacial adhesion performance of polymer coatings

Abstract The interface performance between clay–sand mixtures and concrete structures is governed by the mixture's composition and its physical properties. Moisture content and particle-size distribution play important roles in deciding the mixture's arrangement of soil particles, porosity, hydraulic conductivity and behaviour under various mechanical loadings. Application of a polymer interfacial coating can improve the bond performance between soils and concrete mainly via interfacial friction/mechanical interlocking. The present work analyses the development of interfacial strength between clay–sand mixtures and a polymer coating with changes in particle gradation. The multi-scale mechanisms at the interface are investigated, giving primary attention to soil porosity. A 50:50 clay–sand mixture exhibited a greater interfacial adhesive performance compared to other soil mixtures. In addition, the moisture-controlled pores and gradation-controlled pores demonstrated differences in macroscale interfacial strength. Both mercury intrusion porosimetry (MIP) and 129Xe nuclear magnetic resonance (NMR) were utilized to detect the pore structure of the mixtures. 129Xe-NMR revealed the pore distribution of the mixtures as ranging from macropores to nanopores, and MIP complemented the pore information by determining the critical pore entry diameter in the macropore regime. Mesopores dominated with increasing fine sand content until a threshold value was reached; thereafter, merging of pores occurred and macropores dominated.

Porosity of Molecularly Imprinted Polymers Investigated by 129Xe NMR Spectroscopy.

Molecularly imprinted polymers (MIPs) display intriguing recognition properties and can be used as sensor recognition elements or in separation. In this work, we investigated the formation of hierarchical porosity of compositionally varied MIPs using 129Xe Nuclear Magnetic Resonance (NMR) and 1H Time Domain Nuclear Magnetic Resonance (TD-NMR). Variable temperature 129Xe NMR established the morphological variation with respect to the degree of cross-linking, supported by 1H TD-NMR determination of polymer chain mobility. Together, the results indicate that a high degree of cross-linking stabilizes the porous structure: highly cross-linked samples display a significant amount of accessible mesopores that instead collapse in less structured polymers. No significant differences can be detected due to the presence of templated pores in molecularly imprinted polymers: in the dry state, these specific shapes are too small to accommodate xenon atoms, which, instead, probe higher levels in the porous structure, allowing their study in detail. Additional resonances at a high chemical shift are detected in the 129Xe NMR spectra. Even though their chemical shifts are compatible with xenon dissolved in bulk polymers, variable temperature experiments rule out this possibility. The combination of 129Xe and TD-NMR data allows attribution of these resonances to softer superficial regions probed by xenon in the NMR time scale. This can contribute to the understanding of the surface dynamics of polymers.

The Rb-Xe coupling effect in optically pumped NMR gyroscopes

Optically pumped nuclear magnetic resonance (NMR) gyroscope has been widely concerned as a future gyroscope with great development potential. In this study, the Rb-Xe coupling effect in this gyroscope between the alkali metal magnetometer and the noble gas NMR is investigated. The theoretical formulae about this effect are obtained based on Bloch equation and demonstrated by experiments. In order to see the influence of this effect on the gyro signal, the Xe NMR frequency changes with the carrier frequency and demodulation phase of the Rb magnetometer are studied with the deduced formulae. More importantly, theoretical analysis shows that this coupling effect can also significantly reduce the efficiency of the dual species scheme, which is used to suppress the influence of magnetic fluctuation. Therefore, to remove the Rb-Xe coupling effect, a simple and preliminary decoupling scheme operated by locking the phase difference to zero is proposed and demonstrated. All this work is of great significance to the improvement of the NMR gyro performance and future applications.

