NMR Investigation of Water in Salt Crusts: Insights from Experiments and Molecular Simulations.

The evaporation of water from bare soil is often accompanied by the formation of a layer of crystallized salt, a process that must be understood in order to address the issue of soil salinization. Here, we use nuclear magnetic relaxation dispersion measurements to better understand the dynamic properties of water within two types of salt crusts: sodium chloride (NaCl) and sodium sulfate (Na2SO4). Our experimental results display a stronger dispersion of the relaxation time T1 with frequency for the case of sodium sulfate as compared to sodium chloride salt crusts. To gain insight into these results, we perform molecular dynamics simulations of salt solutions confined within slit nanopores made of either NaCl or Na2SO4. We find a strong dependence of the value of the relaxation time T1 on pore size and salt concentration. Our simulations reveal the complex interplay between the adsorption of ions at the solid surface, the structure of water near the interface, and the dispersion of T1 at low frequency, which we attribute to adsorption-desorption events.

Modeling Molecular Interactions with Wetting and Non-Wetting Rock Surfaces by Combining Electron Paramagnetic Resonance and NMR Relaxometry.

Three types of natural rocks─Bentheimer and Berea sandstones, as well as Liège Chalk─have been aged by immersion in a bitumen solution for extended periods of time in two steps, changing the surface conditions from water-wet to oil-wet. NMR relaxation dispersion measurements were carried out on water and oil constituents, with saturated and aromatic molecules considered individually. In order to separate the different relaxation mechanisms discussed in the literature, 1H and 19F relaxation times were compared to 2H for fully deuterated liquids: while 2H relaxes predominantly by quadrupolar coupling, which is an intramolecular process, the remaining nuclei relax by dipolar coupling, which potentially consists of intra- and intermolecular contributions. The wettability change becomes evident in an increase of relaxation rates for oil and a corresponding decrease for water. However, this expected behavior dominates only for the spin-lattice relaxation rate R1 at very low field strengths and for the spin-spin relaxation rate R2, while high-field longitudinal relaxation shows a much weaker or even reverse trend. This is attributed in part to a change of radical concentration on the pore surface upon coverage of the native rock surface by bitumen as well as by the change of surface chemistry and roughness. EPR and DNP measurements quantify the change of volume vs surface radical concentration in the rocks, and an improved understanding of the role of relaxation via paramagnetic centers is obtained. By means of comparing different fluids and nuclei in combination with a defined wettability change of natural rocks, a refined model for molecular dynamics in conjunction with NMR relaxation dispersion is proposed.

Three mechanisms of room temperature dynamic nuclear polarization occur simultaneously in an ionic liquid.

Dynamic nuclear polarization is a versatile approach to increasing the sensitivity of NMR measurements and is achieved by any of four different mechanisms which dominate for either liquids or solids, depending on temperature and radical density. In this work, we unequivocally demonstrate for the first time the coexistence, at a comparable magnitude, of several mechanisms, namely the Overhauser effect, solid effect, and cross-effect/thermal mixing in a viscous ionic liquid at ambient temperatures.

Molecular Dynamics and Proton Hyperpolarization via Synthetic and Crude Oil Porphyrin Complexes in Solid and Solution States.

The use of vanadyl porphyrins either in synthetic compounds or naturally occurring in asphaltenes is investigated as a source of proton hyperpolarization via dynamic nuclear polarization (DNP) in nuclear magnetic resonance (NMR) experiments. The features of dynamics and location of the vanadyl VO2+ complex in aggregates within the oil asphaltene molecules are studied by means of DNP, electron paramagnetic resonance (EPR), and NMR field cycling relaxometry. Both the solid effect and Overhauser DNP were observed for the asphaltene solution in benzene, as well as in the solution and solid states for synthetic compounds. By comparison with a solution of synthetic vanadyl porphyrins, it is shown that vanadyl porphyrins in asphaltene aggregates are localized outside of the interface of the asphaltene aggregates and more exposed to the maltene molecules than "free" carbon-centered radicals associated with the core of asphaltene molecules. The perceptible contribution of scalar interaction is observed in solutions for both synthetic and asphaltene vanadyl porphyrins.

