Combining macroscopic and microscopic diffusion studies in zeolites using NMR techniques

In this study the zero length column (ZLC) technique is extended to the case where the decay of the adsorbed phase concentration is observed directly by nuclear magnetic resonance (NMR). An adsorption-desorption apparatus compatible with a 400-MHz NMR spectrometer was developed. It operates with nitrogen or helium as the inert purge gas. The column of the adsorbent material is placed in the sensitive region of the superconducting magnet and the rf coil of the NMR spectrometer. The time scales of the adsorption and desorption processes depend on concentration, temperature and crystal shape and are found to be in the range of 1-10 min. From the desorption branch, the non-equilibrium ZLC-NMR measurements yield intracrystalline diffusion coefficients in the range of 10(-13) to 10(-11) m2/s for different alkanes in silicalite-1. These values are always found to be smaller than the values measured by pulsed field gradient NMR under equilibrium condition indicating that there must be additional transport resistance at the external surface of these silicalite-1 zeolite crystals.     

On the problem of field-gradient NMR measurements of intracrystalline diffusion in small crystallites--water in NaA zeolites as an example

Necessary conditions for measuring intracrystalline diffusion in small crystal size systems via field-gradient NMR are discussed. As an illustrative case self-diffusion coefficients of water adsorbed in NaA zeolites (average crystal diameter about 1 μm) have been measured by ¹H-NMR stimulated echoes in static magnetic field gradients of up to 180 T/m in the temperature range of 254–344 K. Obtaining intracrystalline diffusion coefficients necessitates a sufficiently high spatial resolution only provided by such large field gradients.     

NMR studies of restricted diffusion in lyotropic systems

Tortuosity, 1/alpha, and surface-to-volume ratio, S/V, were determined in aqueous solutions of decylammonium, dodecylammonium and tetradecylammonium chlorides of various concentrations by measuring the apparent diffusion coefficient of water, D(app)(delta). This was found to be much smaller than in the bulk state. Such restricted diffusion is interpreted in terms of the Mitra model, where D(delta) depends on diffusion time and is controlled primarily by S/V. The samples exhibit lamellar (L), hexagonal (H) and isotropic (I) liquid crystalline phases. We observed changes in S/V upon phase transition. In the lamellar and hexagonal phases, the system is ordered, resulting in relatively small S/V ratios, compared to the micellar-isotropic phase. We did not observe a dependence on the diffusion time, delta, in the isotropic phase, because the duration of the experiment was not sufficiently short to observe the change from D(app)(delta) to D(eff). We observed the effective diffusion coefficient of water, which directly probes the tortuosity of the system. The S/V ratios were obtained by fitting the Mitra model, using known values of the bulk water diffusion coefficients, and the assumption that D(app) --> D0 for delta --> 0. S/V is correlated with the type of structure, increasing on transition to the isotropic phase and decreasing on transition to other phases. The change in tortuosity is small, but slightly larger for the isotropic phase.     

NMR of water in biological systems

Summary A simple relationship has been thought to exist between the dynamics of water in heterogeneous (liquid-solid) systems and the NMR response. This relationship is usually expressed by the Bloembergen-Purcell-Pound (BPP) equations for relaxation and the phase model. However, a requirement for the use of the BPP theory is that motions take place in an isotropic, infinite and three-dimensional space. It is shown that the mere presence of solid surfaces causes the appearance of solidlike features in the NMR response of the liquid even if its dynamics is directly affected by the surfaces. Some of these ?topological? or ?indirect? surface effects are of the same kind as the low-dimensionality effects. Their order of magnitude is estimated for simple geometries and by treating the liquid motion in a hydrodynamic approximation. Comparison with the experiment is carried on in a companion paper. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

A novel application of NMR microscopy: measurement of water diffusion inside bioadhesive bonds.

The self-diffusion coefficient of water (D) inside bioadhesive bonds formed by dry and prehydrated hydrophilic matrices has been spatially resolved using nuclear magnetic resonance (NMR) microscopy. One-dimensional profiles showing the variation of D inside bioadhesive bonds were calculated from nine diffusion-weighted profiles obtained immediately after bond formation and every 5 min for 30 min. The resulting data indicated that the hydration state of a hydrophilic matrix can significantly and dramatically influence the dynamics of water movement inside a bioadhesive bond.     

NMR studies of water diffusion and relaxation in hydrating slag-based construction materials.

The NMR relaxation properties of hydrating blast-furnace slag cements have recently been shown to be dominated by the effect of water self-diffusion in internal magnetic field gradients in the pastes. While this was suggested on the basis of NMR relaxometry and magnetic susceptibility data, we report here the results from first direct studies of the water self-diffusion in the hydrating paste using a specialized PFG sequence and very intensive magnetic field gradient pulses.     

