Pulsed Nuclear Magnetic Resonance Research Articles

Transport Properties and Local Structure of an Imidazole/Protic Ionic Liquid Mixture Confined in the Mesopores of Hydrophobic Silica

The local structure and the molecular dynamics of an imidazole/protic ionic liquid mixture have been investigated while confined in only ca. 5 nm mesopores of silica particles. The walls of the silica pores were functionalized with trioctyl groups to ensure a hydrophobic character, and a series of hybrid materials with varying liquid-to-silica ratios were investigated. Results from vibrational spectroscopy (both Raman and infrared) indicate that the local ion–ion interactions as well as the nature of hydrogen bonds inside the nanopores are not significantly different from the case of the bulk liquid mixture. Nevertheless, the ionic conductivity decreases rapidly and monotonically with decreasing amount of liquid, while the self-diffusion coefficients measured by pulsed field gradient nuclear magnetic resonance (NMR) show a distinct dependence on composition. The population of molecules outside the particles seems to contribute with an enhanced diffusivity, while the molecules inside the mesopores diffuse at a rate comparable to that observed in the bulk liquid. In addition, when experimentally possible, we have measured higher diffusivities for the exchangeable −NH proton than for any other molecular species, which is indicative of a decoupled proton motion. Results from X-ray scattering, employed to elucidate the local molecular structure, reveal an additional feature characteristic of the nanoconfined state, which is associated with a real space distance of about 3.5 nm. This distance describes a specific molecular organization inside the mesopores and may reflect the formation of a monolayer of the octyl-imidazolium cations self-assembled at the hydrophobic silica surface. Such a local structure would favor the localization of charges, including the exchangeable protons. In addition, the analysis of molar conductivity suggests that a major problem with a low pore filling is the emergence of discontinuities throughout the liquid phase.

Catalytic Oxidation of Heavy Residual Oil by Pulsed Nuclear Magnetic Resonance

A study on the catalytic oxidation of heavy residual oil (HRO) was carried out. The thermodynamic parameters of components of HRO oxidation products were studied by pulsed nuclear magnetic resonance (NMR). A method for the quantitative assessment of thermodynamic parameters of HRO components and oxidized bitumen using pulsed NMR is presented. The relationship between NMR parameters and the viscosity of HRO and its oxidation products is established. The obtained results prove the possibility of using pulsed NMR as a flow-line method for rapid analysis of intermediates and products of the heavy residual oil oxidation.

PFG NMR time-dependent diffusion coefficient analysis of confined emulsion: Post drainage phase conformation

In this work, we present a characterization of phase configuration in water-saturated sintered glass bead samples after oil injection, through the analysis of time-dependent diffusion coefficients obtained from sets of one-dimensional pulsed field gradient nuclear magnetic resonance (PFG NMR) measurements, pre and post drainage. Estimates of samples' surface-to-volume ratio and permeability from pre drainage PFG measurements in a water-saturated sample were compared with analytical and reported values, respectively, and a fair agreement was found in both cases. Short-time analysis of diffusion coefficients extracted from PFG measurements was used to quantify the increase in surface-to-volume ratio probed by the wetting phase after drainage. Analysis of water and oil diffusion coefficients from post drainage PFG experiments were carried out using a bi-Gaussian model, and two distinct scenarios were considered to describe fluids conformation within pores. For the case where non-wetting phase was considered to exhibit a poorly connected geometry, an analysis assuming the formation of oil-in-water droplets within pores was performed, and a Gaussian distribution of droplets radii was determined.

Singlet to triplet conversion in molecular hydrogen and its role in parahydrogen induced polarization.

Detailed experimental and comprehensive theoretical analysis of singlet-triplet conversion in molecular hydrogen dissolved in a solution together with organometallic complexes used in experiments with parahydrogen (the H2 molecule in its nuclear singlet spin state) is reported. We demonstrate that this conversion, which gives rise to formation of orthohydrogen (the H2 molecule in its nuclear triplet spin state), is a remarkably efficient process that strongly reduces the resulting NMR (nuclear magnetic resonance) signal enhancement, here of 15N nuclei polarized at high fields using suitable NMR pulse sequences. We make use of a simple improvement of traditional pulse sequences, utilizing a single pulse on the proton channel that gives rise to an additional strong increase of the signal. Furthermore, analysis of the enhancement as a function of the pulse length allows one to estimate the actual population of the spin states of H2. We are also able to demonstrate that the spin conversion process in H2 is strongly affected by the concentration of 15N nuclei. This observation allows us to explain the dependence of the 15N signal enhancement on the abundance of 15N isotopes.

