M. Engelsberg

The isotropic-nematic interface in suspensions of Na-fluorohectorite synthetic clay.

Colloidal suspensions of Na-fluorohectorite synthetic clay platelets in saline water exhibit coexisting isotropic and nematic phases, due to gravitational separation of the polydisperse particles. We study the ordering of the platelets at the interfaces between various coexisting phases. Four different experimental techniques are employed: visual observation of birefringence, synchrotron wide angle and small-angle X-ray scattering, and magnetic resonance imaging. We find that at the narrow isotropic sol-nematic sol interface the platelets are lying horizontally, i.e. with their mean platelet normal along the vertical direction. The experiments indicate that the platelets align homeotropically both at the isotropic sol-nematic sol interface and at the nematic sol-wall interface. We further investigate the complex alignment effect of a horizontally applied magnetic field in the nematic sol, and we compare it with the adjacent nematic gel.

Continuous wave free precession. Practical analytical tool for low-resolution nuclear magnetic resonance measurements

The use of continuous wave free precession (CWFP) as a practical analytical tool for quantitative determinations in low-resolution nuclear magnetic resonance (LRNMR) is examined. The requirements of this technique are shown to be no more demanding than those prevailing in free-induction decay or spin-echo measurements. It is shown that the substantial gain in signal to noise ratio for a given acquisition time permitted by CWFP, can be exploited with advantage in practically any application of LRNMR. This applies not only to homogeneous low viscosity liquid samples but also to multi-component systems where differences in relaxation times of each component permit a separation of the individual contributions. As an example, the use of CWFP for fast quantitative determination of oil and moisture in various seeds is presented.

Intercalated Water in Synthetic Fluorhectorite Clay

(7)Li and (1)H nuclear magnetic resonance together with X-ray diffraction measurements in powdered samples and pseudocrystalline films of synthetic fluorhectorite as a function of relative ambient humidity permit to address several aspects of the structure and dynamics of intercalated water molecules. The role of proton exchange as a possibly dominant mechanism of charge transport in the one-water layer regime of hydration is reexamined. The experimental results in Li-fluorhectorite support the result of molecular simulations which predict, for Li-montmorillonite, the existence of an intermediate regime, between one-water layer and two-water layer states.

The limitations of a target field approach to coil design

The limitations of a recently proposed target field approach for the design of solenoidal magnets with highly uniform fields over large volumes are examined. Numerical results of the uniformities attainable and the corresponding current distributions are presented.

Low field intermolecular double-quantum coherence imaging via the Overhauser effect

Intermolecular double-quantum coherence (i-DQC) signals in liquids are usually associated with high magnetic fields. We demonstrate that, in a magnetic field of only 16mT, i-DQC imaging of water protons is feasible thanks to the nuclear magnetization enhancement produced by the Overhauser effect. i-DQC images of a phantom containing an aqueous solution of a trityl free radical, with phase encoding in the DQC evolution period or in the acquisition period, are presented. Possible applications of low field i-MQC images are proposed.

Structural determination of hydrogen site occupation in proton‐exchanged LiNbO3 by nuclear magnetic resonance

1H nuclear magnetic resonance measurements in proton‐exchanged LiNbO3 were employed to determine structural positions of sites believed to be responsible for the desirable optical properties of this material. Contributions from other proton sites were first subtracted from the line shape which was subsequently analyzed by Van Vleck’s second moment method using a spin‐echo technique to separate like‐spin from unlike‐spin contributions. Among various models examined, only one in which protons are located on oxygen planes nearest to normal lithium sites, midway between two oxygen anions in an oxygen triangle, was found to be consistent with the data.

Surface pattern formation in a swelling gel

We study the time evolution of surface pattern formation in a swelling spherical gel using proton nuclear magnetic resonance microimaging of water molecules migrating into the gel. Relaxation-weighted images permit mapping of the local evolution of the swelling ratio which, within the framework of a nonlinear poroelastic theory, can be identified with the determinant of the deformation gradient tensor. The wavelength of the surface pattern is examined and some analogies with the extensively studied problem of the wavelength at threshold for swelling rings are pointed out. By controlling the degree of cross-linking within a surface layer of the gel it is possible to monitor the transition from an undulating folding pattern to the formation of singular creases.

Distant-dipole field in liquids and diffusion: a perturbative approach.

A perturbative approach is employed to solve the Bloch-Torrey equations in the presence of distant-dipole fields in nuclear magnetic resonance. The procedure, although only carried out to first order in the perturbation parameter a=1/k2Dtaud, could, in principle, be generalized to higher orders. Here D is the diffusivity, taud the dipolar demagnetization time, and k is the wave vector of the spatial modulation of magnetization produced by the magnetic field gradient. The results are especially interesting for dilute binary mixtures consisting of molecular species with different diffusivities. In this case the calculated two-dimensional correlation spectroscopy revamped by asymmetric Z-gradient echo detection spectra are shown to be free from some inadequacies resulting from a simplistic application of standard approximations.

Diffusion, reaction kinetics and exchange of sodium in aqueous solutions containing a crown ether

Abstract Sodium self-diffusion coefficients in aqueous solutions containing various amounts of crown ether (18-crown-6) were measured using pulsed field gradient nuclear magnetic resonance (NMR) techniques. The fast exchange on a NMR time scale, prevailing in these systems as a result of the reaction kinetics, was monitored using 23 Na chemical shift and spin–spin relaxation rate measurements. The addition of very small amounts of 18-crown-6, while having a negligible effect on the macroscopic viscosity, can cause an anion-sensitive suppression of sodium diffusion which, for some electrolytes, was found to be quite significant.

Digital NMR imaging system for ultralow magnetic fields

A low-cost, full-body, ultralow magnetic NMR imaging system employing digital detection and RF synthesis is described. The data processing, which involves analogue to digital conversion at the Larmor rate of 680 kHz, followed by digital quadrature detection and digital smoothing with undersampling, is analysed. The performance of the system exhibits significant improvements over more conventional systems operating at ultralow magnetic fields.