NMR study of hydrogen diffusion in uranium hydride

The diffusion of hydrogen in uranium hydride is studied employing the NMR technique. From measurements of spin-spin relaxation time T₂, the activation energy for hydrogen diffusion in β-UH₃ is determined to be $\text{E}{}_{\mathrm{a}}\text{= (19.25 ± 0.4)}\text{kcal}\text{mole}$ and the preexponential factor to be A₀ ≈ 5 × 10¹⁴ Hz. It is shown that these results are in fair agreement with spin-lattice relaxation time T₁ data. Assuming that hydrogen diffusion proceeds via vacancies whose concentration is temperature dependent, it is concluded that E_a is the sum of the energies of vacancy formation and barrier height, and that A₀ contains an entropy change factor. Using vacancy concentration data calculated by Libowitz, we estimate the barrier height energy to be E_b ≈7 kcal/mole. Using a value for the frequency of hydrogen vibration v₀ determined from inelastic neutron scattering by Rush et al., we estimate the entropy change due to vacancy formation and the hydrogen atom jump to be about $\text{S}\text{k}{}_{\mathrm{B}}\text{≈3}$. Similar measurements on samples containing less hydrogen than is needed to compose stoichiometric UH₃, show that the rate of diffusion is enhanced by the presence of excess metal in the sample. The jump frequency at 500°K in UH₃ is found to be approximately 10⁶ Hz while for the two-phase samples of H/U = 2.8 and 2.5, it is larger by a factor of about 3 and 3.5, respectively.     

1H NMR study of water relaxation and self-diffusion in human serum albumin aqueous solutions

Summary Water proton spin-lattice relaxation and self-diffusion in aqueous solutions of human serum albumin have been studied by¹H NMR as a function of the protein concentration. Spin-lattice relaxation data, which display a nonlinear behaviour with the protein concentration, could be fitted with a two-phase model taking into account the experimentally determined hydration (?bound?) water values. Despite a similar trend is registered for the water self-diffusion coefficient, such a model has been found unable to explain the related experimental data taken as a function of the biomolecule concentration. On the other hand, the solute-induced proton self-diffusion decrease could be satisfactorily interpreted by postulating an enhanced probability of hydrogen-bond formation occurring within the ?vicinal? water surrounding the biomolecules for several hydration shells. The consistency within the two models is discussed in connection with the magnetic interactions occurring within the solute-solvent systems.     

NMR study of Li diffusion in β-LiAl

Diffusion of a Li atom in an anode material β-LiAl was studied over the composition range of 48.0–50.2 at % Li by the ⁷Li magnetic relaxation times. Dependence of T₁ upon temperature and the resonance frequency was successfully explained by a vacancy diffusion on the Li sublattice involving a distribution of the activation energy for migration, E_a. The central value of E_a was almost independent of the composition (? 13.8 kJ mol^-1), while the width of distribution increased with increase of the number of substituted Li on the Al sublattice. The obtained diffusion constants, extrapolated to 415°C, were larger by a factor of 1.8–3.4 than those obtained by the electrochemical method.     

Translational diffusion of water in compacted clay systems

The translational diffusion of water in compacted clays at a high hydration level has been investigated by quasielastic neutron scattering at a time-of-flight spectrometer FOCUS (SINQ). Four compacted clays with systematic structural differences have been studied: Na-montmorillonite, Na-illite, kaolinite and pyrophyllite. The QENS experiments were performed using two different incident wavelengths in order to access a larger Q range and verify the data analysis. The translational diffusion coefficient for water in Na-montmorillonite and Na-illite are lower than those for bulk water, whereas the preliminary results for kaolinite and pyrophyllite show larger diffusion coefficient.     

Measurement of diffusion coefficients in polystyrene using xenon-129 NMR

A sample of polystyrene beads, 18 μm in diameter, has been sealed in an NMR tube under 10 atm of xenon gas. Two dimensional,¹²⁹Xe NMR spectra show cross peaks between the resonances corresponding to xenon in the free gas and the sorbed state, indicating that appreciable exchange occurs during the mixing time of the NMR experiment. Selective saturation of the free gas resonance attenuates the integrated intensity of the sorbed xenon resonance as a function of saturation time, thus allowing the accurate measurement of the exchange rates between the gas and the sorbed states. A model has been developed using a slightly modified form of Crank’s treatment of diffusion in a sphere which allows for the accurate determination of the diffusion coefficient for xenon in the sorbed state. The diffusion coefficient for xenon in polystyrene at 25°C is determined to be 2.9·10^?9 cm²/s.     

