Spatially-resolved 1H NMR relaxation-exchange measurements in heterogeneous media

Abstract

In the last decades, the 1 H NMR relaxation-exchange (REXSY) technique has become an essential tool for the molecular investigation of simple and complex fluids in heterogeneous porous solids and soft matter, where the mixing-time-evolution of crosscorrelated peaks enables a quantitative study of diffusive exchange kinetics in multicomponent systems. Here, we present a spatially-resolved implementation of the correlation technique, named , based on one-dimensional spatial mapping along using a rapid frequency-encode imaging scheme. Compared to other phase-encoding methods, the adopted MRI technique has two distinct advantages: (i) is has the same experimental duration of a standard (bulk) measurement, and (ii) it provides a high spatial resolution. The proposed method is first validated against bulk measurements on homogeneous phantom consisting of cyclohexane uniformly imbibed in finely-sized α-Al 2 O 3 particles at a spatial resolution of 0.47 mm; thereafter, its performance is demonstrated, on a layered bed of multi-sized α-Al 2 O 3 particles, for revealing spatiallydependent molecular exchange kinetics properties of intra-and inter-particle cyclohexane as a function of particle size. It is found that localised spectra provide well resolved cross peaks whilst such resolution is lost in standard bulk data. Future prospective applications of the method lie, in particular, in the local characterisation of mass transport phenomena in multi-component porous media, such as rock cores and heterogeneous catalysts.3

Low-Field Functional Group Resolved Nuclear Spin Relaxation in Mesoporous Silica

“…The most data-rich methods for elucidating spin relaxation processes in porous media are multidimensional relaxation correlation measurements. − Indeed, two-dimensional (2D) relaxation–relaxation ( T 1 – T 2 − and T 1 – T 2 , ) and diffusion–relaxation ( D – T 2 − ) measurements are widely applied to characterize the dynamics of confined fluids, while higher-dimensional experiments, − including spatially resolved variants, , have also been reported. Such measurements are of utility in the study of fluids confined to heterogeneous solids as they are essentially independent of the detailed chemical shift phenomena associated with traditional frequency-based NMR methods, wherein spectral peaks are likely to be (i) dominated by species away from the interface of interest and (ii) unfavorably broadened due to the effects of magnetic susceptibility differences at the solid–liquid interface, often requiring exotic approaches (see e.g., ref ) to avoid an intractable loss of spectral resolution.…”

Functional Group Resolved Nuclear Spin Relaxation in Porous Media

“…16 The most data-rich methods for elucidating spin relaxation processes in porous media are multidimensional relaxation correlation measurements. [17][18][19] Indeed, two-dimensional (2D) relaxation-relaxation ( 1 − 2 [20][21][22] and 2 − 2 23,24 ) and diffusion-relaxation ( − 2 [25][26][27][28] ) measurements are widely applied to characterise the dynamics of confined fluids, while higher-dimensional experiments, [29][30][31][32] including variants exhibiting spatial resolution, 33,34 have also been reported. Such measurements are of particular utility in the study of fluids confined to heterogeneous solids as they are essentially independent of the detailed chemical shift phenomena associated with traditional frequency-based NMR methods, wherein spectral peaks are likely to be (i) dominated by species away from the interface of interest, and (ii) unfavourably broadened due to the effects of magnetic susceptibility differences at the solid-liquid interface, 35 often requiring exotic approaches (see e.g.…”