Influence of the number and type of functional groups on self-diffusion of some aromatic compounds in acetone: Nuclear magnetic resonance and molecular dynamics simulations

Self-diffusion of parabens (methylparaben and propylparaben), phenolic acids (syringic and gallic acids) and their derivative (protocatechualdehyde), hydroxycinnamic acids (caffeic, ferulic, sinapic, and p-coumaric acids), and tetramethylsilane in acetone has been investigated by the pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) method and molecular dynamics simulation in ambient conditions. The self-diffusion coefficients obtained by the experimental and theoretical methods are in good agreement with each other. A detailed analysis of the average number of solute – acetone hydrogen bonds has made it possible to trace the influence of the molecular heteroassociation on the self-diffusion of the aromatic compounds. The degree of influence of solute – acetone molecular association on the solute self-diffusion defined as the ratio of the solute self-diffusion coefficient to the self-diffusion coefficient of its monomer has been calculated using the Stokes-Einstein equation. It has been found that for a number of aromatic compounds with the same type and number of functional groups, the parameter expressing the degree of influence of molecular association has similar values. This fact has allowed us to calculate the self-diffusion coefficients of the compounds in the given series at a known self-diffusion coefficient value of one of the series members.

Diffusion Spectrum of Polymer Melt Measured by Varying Magnetic Field Gradient Pulse Width in PGSE NMR.

The translational motion of polymers is a complex process and has a big impact on polymer structure and chemical reactivity. The process can be described by the segment velocity autocorrelation function or its diffusion spectrum, which exhibit several characteristic features depending on the observational time scale—from the Brownian delta function on a large time scale, to complex details in a very short range. Several stepwise, more-complex models of translational dynamics thus exist—from the Rouse regime over reptation motion to a combination of reptation and tube-Rouse motion. Accordingly, different methods of measurement are applicable, from neutron scattering for very short times to optical methods for very long times. In the intermediate regime, nuclear magnetic resonance (NMR) is applicable—for microseconds, relaxometry, and for milliseconds, diffusometry. We used a variation of the established diffusometric method of pulsed gradient spin-echo NMR to measure the diffusion spectrum of a linear polyethylene melt by varying the gradient pulse width. We were able to determine the characteristic relaxation time of the first mode of the tube-Rouse motion. This result is a deviation from a Rouse model of polymer chain displacement at the crossover from a square-root to linear time dependence, indicating a new long-term diffusion regime in which the dynamics of the tube are also described by the Rouse model.

Impact of intermolecular attraction and repulsion on molecular diffusion and virial coefficients of spheroidal and rod-shaped proteins

Recognition of protein intermolecular interactions is a key factor for the study and manipulation of protein activity in living systems and biotechnology formulations. Experimental parameter that is sensitive to intermolecular interactions is molecular diffusion. This work presents an extension of an earlier work where we proposed the complementary use of pulsed field gradient nuclear magnetic resonance (PFG NMR) and dynamic light scattering (DLS) to study the specific features of intermolecular interactions via protein translational diffusion. PFG NMR methodology characterizes the thermal motion of protein by means of the self-diffusion coefficient and friction formalism. DLS experiments provide the protein collective (or mutual) diffusion coefficient, which evaluates the direct, non-specific intermolecular interactions of proteins by means of virial coefficients. Theoretical analysis of obtained experimental diffusion data is based on the combination of the friction formalism of nonequilibrium thermodynamics (Vink approach) with the Deryaguin-Landau-Verwey-Overbeek (DLVO) theory. To evaluate the electrostatic potential of proteins with different shape - spheroidal alfa-chymotrypsin (ChTr) and the rod-shaped fibrinogen (Fg) we applied the model of “porous” colloid particles. Our analysis has shown that the atypical local decrease of collective diffusion relative to self-diffusion is a consequence of the dominance of van der Waals attraction over electrostatic repulsion for the rod-shaped Fg.