Non-Exponential 1H and 2H NMR Relaxation and Self-Diffusion in Asphaltene-Maltene Solutions.

The distribution of NMR relaxation times and diffusion coefficients in crude oils results from the vast number of different chemical species. In addition, the presence of asphaltenes provides different relaxation environments for the maltenes, generated by steric hindrance in the asphaltene aggregates and possibly by the spatial distribution of radicals. Since the dynamics of the maltenes is further modified by the interactions between maltenes and asphaltenes, these interactions-either through steric hindrances or promoted by aromatic-aromatic interactions-are of particular interest. Here, we aim at investigating the interaction between individual protonic and deuterated maltene species of different molecular size and aromaticity and the asphaltene macroaggregates by comparing the maltenes' NMR relaxation (T1 and T2) and translational diffusion (D) properties in the absence and presence of the asphaltene in model solutions. The ratio of the average transverse and longitudinal relaxation rates, describing the non-exponential relaxation of the maltenes in the presence of the asphaltene, and its variation with respect to the asphaltene-free solutions are discussed. The relaxation experiments reveal an apparent slowing down of the maltenes' dynamics in the presence of asphaltenes, which differs between the individual maltenes. While for single-chained alkylbenzenes, a plateau of the relaxation rate ratio was found for long aliphatic chains, no impact of the maltenes' aromaticity on the maltene-asphaltene interaction was unambiguously found. In contrast, the reduced diffusion coefficients of the maltenes in presence of the asphaltenes differ little and are attributed to the overall increased viscosity.

Effect of Initial Conformation on the Starch Biopolymer Film Formation Studied by NMR.

The formation of a rigid porous biopolymer scaffold from aqueous samples of 1% w/v (suspension) and 5% w/v (gel) corn starch was studied using optical and nuclear magnetic resonance (NMR) techniques. The drying process of these systems was observed using a single-sided NMR scanner by application of the Carr-Purcell-Meiboom-Gill pulse sequence at different layer positions. The echo decays were analyzed and spin-spin relaxation times (T2) were obtained for each layer. From the depth dependent T2 relaxation time study, it was found that the molecular mobility of water within the forming porous matrix of these two samples varied notably at different stages of film formation. At an intermediate stage, a gradual decrease in mobility of the emulsion sample towards the air-sample interface was observed, while the gel sample remained homogeneous all along the sample height. At a later stage of drying, heterogeneity in the molecular dynamics was observed in both samples showing low mobility at the bottom part of the sample. A wide-angle X-ray diffraction study confirmed that the structural heterogeneity persisted in the final film obtained from the 5% corn starch aqueous sample, whereas the film obtained from the 1% corn starch in water was structurally homogeneous.

The Effect of Silica Fume and Organosilane Addition on the Porosity of Cement Paste.

The present work systematically investigates the influence of silica fume and organosilane addition on the hydration dynamics and the capillary pore formation of a cement paste. The cement samples were prepared with two water-to-cement ratios with increasing amounts of silica fume and of (3-Aminopropyl)triethoxysilane (APTES) organosilane. Low-field 1H nuclear magnetic resonance (NMR) relaxation measurements were performed during the hydration of the samples and after hydration, in order to reveal the dynamics of water molecules and the pore distribution. Increasing concentrations of silica fume impact the perceived hydration dynamics through the addition of magnetic impurities to the pore solution. However, there is a systematic change in the capillary pore size distribution with an increase in silica fume concentration. The results also show that the addition of APTES majorly affects the hydration dynamics, by prolonging the dormancy and hardening stages. While it does not influence the pore size distribution of capillary pores, it prevents cyclohexane from saturating the capillary pores.

Multicomponent analysis of T1 relaxation in bovine articular cartilage at low magnetic fields.