MR microscopy of water diffusion tensor in biological systems

Diffusion tensor imaging (DTI) of the control and mechanically injured spinal cord of a ratin vitro andin vivo are reported.In vitro experiments were done on a home-built 6.4 T magnetic resonance microscope. Results for formaline-fixed samples show significant differences in diffusion tensor components between gray matter and white matter of the control spinal cord. Moreover, it is shown that already 6–10 min after the injury DTI can detect changes in water diffusion in areas extending far beyond the region of primary tissue damage.In vivo experiments were performed using a 9.4 T Magnex magnet and Bruker Medspec imaging system. Good-quality DTI images, free from motion artifacts were obtained. Results from control samples confirm differences in water diffusion between white matter and gray matter, observedin vitro. In vivo experiments show that characteristic changes in water diffusion observedin vitro 6–10 min after injury are preserved 60–360 min after injury, without significant alteration during this time.     

Separation of velocity distribution and diffusion using PFG NMR

Pulsed field gradient (PFG) NMR is applied to investigate flow processes. In this case the NMR signal experiences phase modulation due to flow and signal attenuation due to the distribution of velocities. The velocity distribution consists of one part originating from diffusion and of a second part, the distribution of the directed motion. The usual PFG-experiment in which the gradient strength is incremented cannot distinguish between both. Incrementing velocity at constant gradient strength keeps the contribution from diffusion constant but changes the absolute width of the velocity distribution. So the signal is attenuated again, but only due to the distribution of the directed motion. The phase modulation as a signature of flow is not affected by this strategy, because velocity and gradient strength are Fourier conjugated. The key advantage of this approach is the possibility of measuring very low velocities, which only cause a very slight phase modulation that is easily covered by diffusion. The method is discussed here for very slow flow in a rheometer cell.     

Thermodynamic properties of aqueous non-electrolyte mixtures. Alkanols + + water systems.

The molar excess enthalpies, H^E, at 298.15 K, and the liquid-liquid equilibrium, LLE, of aqueous mixtures of alkanols have been measured.The results are interpreted qualitatively in terms of intercomponent molecular interactions and hydrophobic stabilization of the water structure by the alkanol molecules. The results obtained indicate that the geometry, the number of alkyl groups in the aliphatic chain and the relative position of the functional group -OH have an important action on these two effects.     

Solid-state NMR studies for the determination of 11B electric field-gradient tensor orientations in P/B Frustrated Lewis Pairs and related systems

A solid state NMR method is described for measuring the angle Θ specifying the orientation of the principal component of the ¹¹B electric field gradient tensor relative to the ¹¹B^…31P internuclear vector of ¹¹B–³¹P spin pairs. It is based on the anisotropic dephasing of ¹¹B spins in the dipolar field of ³¹P nuclei via ¹¹B{³¹P} Rotational Echo DOuble Resonance (REDOR) experiments. The method is applied to four solid borane–phosphane compounds related to Frustrated Lewis Pair (FLP) chemistry. Results determined by numerical line shape simulations are found in excellent agreement with theoretically calculated values using advanced DFT methods. The angle Θ, which can be measured with an estimated precision of ±5°, offers a clear spectroscopic distinction between classical Lewis-acid/base adducts and active Frustrated Lewis pairs (FLPs).     

A pulsed NMR study of hydrogen diffusion in hydrides of group IVa transition metals

Activation energies E_A for hydrogen diffusion in hydrides of Group IVa transition metals have been determined by ¹H nuclear magnetic resonance measurements of spin lattice relaxation in both the laboratory (T₁) and rotating (T_1p) frames. For the HfHx system both activation energies obtained from T₁ data, and the value of T₁ at the minimum appear to be insensitive to hydrogen content x in the range 1.58 ? x ? 1.98. For hydrides of titanium and zirconium of approximately stoichiometric composition MH₂ (where M = metal), there is excellent agreement between activation energies obtained from T₁ and T_1p data. Mean activation energies obtained were 0.51 eV for TiH_1.98 and 0.83 eV for ZrH_1.96, consistent with a single diffusion mechanism in each case over the temperature range 260–600K and 400–800K respectively. In the case of HfH_1.98 the agreement was less good, values of 0.64 and 0.55 eV being obtained from T₁ and T_1p respectively.     

NMR diffusion of hyperpolarised 3He in aerogel: a systematic pressure study.

Diffusion of hyperpolarised 3He in a silica aerogel sample with 98% porosity was measured by NMR, and systematically studied as a function of pressure P in the range 20 mbar -1 bar. Spin-echo amplitudes, recorded with pulsed gradients in the usual CMPG sequence, follow a monoexponential decay, and the decay rate varies quadratically with the gradient intensity: thus the apparent diffusion coefficient of the gas D can be evaluated. Our results show a power-law (1/D proportional to P(0.55)), which could be consistent with a fairly broad mean free path distribution, implying some inhomogeneity in the aerogel structure. We have observed a spatial non-uniformity of D, and some deviation to the quadratic dependence of the decay rate with the gradient intensity, possibly related to sample structure inhomogeneity.