Thermodynamic and Structural Investigation of Xe and Kr Hydrates Containing Tetrahydrofuran for Applications in Gas Capture and Storage

Noble gases, such as xenon (Xe) and krypton (Kr), can be captured in hydrate cages, and thus, gas hydrates can be used as capture and storage media for these gases. This study examined the phase equilibria, crystal structure, and guest distributions of Xe and Kr hydrates in the presence of tetrahydrofuran (THF). The three-phase [hydrate (H)–liquid (L)–vapor (V)] equilibria of the Xe + THF + water and Kr + THF + water systems were measured at two different concentrations of THF (1.0 and 5.6 mol %) to examine the thermodynamic promotion effect of THF on Xe and Kr hydrates. The inclusion of THF in the hydrate cages led to significant shifts in the equilibrium curves of the Xe and Kr hydrates to lower pressure and higher temperature regions. The powder X-ray diffraction patterns and the 129Xe and 13C nuclear magnetic resonance spectra revealed that THF was enclathrated in the large (51264) cages of structure II hydrates and that the small (512) cages were occupied by Xe (or Kr). In particular, at 1.0 mol % THF, the large 51264 cages were partly occupied by Xe (or Kr) as a result of tuning behavior. The experimental results expand the basic understanding of hydrate-based gas capture and storage for Xe and Kr.

Effect of electrochemical cycling on microstructures of nanocomposite silicon electrodes using hyperpolarized 129Xe and 7Li NMR spectroscopy

The microstructural stability of composite electrodes during electrochemical cycling is critically important as it dictates the performance of Li-ion batteries. The issue becomes even more important for the high capacity alloying anode such as silicon that typically exhibits dramatic lithiation–delithiation-induced volume changes. The solid electrolyte interphase (SEI) layer formed on the active electrode surface has a profound effect on the overall microstructural stability of composite electrodes. An ideal SEI layer allows Li+ ions in and out of the electrode, but is an insulator to electrons, preventing the electrolyte from being further reduced. However, the SEI layers formed during initial lithiation may experience changes or degradation with subsequent cycling, adversely affecting the electrode performance. A combination of hyperpolarized 129Xe and 7Li nuclear magnetic resonance spectroscopies was applied to probe the microstructures of nanocomposite silicon electrodes at various stages of the lithiation–delithiation cycle. The results obtained from this study shed light on the degradation mechanism of nanocomposite Si electrodes upon electrochemical cycling and should prove useful in the effort to design more robust electrodes in the future.

Microsized Pore Structure Determination in EPDM Rubbers Using High-Pressure 129Xe NMR Techniques.

Microsized pore parameters, such as pore size and distance between pores in a series of model EPDM rubbers, were determined in situ under the pressure of 500 psi using 129Xe nuclear magnetic resonance (NMR) techniques: spin-lattice (T1) and spin-spin (T2) relaxation measurements, pulsed-field gradient (PFG) NMR, and two-dimensional exchange spectroscopy (2D EXSY). The T1/T2 (≫1) ratio for the xenon confined in the pores is larger than that for nonconfined free xenon. This suggests that almost the entire pore surface interacts with xenon atoms like a closed pore. While these pores still connect each other through very narrow diffusion/exchange channels, it is possible to observe the echo decay in PFG-NMR and cross-peaks in 2D EXSY. The results show that both diffusion (Dpore ≈ 2.1 × 10-10 m2/s) and exchange (exchange rate, τexch = a few tens of milliseconds) of xenon between a pore within the material and outer surface are prolonged. The exchange distances (l), which correspond to the xenon gas penetration depth, were estimated to be 70-100 μm based on the measured diffusion coefficients and exchange rate (1/τexch). NMR diffraction analysis reveals that pore size (a) and pore distance (b) are on the order of magnitude of micrometers and tens of micrometers, while the diffusion coefficients of xenon gas in the diffusion channels (Deff) are about 10-8 m2/s. Overall, this study suggests that the pores with a few micrometers connected through very narrow flowing channels with the length of several tens of micrometers are developed 70 to 100 μm below the rubber surface. Furthermore, the overall steady-state diffusion of xenon is slower, approximately 2 orders of magnitudes, than the diffusion in the channel between the pores. The pore and exchange distances correlated with the composition of rubbers showed that the properties of EPDM rubber as a high-pressure gas barrier could be improved by reducing the size of cracks and the depth of gas penetration by the addition of both carbon black and silica fillers.