PURPOSE: The multi-exponential character of T1 relaxation in bovine articular cartilage was investigated at low magnetic fields below 0.5 T. The ultimate aim was to identify a parameter based on the T1 relaxation time distribution as a biomarker to biochemical features of osteoarthritis. METHODS: Osteoarthritis conditions were simulated by enzymatic digestion of cartilage with trypsin. Fast-field cycling NMR relaxometry was carried out in the magnetic field range B0 = 70 µT to 600 mT. The data were analyzed in terms of T1 distributions on a log-time scale using inverse Laplace transform, whereas integral properties such as mean T1 s and distribution widths were obtained without data inversion from logarithmic moment analysis and a stretched-exponential fit to the data. Attempts were also made to differentiate between water dynamic components through multi-Lorentzian decomposition of average relaxation-rate dispersions. RESULTS: T1 distribution in bovine articular cartilage was found to be bimodal, with the dominating, long component shifting toward larger values following trypsin digestion. The effect is more prominent toward lower magnetic field strength. This shift leads to an overall increase of the distribution width and an equivalently more pronounced deviation from exponential behavior. CONCLUSION: The logarithmic width of T1 distribution functions at fields of 0.5 T and below, and the stretched-exponential decay fit exponent ß, show a significant trend after trypsin digestion of cartilage. These 2 parameters are suggested as possible biomarkers for osteoarthritis in humans and can be acquired entirely in vivo, with increasing significance for lower magnetic field strengths.

Molecular Dynamics in the Lyophases of Copolymer P123 Investigated with FFC NMR Relaxometry.

Associative block copolymers of the type (EO) x(PO) y(EO) x (where EO and PO represent ethylene and propylene oxides, respectively) in aqueous solution have far reaching commercial applications such as solubilization, controlled-drug delivery, etc. The molecular dynamics of a self-associating triblock copolymer (EO)20(PO)70(EO)20 (known as P123 with a molecular weight of ∼5800), in aqueous solution (D2O), consisting of various lyotropic liquid crystalline phases such as isotropic micellar, cubic, hexagonal, and lamellar phases, is investigated using the fast field cycling nuclear magnetic resonance (FFC NMR) relaxometry technique in the Larmor frequency range from 5 kHz to 30 MHz. A nuclear spin-relaxation model consisting of chain modes (Rouse modes) and order fluctuation (OF) modes typical for polymers and liquid crystals, respectively, is considered to explain the observed proton magnetic relaxation dispersion (PMRD) data in the lyophases under investigation. The PMRD analysis in both isotropic micellar and cubic phases revealed a Rouse frequency dependence of spin-lattice relaxation rate ( R1), i.e., R1 ∝ -τs ln(ωτs), in the entire frequency range of study. Hexagonal and lamellar phase data show Rouse modes as well as OF modes, leaving the signature of the latter as R1 ∝ ω- p, where p ∼ 0.5 is typical for nematic mesogens. The activation energies were also determined from segmental correlation times in the lyophases of study. To the best of our knowledge, the present FFC NMR relaxometry study is unique and quantitative in unraveling molecular dynamics of the associative copolymer P123 in aqueous solution.

Nuclear magnetic relaxation and diffusion study of the ionic liquids 1-ethyl- and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide confined in porous glass.

The molecular dynamics of the room-temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (Bmim Tf2N) confined in porous glass is studied by nuclear magnetic resonance (NMR) relaxometry and diffusometry and is compared with the bulk dynamics over a wide temperature range. The molecular reorientation processes for anions and cations alike are found to be significantly affected by the presence of the glass interface at high temperatures. In this respect, the ionic liquid behaves similarly to polar liquids where proton NMR relaxation is governed by reorientations mediated by translational displacements (RMTDs). This process becomes less significant towards lower temperatures when the characteristic translational correlation times of the ions approach a timescale comparable with those of the RMTD process, and the relaxation dispersions in bulk and in confinement become similar below a temperature corresponding to about 1.2Tg , a value where the onset of dynamic heterogeneity has been observed before. The self-diffusion coefficient, on the other hand, is found to be strongly reduced than the bulk within the accessible temperature range of 248 K and above and is significantly slower than expected from the tortuosity effect, suggesting that ion-surface interactions affect the macroscopic properties.

