Rex E. Gerald

Microemulsions of water in supercritical carbon dioxide : an in-situ NMR investigation of micelle formation and structure.

High-pressure NMR spectroscopy was used for the first time to investigate microemulsions of water in supercritical carbon dioxide. The emulsions were formed using a family of anionic perfluoropolyether ammonium carboxylate surfactants. This system holds promise as a reaction medium for conducting homogeneous catalytic reactions within the aqueous micellular cores while, at the same time, exploiting the facile mass transfer properties of the supercritical fluid. Ammonium hexafluorophosphate was used as a water-soluble ionic guest to investigate micelle formation and structure. Under micelle-forming conditions, the PF{sub 6}{sup -} guest, surfactant, and water were uniformly dispersed throughout the CO{sub 2} phase, as demonstrated by in situ NMR imaging. In addition, the micelles were observed to form even in the absence of mechanical stirring. This spontaneous formation of micelles demonstrates that the NMR spectral properties were obtained under conditions that result in the production of thermodynamically stable microemulsions. The nuclear overhauser effect (NOE) was used to probe the micellular structure through dipole-dipole interactions between the PF{sub 6}{sup -} anion and the fluorinated backbone of the surfactant. A strong negative homonuclear NoE was observed between the PF{sub 6}{sup -} guest and the fluorine moiety that is located directly adjacent to the surfactants carboxylate head group.morexa0» This highly specific negative NOE indicates an ordered arrangement, where the PF{sub 6}{sup -} anion and carboxylate ion are located in close proximity to one another. This close association of two negatively charged ionic groups in an aqueous environment is unusual and suggests that the PF{sub 6}{sup -} guest is concentrated within the electric double layer that forms at the micellular interface.«xa0less

COMPUTATION OF DISTORTIONS IN MAGNETIC FIELD AND SPECTRUM FOR NUCLEAR MAGNETIC RESONANCE INSTRUMENTS

A method for highly accurate modeling of induced field distortions in nuclear magnetic resonance (NMR) is developed based on the partial field formulation of the magnetostatic problem and use of the surface integral equation technique. A bound for the error in the NMR spectrum dispersion computation is derived in terms of the energy norm error associated with the static field calculations. It is shown that distortion fields can be computed with sufficient accuracy in many NMR applications using only the equivalent sources induced by the applied uniform static field.

RIDE’n RIPT—ring down elimination in rapid imaging pulse trains ☆

Abstract A new pulse sequence is introduced for compensation of acoustic ringing effects, which occur in rotating-frame images obtained with the rapid imaging pulse trains (RIPT). The new sequence (RIDE’n RIPT) combines features of ring down elimination (RIDE), the most common difference-spectroscopy sequence for acoustic-ringing compensation, with the advantages of RIPT for fast acquisition of magnetization profiles in B 1 field gradients. For even greater time efficiency in many experiments, the two transients of RIDE’n RIPT are combined to a single transient in which data for the difference spectroscopy are collected sequentially. RIDE’n RIPT was used to record one-dimensional profiles of the proton magnetization in supercritical fluid samples of methane in carbon dioxide. The profiles showed substantial improvements over profiles obtained from standard RIPT. To withstand the high pressures required for the supercritical carbon dioxide mixtures, a toroid cavity autoclave (TCA) was used as the NMR resonator and pressure vessel. The well-defined, strong, and nonuniform B 1 field of the TCA was used to resolve distances along the radial dimension.