Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation.

The molecular dynamics study of thermotropic mesogens exhibiting the crystal phases is valuable in unraveling the complex global (collective) and local (noncollective) motions executed by liquid crystal molecules, which would further advance the existing knowledge on orientationally disordered crystalline (ODIC) phases. Toward the fulfillment of such a task, a combined nuclear magnetic resonance (NMR) relaxometry approach employing the fast field cycling (FFC) NMR (10 kHz-30 MHz) and high-field pulsed NMR (400 MHz) techniques is utilized to sample the broad frequency range offered by molecular motions in the crystal phase of 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). The validity of the observed relaxation data is tested and interpreted by the Bloembergen-Purcell-Pound (BPP) model involving the superposition of four mutually independent Lorentzian spectral densities, reflecting molecular dynamical processes on different time scales. The salient feature of the detailed analysis reveals that the lengthening of temporal dynamics in the crystal phase due to molecular rotations by jumps, which are of intermolecular origin, is evident and further supports the presence of collective-like local dynamics. The analysis does permit decoupling of the molecular reorientations about their short axes (∼100 ns) as well as long axes (∼50 ns) and methyl group rotations (∼0.5 ns) on distinct time scales. The activation energies for reorientations about the short axes and methyl group rotations are found to be 27.3 ± 2.7 and 15.8 ± 1.1 kJ/mol, respectively. The fast methyl rotations in the crystal phase of 6CHBT obtained from FFC NMR are further well complemented by high-field NMR, where 1H NMR line shapes are relatively narrow when compared to those of the nematic phase.

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.

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.

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.

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.

Low-field and variable-field NMR relaxation studies of H2O and D2O molecular dynamics in articular cartilage.

Osteoarthritis (OA) as the main degenerative disease of articular cartilage in joints is accompanied by structural and compositional changes in the tissue. Degeneration is a consequence of a reduction of the amount of macromolecules, the so-called proteoglycans, and of a corresponding increase in water content, both leading to structural weakening of cartilage. NMR investigations of cartilage generally address only the relaxation properties of water. In this study, two-dimensional (T1-T2) measurements of bovine articular cartilage samples were carried out for different stages of hydration, complemented by molecular exchange with D2O and treatment by trypsin which simulates degeneration by OA. Two signal components were identified in all measurements, characterized by very different T2 which suggests liquid-like and solid-like dynamics. These measurements allow the quantification of separate hydrogen components and their assignment to defined physical pools which had been discussed repeatedly in the literature, i.e. bulk-like water and a combination of protein hydrogens and strongly bound water. The first determination of 2H relaxation dispersion in comparison to 1H dispersion suggests intramolecular interactions as the dominating source for the pronounced magnetic field dependence of the longitudinal relaxation time T1.

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.

Molecular Dynamics in Ionic Liquid/Radical Systems.

Molecular dynamics of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (Emim-Tf2N) with either of the four organic stable radicals, TEMPO, 4-benzoyloxy-TEMPO, BDPA, and DPPH, is studied by using Nuclear Magnetic Resonance (NMR) and Dynamic Nuclear Polarization (DNP). In complex fluids at ambient temperature, NMR signal enhancement by DNP is frequently obtained by a combination of several mechanisms, where the Overhauser effect and solid effect are the most common. Understanding the interactions of free radicals with ionic liquid molecules is of particular significance due to their complex dynamics in these systems, influencing the properties of the ion-radical interaction. A combined analysis of EPR, DNP, and NMR relaxation dispersion is carried out for cations and anions containing, respectively, the NMR active nuclei 1H or 19F. Depending on the size and the chemical properties of the radical, different interaction processes are distinguished, namely, the Overhauser effect and solid effect, driven by dominating dipolar or scalar interactions. The resulting NMR relaxation dispersion is decomposed into rotational and translational contributions, allowing the identification of the corresponding correlation times of motion and interactions. The influence of electron relaxation time and electron-nuclear spin hyperfine coupling is discussed.

Field-cycling NMR and DNP - A friendship with benefits.

Field-cycling relaxometry, or rather its electronic version with a resistive magnet which requires signal detection at a field strength of 1 Tesla or below, remains an inherently insensitive technique due to the construction compromise that goes along with the need for a fast-switching, low-inductance magnet. For the same reasons, signal lifetime is short and frequency resolution is typically not given, at least for the predominantly used hydrogen nuclei. Dynamic Nuclear Polarization (DNP) bears the potential to circumvent these disadvantages: not only has it been demonstrated to enhance magnetization by up to three orders of magnitude beyond its thermal value, but it also provides the possibility to address particular parts of a molecule, thus generating selectivity even in the absence of spectral resolution. At the same time, DNP requires the introduction of stable radicals giving rise to additional relaxation contributions. This article presents a straightforward way to recover the native relaxation rates of the undisturbed system, and shows examples in different research fields where field-cycling relaxometry is traditionally used for refining models of molecular dynamics and interactions.

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.

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.

Hyperpolarization by DNP and Molecular Dynamics: Eliminating the Radical Contribution in NMR Relaxation Studies.

Fast field cycling NMR relaxation dispersion represents a versatile method to elucidate the distribution of timescales of molecular motion for systems as diverse as polymers, proteins, and complex fluids. While electronic field switching accesses magnetic field strengths between about 1 T and Earth field, the method remains fundamentally insensitive and unspecific due to the low signal intensity at low fields and the inherently large field inhomogeneity that prohibits spectral resolution for most nuclei. These conditions limit the accessible concentrations and the detection of insensitive X-nuclei. Dynamic nuclear polarization (DNP) has been demonstrated to significantly enhance sensitivity, favoring low-field applications due to the increase in enhancement factors under these conditions. However, the required presence of radicals adds a significant and often dominating relaxivity to the system of nuclei, which has mostly precluded relaxation studies under DNP because of the need to separate several competing relaxation mechanisms. In this study, we present proof that the intrinsic relaxation dispersion of a substance can be completely recovered from experiments with different concentrations of radicals, irrespective of the nature of the DNP effect. This approach not only enhances detection sensitivity by at least one order of magnitude but also provides information about selective radical/molecule interaction that allows the separation of contributions from different molecular moieties from their differential enhancement and relaxation time.