Effect of nitroxide spin probes on the transport properties of Nafion membranes.

Nafion is the most common material used as a proton exchange membrane in fuel cells. Yet, details of the transport pathways for protons and water in the inner membrane are still under debate. Overhauser Dynamic Nuclear Polarization (ODNP) has proven to be a useful tool for probing hydration dynamics and interactions within 5-8 Å of protein and soft material surfaces. Recently it was suggested that ODNP can also be applied to analyze surface water dynamics along Nafion's inner membrane. Here we interrogate the viability of this method for Nafion by carrying out a series of measurements relying on 1H nuclear magnetic resonance (NMR) relaxometry and diffusometry experiments with and without ODNP hyperpolarization, accompanied by other complementary characterization methods including small angle X-ray scattering (SAXS), thermal gravimetric analysis (TGA) of hydration, and proton conductivity by AC impedance spectroscopy. Our comprehensive study shows that commonly used paramagnetic spin probes-here, stable nitroxide radicals-for ODNP, as well as their diamagnetic analogues, reduce the inner membrane surface hydrophilicity, depending on the location and concentration of the spin probe. This heavily reduces the hydration of Nafion, hence increases the tortuosity of the inner membrane morphology and/or increases the activiation barrier for water transport, and consequently impedes water diffusion, transport, and proton conductivity.

Dynamics of binary mixtures of an ionic liquid and ethanol by NMR.

A study of molecular dynamics of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoro-methylsulphonyl)imide ([Emim][Tf2N]) in solution with deuterated ethanol at different molar concentration and temperatures is presented. The study was performed using 1 H and 2 H nuclear magnetic relaxation and 2 H 1D spectroscopy. The temperature dependence of the spin-lattice relaxation time T1 of the cations allows the evaluation of the activation energies of the rotational degree of freedom of these molecules. The viscosity in the binary system increases with the concentration of ionic liquid. However, the activation energy in the cation molecules decreases when the concentration of the ionic liquid increases, indicating that the rotational dynamics is facilitated. This behavior is explained from the fact that the presence of the ionic liquid in the system disrupts the degree of intermediate range order expected in the alcohol system. Copyright © 2017 John Wiley & Sons, Ltd.

Correlations of low-field NMR and variable-field NMR parameters with osteoarthritis in human articular cartilage under load.

NMR experiments carried out at magnetic fields below 1 T provide new relaxation parameters unavailable with conventional clinical scanners. Contrast of T1 generally becomes larger towards low fields, as slow molecular reorientation processes dominate relaxation at the corresponding Larmor frequencies. This advantage has to be considered in the context of lower sensitivity and frequently reduced spatial resolution. The layered structure of cartilage is one example where a particularly strong variation of T1 across the tissue occurs, being affected by degenerative diseases such as osteoarthritis (OA). Furthermore, the presence of 1 H-14  N cross-relaxation, leading to so-called quadrupolar dips in the 1 H relaxation time dispersion, provide insight into the concentration and mobility of proteoglycans and collagen in cartilage, both being affected by OA. In this study, low-field imaging and variable-field NMR relaxometry were combined for the first time for tissue samples, employing unidirectional load to probe the mechanical properties. 20 human knee cartilage samples were placed in a compression cell, and studied by determining relaxation profiles without and with applied pressure (0.6 MPa) at 50 µm in-plane resolution, and comparing with volume-averaged T1 dispersion. Samples were subsequently stored in formalin, prepared for histology and graded according to the Mankin score system. Quadrupolar dips and thickness change under load showed the strongest correlation with Mankin grade. Average T1 and change of maximum T1 under load, as well as its position, correlate with thickness and thickness change. Furthermore, T1 (ω) above 25 mT was found to correlate with thickness change. While volume-averaged T1 is not a suitable indicator for OA, its change due to mechanical load and its extreme values are suggested as biomarkers available in low-field MRI systems. The shape of the dispersion T1 (ω) represents a promising access to understanding and quantifying molecular dynamics in tissue, pointing toward future in vivo tissue studies.