Omega‐3 fatty acid‐fortified butter: Preparation and characterisation of textural, sensory, thermal and physico‐chemical properties

The influence of adding flaxseed oil (2.9–5.1%) and flaxseed–whey protein concentrate (WPC) emulsion (4.8–8.6%) to cream as an omega‐3 fatty acid source was evaluated on the thermal, sensory and physico‐chemical properties of the developed butter. Pulsed nuclear magnetic resonance analysis revealed lower saturated fat content in the fortified butter than control butter. Differential scanning calorimetry exotherms and endotherms also corroborated these findings. Fortified butter prepared with 6.8% flaxseed oil–WPC emulsion had 3.7 times more alpha‐linolenic acid content than control and provided nearly 25% of the recommended dietary allowance in one serving. The developed butter exhibited improved spreadability.

Effect of Milk Fat Concentration on Fat Crystallization of Palm Oil‐Based Shortenings

AbstractThe effect of different dosages of anhydrous milk fat (AMF) (25%, 50% and 75%, w/w) on shear‐crystallization of fat blends made of refined palm oil, refined palm stearin, and rapeseed oil was studied. Classical techniques as differential scanning calorimetry (DSC), pulsed field gradient nuclear magnetic resonance (pfg‐NMR), rheometer, and X‐ray diffraction (XRD) were applied to evaluate the crystallization kinetics of fat blends as well as the fat compatibility between components in rapid cooling (15 °C min−1), isothermal crystallization (at 15 °C), and storage (5 °C). Obtained results revealed that the mixtures of palm oils and milk fat had a low compatibility. The co‐crystallization between triacylglycerols (TAG) of milk fat and of palm oil occurred during isothermal crystallization and storage resulting in slower crystallization kinetics and the formation of some eutectic mixtures.

Structural and Ion Dynamics in Fluorine-Free Oligoether Carboxylate Ionic Liquid-Based Electrolytes.

Here, we investigate the physicochemical and electrochemical properties of fluorine-free ionic liquid (IL)-based electrolytes with two different cations, tetrabutylphosphonium, (P4,4,4,4)+, and tetrabutylammonium, (N4,4,4,4)+, coupled to a new anion, 2-[2-(2-methoxyethoxy)ethoxy]acetate anion (MEEA)−, for both neat and (P4,4,4,4)(MEEA) also doped with 10–40 mol % of Li(MEEA). We find relatively weaker cation–anion interactions in (P4,4,4,4)(MEEA) than in (N4,4,4,4)(MEEA), and for both ILs, the structural flexibility of the oligoether functionality in the anion results in low glass transition temperatures, also for the electrolytes made. The pulsed field gradient nuclear magnetic resonance (PFG NMR) data suggest faster diffusion of the (MEEA)− anion than (P4,4,4,4)+ cation in the neat IL, but the addition of a Li salt results in slightly lower mobility of the former than the latter and lower ionic conductivity. This agrees with the combined 7Li NMR and attenuated total reflection–Fourier transform infrared (ATR–FTIR) spectroscopy data, which unambiguously reveal preferential interactions between the lithium cations and the carboxylate groups of the IL anions, which also increased as a function of the lithium salt concentration. In total, these systems provide a stepping stone for further design of fluorine-free and low glass transition temperature IL-based electrolytes and also stress how crucial it is to control the strength of ion–ion interactions.

Propylene carbonate-nitrile solvent blends for thermally stable gel polymer lithium ion battery electrolytes