129Xe NMR analysis reveals efficient gas transport between inborn micro-, meso- and macropores in geopolymers

Geopolymers are alumino-silicates with inborn mesoporous structures. For construction applications, the knowledge on the porous structure and fluid dynamics is important. In this work, the interconnected pore structures of metakaolin-based geopolymers and dynamics of guest molecules were studied by 129Xe nuclear magnetic resonance (NMR), by exploiting the high sensitivity of the 129Xe chemical shift to its local environment. The 129Xe NMR spectra revealed three different pore structures experienced by xenon gas in the geopolymers: the first associated with microporous zeolite-analogous cages (≤2 nm), the second with the mesopores (2–50 nm) and the third with free Xe in larger voids. The sizes of mesopores, heat of adsorption as well as pore connectivity were determined as a function of water-to-solid (w/s) ratio. The results show that the pore structures can be tailored by changing the w/s ratio, providing tunability both for potential construction, catalysis, and adsorbent applications.

129Xe: A Wide-Ranging NMR Probe for Multiscale Structures

Porous materials are ubiquitous systems with a large variety of applications from catalysis to polymer science, from soil to life science, from separation to building materials. Many relevant systems of biological or synthetic origin exhibit a hierarchy, defined as spatial organization over several length scales. Their characterization is often elusive, since many techniques can only be employed to probe a single length scale, like the nanometric or the micrometric levels. Moreover, some multiscale systems lack tridimensional order, further reducing the possibilities of investigation. 129Xe nuclear magnetic resonance (NMR) provides a unique and comprehensive description of multiscale porous materials by exploiting the adsorption and diffusion of xenon atoms. NMR parameters like chemical shift, relaxation times, and diffusion coefficient allow the probing of structures from a few angstroms to microns at the same time. Xenon can evaluate the size and shape of a variety of accessible volumes such as pores, layers, and tunnels, and the chemical nature of their surface. The dynamic nature of the probe provides a simultaneous exploration of different scales, informing on complex features such as the relative accessibility of different populations of pores. In this review, the basic principles of this technique will be presented along with some selected applications, focusing on its ability to characterize multiscale materials.

Origin of the Hierarchical Structure of Dendritic Fibrous Nanosilica: A Small-Angle X-ray Scattering Perspective.

The discovery of dendritic fibrous nanosilica (DFNS) has attracted great attention to the field of catalysis, CO2 capture, drug delivery due to its distinct morphology, and pore size distribution. Despite extensive research, the understanding of the DFNS formation process and its internal structure remains incomplete as microscopy and gas sorption techniques were not able to provide necessary in-depth structural information due to their inherent limitations. In the current work, we present a structural model of DFNS derived using small-angle X-ray scattering (SAXS) supported by 129Xe nuclear magnetic resonance (NMR), which provided intricate details of DFNS and its internal structure. Mechanistic understanding of the DFNS formation and growth process was achieved by performing time-resolved SAXS measurements during the synthesis of DFNS, which unveils the evolution of two levels of a bicontinuous microemulsion structure responsible for intricate DFNS morphology. The validity and the accuracy of the SAXS method and the model were successfully established through a direct correlation among the functionality of the DFNS scattering profile and its pore size distribution, as well as results obtained from the 129Xe NMR studies. It has been established that the DFNS structure originates from direct modulation of the bicontinuous structure controlled by a surfactant, a co-surfactant, and the silicate species formed during hydrolysis and the condensation reaction of the silica precursor.

Identification of extracellular nanoparticle subsets by nuclear magnetic resonance.

Exosomes are a subset of secreted lipid envelope-encapsulated extracellular vesicles (EVs) of 50–150 nm diameter that can transfer cargo from donor to acceptor cells. In the current purification protocols of exosomes, many smaller and larger nanoparticles such as lipoproteins, exomers and microvesicles are typically co-isolated as well. Particle size distribution is one important characteristics of EV samples, as it reflects the cellular origin of EVs and the purity of the isolation. However, most of the physicochemical analytical methods today cannot illustrate the smallest exosomes and other small particles like the exomers. Here, we demonstrate that diffusion ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR) method enables the determination of a very broad distribution of extracellular nanoparticles, ranging from 1 to 500 nm. The range covers sizes of all particles included in EV samples after isolation. The method is non-invasive, as it does not require any labelling or other chemical modification. We investigated EVs secreted from milk as well as embryonic kidney and renal carcinoma cells. Western blot analysis and immuno-electron microscopy confirmed expression of exosomal markers such as ALIX, TSG101, CD81, CD9, and CD63 in the EV samples. In addition to the larger particles observed by nanoparticle tracking analysis (NTA) in the range of 70–500 nm, the DOSY distributions include a significant number of smaller particles in the range of 10–70 nm, which are visible also in transmission electron microscopy images but invisible in NTA. Furthermore, we demonstrate that hyperpolarized chemical exchange saturation transfer (Hyper-CEST) with 129Xe NMR indicates also the existence of smaller and larger nanoparticles in the EV samples, providing also additional support for DOSY results. The method implies also that the Xe exchange is significantly faster in the EV pool than in the lipoprotein/exomer pool.