Dynamic Nuclear Polarization Fast Field Cycling Method for the Selective Study of Molecular Dynamics in Block Copolymers.

Dynamic nuclear polarization (DNP) is one of the most useful methods to increase sensitivity in NMR spectroscopy. It is based on the transfer of magnetization from an electron to the nuclear spin system. Based on previous work that demonstrated the feasibility of integrating DNP with fast field cycling (FFC) relaxometry and the possibility to distinguish between different mechanisms, such as the Overhauser effect (OE) and the solid effect (SE), the first FFC study of the differential relaxation properties of a copolymer is presented. For this purpose, concentrated solutions of the polystyrene-block-polybutadiene-block-polystyrene (SBS) triblock copolymer and the corresponding homopolymers were investigated. T1 -T2 relaxation data are discussed in terms of molecular mobility and the presence of radicals. The DNP selective data indicate a dominant SE contribution to the enhancement of the NMR signal for both blocks of the triblock copolymer and for the homopolymer solutions. The enhancement factors are different for both polymer types and in the copolymer, which is explained by the individual 1 H T1 relaxation times and different electron-nucleus coupling strength. The T1 relaxation dispersion measurements of the SE enhanced signal were performed, which led to improved signal-to-noise ratios that allowed the site-specific separation of relaxation times and their dependence on the Larmor frequency with a higher accuracy.

Enhanced Surface Interaction of Water Confined in Hierarchical Porous Polymers Induced by Hydrogen Bonding.

Hierarchical porous polymer systems are increasingly applied to catalysis, bioengineering, or separation technology because of the versatility provided by the connection of mesopores with percolating macroporous structures. Nuclear magnetic resonance (NMR) is a suitable technique for the study of such systems as it can detect signals stemming from the confined liquid and translate this information into pore size, molecular mobility, and liquid-surface interactions. We focus on the properties of water confined in macroporous polymers of ethylene glycol dimethacrylate and 2-hydroxyethyl methacrylate [poly(EGDMA-co-HEMA)] with different amounts of cross-linkers, in which a substantial variation of hydroxyl groups is achieved. As soft polymer scaffolds may swell upon saturation with determined liquids, the use of NMR is particularly important as it measures the system in its operational state. This study combines different NMR techniques to obtain information on surface interactions of water with hydrophilic polymer chains. A transition from a surface-induced relaxation in which relaxivity depends on the pore size to a regime where the organic pore surface strongly restricts water diffusion is observed. Surface affinities are defined through the molecular residence times near the network surface.

Spatially Resolved Monitoring of Drying of Hierarchical Porous Organic Networks.

Evaporation kinetics of water confined in hierarchal polymeric porous media is studied by low field nuclear magnetic resonance (NMR). Systems synthesized with various degrees of cross-linker density render networks with similar pore sizes but different response when soaked with water. Polymeric networks with low percentage of cross-linker can undergo swelling, which affects the porosity as well as the drying kinetics. The drying process is monitored macroscopically by single-sided NMR, with spatial resolution of 100 µm, while microscopic information is obtained by measurements of spin-spin relaxation times (T2). Transition from a funicular to a pendular regime, where hydraulic connectivity is lost and the capillary flow cannot compensate for the surface evaporation, can be observed from inspection of the water content in different sample layers. Relaxation measurements indicate that even when the larger pore structures are depleted of water, capillary flow occurs through smaller voids.

Diffusion tensor MR microscopy of tissues with low diffusional anisotropy.

BACKGROUND: Diffusion tensor imaging exploits preferential diffusional motion of water molecules residing within tissue compartments for assessment of tissue structural anisotropy. However, instrumentation and post-processing errors play an important role in determination of diffusion tensor elements. In the study, several experimental factors affecting accuracy of diffusion tensor determination were analyzed. MATERIALS AND METHODS: Effects of signal-to-noise ratio and configuration of the applied diffusion-sensitizing gradients on fractional anisotropy bias were analyzed by means of numerical simulations. In addition, diffusion tensor magnetic resonance microscopy experiments were performed on a tap water phantom and bovine articular cartilage-on-bone samples to verify the simulation results. RESULTS: In both, the simulations and the experiments, the multivariate linear regression of the diffusion-tensor analysis yielded overestimated fractional anisotropy with low SNRs and with low numbers of applied diffusion-sensitizing gradients. CONCLUSIONS: An increase of the apparent fractional anisotropy due to unfavorable experimental conditions can be overcome by applying a larger number of diffusion sensitizing gradients with small values of the condition number of the transformation matrix. This is in particular relevant in magnetic resonance microscopy, where imaging gradients are high and the signal-to-noise ratio is low.