Gel polymer electrolytes (GPEs) based on poly (vinylidene difluoride-co-hexafluoropropylene) (PVdF-HFP) containing propylene carbonate (PC), isobutyronitrile (IBN) and trimethyl acetonitrile (TMAN) solvent blend electrolytes were developed to enhance high temperature electrochemical performance and to improve safety of LiNi3/5Mn1/5Co1/5O2 (NMC622)‖graphite lithium ion battery (LIB) cells. These liquid electrolytes (LE) consist of lithium bis(trifluoro-methane)sulfonimide (LiTFSI) as conducting lithium salt and PC/nitrile (1:1, v/v) solvent blends. Ethylene sulfite (ES) and vinylene carbonate (VC) were used as solid electrolyte interphase (SEI) forming additives and contributing to an excellent cycling stability of electrolytes comprising nitrile solvents in NMC622‖graphite cells. Electrochemical impedance spectroscopy (EIS), pulsed field gradient nuclear magnetic resonance spectroscopy (PFG NMR) and relative permittivity determination reveal remarkable ion conducting properties of IBN and TMAN solvents. Since their electrolytes are incompatible with lithium metal electrodes, their electrochemical stability window (ESW) was determined using lithiated lithium titanate (Li7Ti5O12) (LTO) as counter and reference electrodes and the obtained values were confirmed by quantum mechanical computation. Differential scanning calorimetry (DSC) and accelerating rate calorimetry (ARC) confirmed significantly improved safety of IBN and TMAN containing electrolytes. Therefore, PVdF-HFP-based GPEs containing PC/nitrile solvent blends are a promising alternative to SOTA electrolyte for battery cycling at elevated temperature (60 °C) in NMC622‖graphite cells.

Surprising absence of strong homonuclear coupling at low magnetic field explored by two-field nuclear magnetic resonance spectroscopy.

Strong coupling of nuclear spins, which is achieved when their scalar coupling is greater than or comparable to the difference in their Larmor precession frequencies in an external magnetic field, gives rise to efficient coherent longitudinal polarization transfer. The strong coupling regime can be achieved when the external magnetic field is sufficiently low, as is reduced proportional to the field strength. In the present work, however, we demonstrate that in heteronuclear spin systems these simple arguments may not hold, since heteronuclear spin-spin interactions alter the value. The experimental method that we use is two-field nuclear magnetic resonance (NMR), exploiting sample shuttling between the high field, at which NMR spectra are acquired, and the low field, where strong couplings are expected and at which NMR pulses can be applied to affect the spin dynamics. By using this technique, we generate zero-quantum spin coherences by means of a nonadiabatic passage through a level anticrossing and study their evolution at the low field. Such zero-quantum coherences mediate the polarization transfer under strong coupling conditions. Experiments performed with a -labeled amino acid clearly show that the coherent polarization transfer at the low field is pronounced in the spin subsystem under proton decoupling. However, in the absence of proton decoupling, polarization transfer by coherent processes is dramatically reduced, demonstrating that heteronuclear spin-spin interactions suppress the strong coupling regime, even when the external field is low. A theoretical model is presented, which can model the reported experimental results.

Benchmarking of Pulsed Field Gradient Nuclear Magnetic Resonance as a Demulsifier Selection Tool with Arabian Light Crude Oils

Summary The use of chemical demulsifiers in the treatment of crude oil emulsions is an essential step in processing facilities worldwide. Each production facility requires specific demulsifier reformulations as the crude characteristics change. The assessment of candidate demulsifiers before online field trials is currently done with bottle tests. Such tests are manual, based on water dropout visually measured by operators. The development of a method that can automatically determine the speed and amount of water dropout without the laborious need to manually record water separation would significantly decrease human error. Pulsed field gradient nuclear magnetic resonance (PFG-NMR) is used as a classification tool to qualitatively rank the efficiency of different demulsifiers in breaking Arabian Light emulsions. This imaging method can evaluate demulsifier action based on the emulsion characteristics; for example, rate of sedimentation and coalescence and formation of a dense packed zone (rag layer). The results are validated against field trials performed in gas-oil separation plants (GOSPs) at two Saudi Arabian facilities. There was good agreement between the PFG-NMR method and field trials. The results were found to correspond to the water dropout in the first stage of crude oil treatment in processing plants (production traps).

