Advanced Interpretation of Field Cycling NMR Relaxometry Dispersion Profiles From Hard and Soft Materials

Abstract

A fast field-cycling NMR (FFC NMR) experiment measures the longitudinal (spin–lattice) relaxation rate as a function of applied magnetic field to yield a relaxation rate dispersion curve, $R_1\left(f\right),$ where $f$ is the proton Larmor frequency. The $R_1\left(f\right)$ dispersions, or NMRD profiles, are exquisitely sensitive to the relative dynamics of proton spins across timescales in the range 10⁻⁹–10⁻⁴ s. These timescales span the translational and rotational dynamics of proton-bearing fluids to dynamics at the surfaces of solids, soft material and macromolecules. FFC NMR is useful for studying fluid-filled rocks and soils, porous silica and cementitious material, polymer systems, foodstuffs, protein systems, biological tissues and biofluids. The NMRD profiles are rich with information, but interpretation is challenging. A parametrized relaxometry model must generate an NMRD profile $R_1\left(f\right)$ that can be fit to experimental data across three to four orders of magnitude of frequency. The 3-Tau model has emerged as a model capable of fitting NMRD profiles from a broad range of material types yielding physically meaningful parameters. This review article demonstrates power of the FFC NMR experiment interpreted using the 3-Tau model to reveal properties of hydrated hard and soft material.