Nuclear Magnetic Resonance Dipolar Cross-Relaxation Interaction between Nanoconfined Fluids and Matrix Solids

Abstract

Many different methods have been developed to investigate fluid–solid interactions in nanoporous systems. These methods either only work in the liquid phase or provide an indirect measurement by probing the fluid–solid interaction based on a measured property change of the fluid or solid under different sample conditions. Here, we report a direct measurement technique using NMR dipolar cross-relaxation between the nanoconfined fluids and the matrix solids. The method was tested using a methyl-functionalized mesostructured silica saturated with methanol as a model sample. A formal theory was established to describe the enhanced dipolar cross-relaxation interaction between the nanoconfined fluids and the matrix solids. Both the experiment and theory showed that nanoconfinement of the fluids enhances the dipolar cross-relaxation interaction between the fluid and the matrix solids, which can be applied to investigate the fluid–solid interaction for various materials of a similar nanostructure.

Effect of Nanoconfinement on NMR Relaxation of Heptane in Kerogen from Molecular Simulations and Measurements

“…In the fast-motion (i.e., low-frequency) regime, the simulations would normally predict that T 2 ≃ T 1 lies on the 1–1 diagonal of the 2D map, which is clearly not consistent with T 2 measurements for dissolved heptane in Figure . Similar inadequacies in T 2 have also been noted in other nanoconfined systems. , Upon reflection, we can infer that in contrast to the bulk there is anisotropy in the system originating from tight confinement and strong solid/fluid interactions, leading to additional dephasing (i.e., transverse relaxation) in T 2 . This residual dipolar coupling (RDC) has been hypothesized to play an important role in organic-rich shale. , The RDC is given by the following expression in units of (s –1 ), where r ij is the distance between 1 H dipoles i and j and θ ij is the angle the vector connecting the two dipoles makes with the external magnetic field.…”

“…Cross-relaxation has a tendency to average out and narrow the measured T 1 distribution from all 1 H in contact with each other, via spin diffusion (a.k.a. magnetization transfer). ,,,, In the present case, the 1 H-bearing molecules in contact with each other include intergranular heptane, dissolved heptane, bitumen, and kerogen. The cross-relaxation is evidenced experimentally by the narrower T 1 distribution compared to T 2 in Figure , which makes the pore-size distribution P ( d ) more accurate from T 2 than from T 1 .…”