Spatial molecular-dynamically ordered NMR spectroscopy of intact bodies and heterogeneous systems

Noninvasive evaluation of the spatial distribution of chemical composition and diffusion behavior of materials is becoming possible by advanced nuclear magnetic resonance (NMR) pulse sequence editing. However, there is room for improvement in the spectral resolution and analytical method for application to heterogeneous samples. Here, we develop applications for comprehensively evaluating compounds and their dynamics in intact bodies and heterogeneous systems from NMR data, including spatial z-position, chemical shift, and diffusion or relaxation. This experiment is collectively named spatial molecular-dynamically ordered spectroscopy (SMOOSY). Pseudo-three-dimensional (3D) SMOOSY spectra of an intact shrimp and two heterogeneous systems are recorded to evaluate this methodology. Information about dynamics is mapped onto two-dimensional (2D) chemical shift imaging spectra using a pseudo-spectral imaging method with a processing tool named SMOOSY processor. Pseudo-2D SMOOSY spectral images can non-invasively assess the different dynamics of the compounds at each spatial z-position of the shrimp’s body and two heterogeneous systems.

Establishing Ultralow Activation Energies for Lithium Transport in Garnet Electrolytes.

Garnet-type structured lithium ion conducting ceramics represent a promising alternative to liquid-based electrolytes for all-solid-state batteries. However, their performance is limited by their polycrystalline nature and inherent inhomogeneous current distribution due to different ion dynamics at grains, grain boundaries, and interfaces. In this study, we use a combination of electrochemical impedance spectroscopy, distribution of relaxation time analysis, and solid-state nuclear magnetic resonance (NMR), in order to understand the role that bulk, grain boundary, and interfacial processes play in the ionic transport and electrochemical performance of garnet-based cells. Variable temperature impedance analysis reveals the lowest activation energy for Li transport in the bulk of the garnet electrolyte (0.15 eV), consistent with pulsed field gradient NMR spectroscopy measurements (0.14 eV). We also show a decrease in grain boundary activation energy at temperatures below 0 °C, that is followed by the total conductivity, suggesting that the bottleneck to ionic transport resides in the grain boundaries. We reveal that the grain boundary activation energy is heavily affected by its composition that, in turn, is mainly affected by the segregation of dopants and Li. We suggest that by controlling the grain boundary composition, it would be possible to pave the way toward targeted engineering of garnet-type electrolytes and ameliorate their electrochemical performance in order to enable their use in commercial devices.

Accelerating Restricted Diffusion NMR Studies with Time-Resolved and Ultrafast Methods.

Restricted diffusion of fluids in porous materials can be studied by pulsed field gradient nuclear magnetic resonance (NMR) non-invasively and without tracers. If the experiment is repeated many times with varying diffusion delays, detailed information about pore sizes and tortuosity can be recorded. However, the measurements are very time-consuming because numerous repetitions are needed for gradient ramping and varying diffusion delays. In this paper, we demonstrate two different strategies for acceleration of the restricted diffusion NMR measurements: time-resolved diffusion NMR and ultrafast Laplace NMR. The former is based on time-resolved non-uniform sampling, while the latter relies on spatial encoding of two-dimensional data. Both techniques allow similar 1–2 order of magnitude acceleration of acquisition, but they have different strengths and weaknesses, which we discuss in detail. The feasibility of the methods was proven by investigating restricted diffusion of water inside tracheid cells of thermally modified pine wood.

Unravelling the role of a green rejuvenator agent in contrasting the aging effect on bitumen: A dynamics rheology, nuclear magnetic relaxometry and self-diffusion study

This paper evaluated the potentialities of a green and biocompatible rejuvenator agent (HR) in conferring an appreciable resistance against the effects caused by artificial aging on a given bitumen. Both neat and aged bitumens were analyzed and compared to analogous samples modified with HR. Control samples containing a vegetable oil as softening agent were also tested for comparison. The tested samples were subjected to a second aging cycle. Structural differences between the samples were carried out through an inverse Laplace transform of the NMR spin-echo decay (T2) and self-diffusion measurements by pulsed gradient spin echo nuclear magnetic resonance (PGSE-NMR) spectroscopy. In addition, dynamic rheological analyses were conducted to determine the dependence of the gel-sol transition temperature on both the type of additive and ageing process. The present study clearly highlighted the fact that artificial ageing, realized here by the rolling thin film oven test (RTFOT) and the pressure ageing vessel (PAV) test, induced important structural modifications. The analysis of relaxation times and self-diffusion coefficients indicated that ageing promoted the formation of molecular populations characterized by a shift of the distribution toward higher molecular weights compared to unaged bitumen. Diffusion data showed also an Arrhenius-like temperature dependence. A correlation between all the data was attempted to understand the role of the investigated additives. The eco-friendly biocompatible rejuvenator helped not only to restore the structure of the aged bitumen, but even slowed down the processes of a second aging (aiming at the first aged sample).