129Xe NMR on Porous Materials: Basic Principles and Recent Applications

AbstractA large number of functional and catalytic materials exhibit porosity, often on different length scales and with a hierarchical structure. The assessment of pore sizes, pore geometry, and pore interconnectivity is complex and usually not feasible by classical spectroscopic and diffraction techniques. One of the most powerful methods to probe these parameters is nuclear magnetic resonance (NMR) spectroscopy of xenon, which is introduced into the pore system. Adsorbed to the pore walls, it acts as probe nucleus. In this tutorial review, an introduction into the basic principles of 129Xe NMR spectroscopy and the models developed to determine pore sizes in different materials are given. The possibilities and limitations of this method for obtaining insights into hierarchical structures of functional materials are highlighted and a review of recent works is presented.

A non-interference method for measurement of transverse relaxation time of the alkali metal magnetometer in nuclear magnetic resonance oscillator

The transverse relaxation time τ2 of the built-in alkali metal magnetometer is an important parameter to characterize its performance and the working state of the nuclear magnetic resonance (NMR) oscillator system. The method we propose to measure τ2 is to apply the frequency-scanning magnetic fields along the x-axis and obtain τ2 from the 3 dB bandwidth of the measured x-axis amplitude–frequency response curve of the magnetometer. We give a detailed and rigorous theoretical derivation by using the perturbation-iteration method to demonstrate the rationality of this measuring method. Then, both the actual experiment and the simulation experiment results are presented to verify the validity and good accuracy of the method. All the sampled frequencies in the scanning range should be far away from both the 129Xe and 131Xe NMR frequencies, to avoid introducing any influence on the nuclear spins; therefore, this method can provide a non-interference way for measurement of τ2, helping us monitor the NMR oscillator system’s state under normal operating conditions.

Synthesis of highly active ETS-10-based titanosilicate for heterogeneously catalyzed transesterification of triglycerides.

In this contribution, the preparation of hierarchically structured ETS-10-based catalysts exhibiting notably higher activity in the conversion of triolein with methanol compared to microporous titanosilicate is presented. Triolein, together with its unsaturated analog trilinolein, represent the most prevalent triglycerides in oils. The introduction of mesopores by post-synthetic treatment with hydrogen peroxide and a subsequent calcination step results in the generation of an additional active surface with Brønsted basic sites becoming accessible for triolein and enhancing the rate of transesterification. The resulting catalyst exhibits a comparable triolein conversion (≈73%) after 4 h of reaction to CaO (≈76%), which is reportedly known to be highly active in the transesterification of triglycerides. In addition, while CaO showed a maximum conversion of 83% after 24 h, the ETS-10-based catalyst reached 100% after 8 h, revealing its higher stability compared to CaO. The following characteristics of the catalysts were experimentally addressed – crystal structure (X-ray diffraction, transmission electron microscopy), crystal shape and size (scanning electron microscopy, laser diffraction), textural properties (N2 sorption, Hg porosimetry), presence of hydroxyl groups and active sites (temperature-programmed desorption of NH3 and CO2, 29Si magic angle spinning nuclear magnetic resonance (NMR)), mesopore accessibility and diffusion coefficient of adsorbed triolein (pulsed field gradient NMR), pore interconnectivity (variable temperature and exchange spectroscopy experiments using hyperpolarized 129Xe NMR) and oxidation state of Ti atoms (electron paramagnetic resonance). The obtained results enabled the detailed understanding of the impact of the post-synthetic treatment applied to the ETS-10 titanosilicate with respect to the catalytic activity in the heterogeneously catalyzed transesterification of triglycerides.