X-nuclei hyperpolarization for studying molecular dynamics by DNP-FFC.

Dynamic Nuclear Polarization methods are used for improving the quality of the NMR data, opening new possibilities by increasing both the sensitivity and the selectivity in NMR relaxation experiments. Recently, Fast Field Cycling relaxometry combined with DNP was introduced, demonstrating that molecular dynamics studies in the presence of natural or artificial radicals are indeed feasible under conditions where the signal-to-noise ratio is frequently critical. In this work, the extension of NMR relaxation dispersion beyond 1H NMR, by hyperpolarization of X-nuclei, is demonstrated. Overhauser effect via nitroxide radicals in simple (low viscous) liquids and saline solutions was observed for 2H, 7Li and 13C nuclei at ambient temperature. Substantial NMR signal enhancement up to several hundred was achieved for the studied samples. An advanced approach for reconstructing of the original relaxation dispersion of pure substances is used to eliminate the effect of the additional radical relaxivity of the X-nuclei.

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.

On the influence of wetting behaviour on relaxation of adsorbed liquids - A combined NMR, EPR and DNP study of aged rocks.

The influence of wettability modification in natural rocks has been studied by means of NMR relaxation dispersion of water and several organic liquids, employing 1H, 19F and 2H relaxometry. Berea, Bentheimer and chalk were aged with a bitumen solution, altering the samples from water-wet to mixed-wet. Relaxation measurements were supported by EPR and DNP experiments which are sensitive to bulk radicals and interfacial radicals, respectively. The effect of ageing on relaxation was found to be twofold: first, the change from water-wet to oil-wet affects the dynamics of molecules differently, in particular their immediate interaction with the surface, which is reflected in their relaxation times; second, the bitumen cover includes paramagnetic impurities which act as additional relaxation sinks to all molecules. EPR was used to confirm the amount of deposited material and the total radical content of the rock samples, whereas DNP revealed a small but significant signal enhancement due to the surface-bound bitumen containing stable radicals. The DNP enhancement is dominated by the Solid Effect despite the low viscosity of the interacting fluids.

Overhauser DNP FFC study of block copolymer diluted solution.

Overhauser dynamic nuclear polarization (DNP) is the dominating hyperpolarization technique to increasing the nuclear magnetic resonance signal in liquids and diluted systems. The enhancement obtained depends on the overall mobility of the radical-carrying molecule but also on its specific interaction with the host molecules. Information about the nature of molecular and radical dynamics can be identified from determining the nuclear T1 as a function of Larmor frequency by Fast Field Cycling (FFC) relaxometry. In this work, DNP and FFC methods were combined for a detailed study of 1H Overhauser DNP enhancements at 340 mT (X-band) and 73 mT (S-band) for diluted solutions of a block-copolymer with and without the addition of TEMPO radicals. NMR relaxation dispersions of these solutions are measured at thermal polarization and DNP conditions in the X-band, and the obtained DNP data were analyzed by a model of electron-nucleus interactions modulated by translational diffusion. The coupling factors for the two different blocks of the copolymer are obtained independently from DNP and NMRD experiments. An additional contribution from scalar interactions was found for polystyrene blocks.

Dynamics of ionic liquids in poly(vinyl alcohol) porous scaffold. Low field NMR study.

In this study molecular dynamics of ionic liquids in poly(vinyl alcohol) scaffolds were investigated. The binary poly(vinyl alcohol) - ionic liquid (PVA-IL) compound was prepared from initial solutions of water, ionic liquid (IL) and poly(vinyl alcohol) (PVA) at different concentrations. Subsequently water was evaporated under open conditions, leaving the scaffold/IL system of interest. Low field nuclear magnetic resonance (NMR) relaxation and diffusion measurements, as well as 2D T1-T2 correlated NMR experiments were performed to determine specific local and translational dynamics properties at different time scales. Data suggest that during water evaporation, partial demixing of IL from the polymeric matrix leaves the remaining solvent confined in the porous structure formed by the PVA polymer. The results show that the translational diffusion, as well as the local rotational molecular dynamics is comparable to the bulk liquid state. Moreover, in partial saturation conditions, diffusion shows enhancements relative to the bulk.

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.

Improved miscibility of PA6 and chitosan by the electric-field assisted phase inversion.

A facile and efficient method for the improved miscibility of natural polymer/synthetic polymer blends is reported here based on the electric-field-driven phase inversion. We have employed bioderived chitosan (CS) and Polyamide-6 (PA6) as an example since their blends are known to always result in a large scale phase separation (i.e. CS settles to the bottom of the blends as sediment) during phase inversion procedure. The condensed structure of the polymer blends has been well characterized, notably by the polarized attenuated total reflectance infrared spectroscopy and proton longitudinal relaxation time (T1) distribution. The application of an electric field can orient the polar groups which will hinder the crystallization of blends and also increase the interphase interaction between PA6 and CS. The miscibility of the PA6/CS blends has been characterized by scanning electron microscopy and confocal Raman spectroscopy. It is shown that this controllable hydrogen bonding environment, induced by the presence of electric field, indeed greatly hinder the sedimentation of CS without destroying its chemical structure. The improved miscibility of PA6/CS blends can thus significantly increase the toughness and generate a somewhat larger tensile strength.

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.