Pore-fluid characterizations and microscopic mechanisms of sedimentary rocks with three-dimensional NMR: Tight sandstone as an example

Pore-fluid is directly related to the reservoir quality. Three-dimensional (3D) nuclear magnetic resonance (NMR) can be used for charactering pore-fluid due to its sensitivity to fluid components saturated in sedimentary rocks. In this paper, taken a tight sandstone as an example, investigating pore-fluid characterizations and microscopic mechanisms through 3D NMR. Firstly, a commonly used 3D NMR pulse sequence, tri-window pulse sequence, in actual oilfield was presented and its data processing workflow was exhibited briefly. Then, random-walk method was modified to simulate the pulse sequence measurements at different pore-fluid cases in tight sandstone, such as pure water case, oil-water case, and gas-water case. Finally, pore-fluid 3D NMR responses were exhibited and their microscopic mechanisms were explored. The results show that 3D NMR can identify different fluid components saturated in sedimentary rocks; wetting fluid saturation directly affect wetting fluid relaxation time, but the non-wetting fluid relaxation information is independent of fluid saturation; wetting fluid has an obvious restricted diffusion as a lower saturation; an obvious difference between gas signal and water or oil signal in two-dimensional relaxation spectra due to high longitudinal to transverse relaxation time ratio of pore gas. These simulations provide a theoretical basis for interpreting 3D NMR macroscopic responses, which should be helpful for pore-fluid identifying in tight oil and gas reservoirs.

Application of 1H and 13C NMR Fingerprinting as a Tool for the Authentication of Maltese Extra Virgin Olive Oil.

The application of 1H and 13C nuclear magnetic resonance (NMR) in conjunction with chemometric methods was applied for the discrimination and authentication of Maltese extra virgin olive oils (EVOOs). A total of 65 extra virgin olive oil samples, consisting of 30 Maltese and 35 foreign samples, were collected and analysed over four harvest seasons between 2013 and 2016. A preliminary examination of 1H NMR spectra using unsupervised principle component analysis (PCA) models revealed no significant clustering reflecting the geographical origin. In comparison, PCA carried out on 13C NMR spectra revealed clustering approximating the geographical origin. The application of supervised methods, namely partial least squares discriminate analysis (PLS-DA) and artificial neural network (ANN), on 1H and 13C NMR spectra proved to be effective in discriminating Maltese and non-Maltese EVOO samples. The application of variable selection methods significantly increased the effectiveness of the different classification models. The application of 13C NMR was found to be more effective in the discrimination of Maltese EVOOs when compared to 1H NMR. Furthermore, results showed that different 1H NMR pulse methods can greatly affect the discrimination of EVOOs. In the case of 1H NMR, the Nuclear Overhauser Effect (NOESY) pulse sequence was more informative when compared to the zg30 pulse sequence, since the latter required extensive spectral manipulation for the models to reach a satisfactory level of discrimination.

Study of ion dynamics of LiI·6H2O in the supercooled liquid state using NMR spectroscopy and ionic conductivity measurements

Ion dynamics in the liquid and supercooled liquid state of LiI·6H2O were studied by nuclear magnetic resonance (NMR) spectroscopy and ionic conductivity measurements. The self-diffusion coefficients of the water molecules and lithium ions were measured by 1H and 7Li pulsed gradient spin echo NMR (PGSE-NMR). The temperature dependences of the DC ionic conductivity and diffusion coefficients of lithium ions and water molecules were well-fitted by the Vogel-Tamman-Fulcher (VTF) law, similar to those of the LiCl·RH2O system. The conductivity and lithium diffusion coefficients of the LiI·6H2O system were more than twice those of LiCl·7H2O in the supercooled liquid state below 200 K, which could be attributed to the larger size of the iodide anions than that of chloride anions. The temperature dependence of the correlation times calculated from NMR T1 relaxation deviated from the VTF law at low temperatures because of the local orientational fluctuation of the water molecules.