Exploring Local Disorder within CAU-1 Frameworks Using Hyperpolarized 129Xe NMR Spectroscopy.

The sorption properties of metal-organic frameworks (MOFs) can be influenced by introducing covalently attached functional side chains, which make this subclass of porous materials promising for applications as diverse as gas storage and separation, catalysis, and drug delivery. The incorporation of side groups usually comes along with disorder, as the synthesis procedures rarely allow for one specific position among a larger group of equivalent sites to be selected. For a series of isoreticular CAU-1 frameworks, chosen as model compounds, one out of four positions at every linker is modified with equal probability. Here, we investigate the influence of this disorder on Ar sorption and 129Xe nuclear magnetic resonance spectroscopy using hyperpolarized 129Xe gas. Models used for predicting the pore dimensions as well as their distributions were derived from the unfunctionalized framework by replacing one proton at every linker with either an amino, an acetamide, or a methyl urea functionality. The resulting structures were optimized using density functional theory (DFT) calculations. Results from void analyses and Monte Carlo force field simulations suggest that for available Ar nonlocal DFT (NLDFT) kernels, neither the pore dimensions nor the distributions induced by the side-chain disorder are well-reproduced. By contrast, we found the 129Xe chemical shift analysis for the shift observed at high temperature to be well-suited to develop a detailed fingerprint of the porosity and side-chain disorder within the isoreticular CAU-1 series. After calibrating the 129Xe limiting shift of the amino-functionalized framework with DFT calculations, the downfield shifts for the other two derivatives are an excellent measure for the reduction of the accessible pore space and reveal a strong preference for the side chains toward the octahedral voids for both cases. We expect that the strategy presented here can be commonly applied to disorder phenomena within MOFs in the future.

Investigation of grafted mesoporous silicon sponge using hyperpolarized 129Xe NMR spectroscopy

Abstract

Theoretical study of the NMR chemical shift of Xe in supercritical condition.

In this work we investigate the level of theory necessary for reproducing the non-linear variation of the 129Xe nuclear magnetic resonance (NMR) chemical shift with the density of Xe in supercritical conditions. In detail we study how the 129Xe chemical shift depends under supercritical conditions on electron correlation, relativistic and many-body effects. The latter are included using a sequential-QM/MM methodology, in which a classical MD simulation is performed first and the chemical shift is then obtained as an average of quantum calculations of 250 MD snapshots conformations carried out for Xe n clusters (n = 2 - 8 depending on the density). The analysis of the relativistic effects is made at the level of 4-component Hartree-Fock calculations (4c-HF) and electron correlation effects are considered using second order Møller-Plesset perturbation theory (MP2). To simplify the calculations of the relativistic and electron correlation effects we adopted an additive scheme, where the calculations on the Xe n clusters are carried out at the non-relativistic Hartree-Fock (HF) level, while electron correlation and relativistic corrections are added for all the pairs of Xe atoms in the clusters. Using this approach we obtain very good agreement with the experimental data, showing that the chemical shift of 129Xe in supercritical conditions is very well described by cluster calculations at the HF level, with small contributions from relativistic and electron correlation effects.

The role of water in methane adsorption and diffusion within nanoporous silica investigated by hyperpolarized 129Xe and 1H PFG NMR spectroscopy

Understanding the properties and behavior of water molecules in restricted geometries, such as the nanopores of rocks, is of interest for shale gas exploitation. We present herein ex situ and in situ nuclear magnetic resonance (NMR) studies on the effects of water on the adsorption and diffusion of methane in nanopores. Silica materials with one-dimensional pores of ZSM-22, MCM-41, and SBA-15, with pore sizes ranging from 0.5 to 6 nm, were chosen as models. Hyperpolarized (HP) 129Xe NMR results show that water adsorption does not affect the pore sizes of ZSM-22 and MCM-41 but reduces that of SBA-15. The presence of water suppresses methane adsorption; this suppression effect is stronger in smaller pores. The self-diffusion coefficients of methane within ZSM-22 and MCM-41 are not significantly influenced by the presence of water, as measured by 1H pulsed field gradient (PFG) NMR. However, within SBA-15, which has a pore size of 6 nm, the diffusion coefficient of methane increases as the amount of water adsorption increases, peaks, and then decreases to a constant value with further water adsorption. These experiments reveal the effects of the pore size and the presence of water on methane adsorption and diffusion in constrained spaces, which could have important implications for flow simulations of methane in shales.

