Study of liquid–liquid interfaces by an easily implemented localized NMR sequence

Abstract

To selectively extract heavy metals from solutions containing fission products, it is essential to optimize the liquid-liquid extraction processes. Such an objective requires improving the fundamental knowledge of the different mechanisms that are involved in these processes. In that respect, we propose a localized NMR sequence named LOCSY to assess the concentration profiles of different species involved in these processes. One of the goals of this sequence is to study the products as close as possible to the liquid-liquid interface with the help of a standard NMR spectrometer of chemistry labs. The one-dimensional spatial localization along the NMR tube is obtained by a discrete stepping of the frequency-selective excitation pulses under a pulsed field gradient. Specific data processing has been developed to obtain the 1D NMR spectra as a function of the vertical position in the NMR tube. The LOCSY sequence has been tested and evaluated on three different systems: (i) a cylindrical phantom inserted in the NMR tube containing 4-methylsalicylic acid solution, (ii) D(2)O/olive oil biphasic system, and (iii) the dissolution of solid saccharose in D(2)O. These examples illustrate potential applications of the LOCSY sequence, particularly the possibility to measure concentration profiles and to study phenomena such as diffusion, provided the dynamic range is compatible with NMR timescale and sensitivity.

Origin and correction of magnetic field inhomogeneity at the interface in biphasic NMR samples

“…5 Throughout shimming of the biphasic system, the acetone peaks in the two phases were kept separated approximately by Δ f CS . It is tempting to shim by forcing the acetone peaks to be coincident [16], but that is erroneous. Such a procedure actually warps the field, since it is clear that in a homogeneous field the lines will be separated by their solvent shift difference.…”

“…Techniques other than NMR used to investigate surface properties of biphasic systems include atomic force microscopy [8], scanning electron microscopy [9], nonlinear optics [10], scanning electrochemical microscopy (SECM) [11], and x-ray [12] and neutron [13] scattering. A number of methods have been proposed for generating slice-resolved spectra of biphasic systems prepared in conventional NMR tubes, and run in high resolution liquid probes equipped with z-gradients [3, 14–16]. Here we study consequences of, and remedies for, magnetic field in homogeneity caused by susceptibility mismatch at the interface.…”

“…Mantel et al dealt with this latter issue by offering a program which shears the data sets to cleanly separate spectral and spatial coordinates. They further accounted for axial probe sensitivity variation by weighting intensities of slices according to the experimentally determined probe profile [16]. Lambert et al [15] showed that 1 μ m slice resolution is feasible.…”

“…One is whether slice volumes are large enough to provide adequate signals from lower-concentration-surface-active species [17]. A second is that it can be difficult to isolate diffusive transport from unanticipated slow convection [16, 18], and slice-selection studies [3] have yet to take this into account. A third problem, which we address here, is line shape distortion in the vicinity of the interface caused by susceptibility mismatch between phases.…”

“…Additionally, we present data using a 2D pulse sequence based on stepped-gradients. This approach entirely avoids shearing of the chemical shift/position correlation common to frequency-selective methods [14, 16], while offering sensitivity advantages analogous to those of Fourier over slow passage NMR spectroscopy [19]. …”