NMR dispersion investigations of enzymatically degraded bovine articular cartilage.

PURPOSE: Cross-relaxation of protons with (14) N nuclei in proteins enhances relaxivity in the quadrupolar dip range of typically 2-3 MHz Larmor frequency. The magnitude of these dips was suggested as a means of assessing the degeneracy of articular cartilage during osteoarthritis (OA). However, so far only proteoglycans have been considered whereas collagen nitrogen was neglected. This study addresses the relative importance of glycosaminoglycans (GAG), collagen, and water content for the cross-relaxation effect. METHODS: Relaxation dispersion data were acquired for protons in samples of bovine articular cartilage, collagen, and GAG before and after the addition of trypsin or collagenase, and were compared with spatially resolved dGEMRIC experiments at 0.27 Tesla. RESULTS: Both collagen as well as GAG show quadrupolar dips that strongly depend on hydration. For typical water concentrations in cartilage, the effect of enzymatic activity onto GAG is minor but a strong dependence on water concentration is found. CONCLUSION: Quadrupolar dips in the (1) H relaxation dispersion of cartilage possess similar contributions from both GAG and collagen. The reduction of the cross-relaxation contribution observed in OA tissue is thus not directly proportional to GAG concentration, but maintains a collagen contribution and reflects predominantly the increase in water concentration during OA.

Structure and Dynamics of Solvation Shells of Copper(II) Complexes with N,O-Containing Ligands.

EPR, NMR relaxation methods, and DFT calculations were jointly used to investigate the structural and dynamical characteristics of solvation shells of copper(II) complexes with iminodiacetic acid, glycylglycine, and glycyglycylglycine in comparison with the copper(II) bis-glycinate studied previously. A strong trans influence of deprotonated peptide nitrogen was revealed in EPR spectra parameters of copper(II) complexes with oligopeptides. With models of the experimental NMRD data and literature X-ray structural information, it was suggested that only one water molecule coordinates in axial position of copper(II) complexes with glycine and di- and triglycine (Cu(Gly)2, Cu(GGH(-1)), and Cu(GGGH(-2))(-)), and the copper ion in these complexes is pentacoordinated, while in the iminodiacetate complex, Cu(IDA), both apical positions can be occupied by solute molecules. The obtained structural results were confirmed by DFT calculations of structures of studied compounds using different functionals and basis sets. It was shown that the donor ability of equatorial ligands and trans influence have an effect on the characteristics of the axial water bond. With increasing donor strength of equatorial ligands, pentacoordination of copper(II) complexes in water solutions becomes more preferable.

Dynamics of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide studied by nuclear magnetic resonance dispersion and diffusion.

The dynamics of the imidazolium based room temperature ionic liquid Bmim Tf2N was investigated by means of nuclear magnetic resonance relaxation dispersion (NMRD) and nuclear magnetic resonance pulsed field gradient (NMR-PFG) diffusion experiments on the bulk liquid in a wide range of temperatures. Relaxation and diffusion properties were determined for anions and cations individually, giving evidence of heterogeneities in the dynamics of the ionic liquid. The relevant NMR relaxation mechanisms are the inter- and intramolecular dipolar interactions between the molecular ions reflecting the molecular translational and rotational diffusion. Rotational and translational correlation times could be obtained and showed different dependences on temperature. The experimental diffusion values follow the Vogel-Fulcher-Tammann (VFT) relation above a transition temperature Tc∼ 1.26 Tg, below which a deviation was observed. Differential scanning calorimetry experiments show a transition at the same temperature.