Investigation of Solid Electrolyte Interphase Formed on Si Nanoparticle Composite Electrodes Using Hyperpolarized 129Xe Nuclear Magnetic Resonance Spectroscopy

Solid electrolyte interphase (SEI) plays an important role in determining electrochemical performances of Li-ion batteries. The ideal SEI layer protects the electrolyte from being further reduced on the electrode surface and allows Li-ion diffusion in and out of electrodes without any consumption. However, degradation of the SEI layer over time, which contributes to the thickening of the SEI layer, is a leading pathway for gradual capacity fade. In this study, a hyperpolarized (HP) 129Xe nuclear magnetic resonance (NMR) technique was applied for the first time to probe changes in porosity and connectivity in Si nanoparticle composite electrodes as a result of the SEI formation. Nanopores are present in nanocomposite electrodes as a result of aggregation of the constituting nanoparticles. The connectivity among nanopores greatly affects the ion transport property of the electrode materials, which has a substantial influence on the overall energy output of Li-ion batteries. In this work, information on thic...

An optimized microfabricated platform for the optical generation and detection of hyperpolarized 129Xe

Low thermal-equilibrium nuclear spin polarizations and the need for sophisticated instrumentation render conventional nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) incompatible with small-scale microfluidic devices. Hyperpolarized 129Xe gas has found use in the study of many materials but has required very large and expensive instrumentation. Recently a microfabricated device with modest instrumentation demonstrated all-optical hyperpolarization and detection of 129Xe gas. This device was limited by 129Xe polarizations less than 1%, 129Xe NMR signals smaller than 20 nT, and transport of hyperpolarized 129Xe over millimeter lengths. Higher polarizations, versatile detection schemes, and flow of 129Xe over larger distances are desirable for wider applications. Here we demonstrate an ultra-sensitive microfabricated platform that achieves 129Xe polarizations reaching 7%, NMR signals exceeding 1 μT, lifetimes up to 6 s, and simultaneous two-mode detection, consisting of a high-sensitivity in situ channel with signal-to-noise of 105 and a lower-sensitivity ex situ detection channel which may be useful in a wider variety of conditions. 129Xe is hyperpolarized and detected in locations more than 1 cm apart. Our versatile device is an optimal platform for microfluidic magnetic resonance in particular, but equally attractive for wider nuclear spin applications benefitting from ultra-sensitive detection, long coherences, and simple instrumentation.

Effect of pretreatment temperature on the surface modification of diatomite with trimethylchlorosilane

Diatomite samples from Costa Rica were purified using acidic treatments with hydrochloric acid, thermally treated (400–1000 °C) and then silylated with trimethylchlorosilane in toluene under inert atmosphere. The purification process allows to decrease the concentration of metals presented in the crude diatomite, as is confirmed by X-ray Fluorescence (XRF) Analysis. The silylated materials were analyzed by using Hyperpolarized 129Xe Nuclear Magnetic Resonance Spectroscopy (HP 129Xe NMR), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), rehydration tests, and contact angle measurements. XRD measurements indicate that diatomite is mainly amorphous, but presents several crystalline phases (kaolinite, cristobalite, and quartz). Pretreatments at high temperatures cause changes in those crystalline phases, resulting in more amorphous materials. However, there is no difference in the overall structure of purified and thermally treated diatomite samples with respect to the silylation products. In addition, SEM measurements show no effect over the pore structure of the materials. On the other hand, TGA measurements and rehydration tests show lower losses of water for silylated materials prepared using higher pretreatment temperatures. Moreover, HP 129Xe NMR, FTIR, and contact angle measurements evidence a modification due to covalent attachment of Si(CH3)3-groups to the surface, which increases for higher pretreatment temperatures. The results provide valuable information about external factors that influence the surface modification of diatomite. This can be useful to control modifications that can be achieved in a similar way.