NMR diffusion coefficients to describe complex materials

Nuclear magnetic resonance (NMR) is a spectroscopy that provides a wealth of information on the structure of molecules (chemical shifts, couplings, etc.) and their dynamics (relaxation times, etc.). The development of techniques using magnetic field gradients also provides access to spatially localized information: magnetic resonance imaging and translational molecular mobility. These data are ideally suited to the study of micrometric and nanometric scales.

After recalling the principles of NMR spectroscopy and, in particular, the contribution of field gradients to encode the magnetic field map, we'll show how this technique is used to measure (macro)molecular translational displacements and deduce the associated self-diffusion coefficients.

Two main areas can benefit from gradient field NMR.

Solution characterization: this technique enables the size or molecular mass of entities in solution to be determined. This method is suitable for characterizing molecules/objects with dimensions between a few ångströms and several micrometers (or molecular masses between 10 ² and 10 ⁶ g/mol). It can be used to specify the shape or conformation of objects, but also to :

• spectrally separate information relating to different entities ;

• and/or specify the mass distribution of these objects in the case of a polydisperse mixture.

In the case of molecular assemblies, coordination complexes, micelles or associative systems, these data can also be used to determine equilibrium constants and characterize surface ligands. These measurements can be carried out directly in the medium of interest (solvent, pH, ionic strength, temperature, etc.), and have applications in the fields of chemistry, materials, colloidal chemistry, synthetic and natural polymers, agrifoods, the oil industry, formulation, etc.

Characterization of divided matter (porous, emulsions, mesophases...): this technique enables us to determine the self-diffusion coefficients of molecules or polymers confined in heterogeneous divided media, such as porous or lamellar compounds, networks, emulsions, mesophases, gels, coacervates... Analysis of these data enables us to determine the interactions between confined (macro)molecules and the confining space (or its surface), and to access transport properties in these spaces of reduced dimensionality. These measurements have applications in catalysis, energy materials, filtration materials, soft and biological matter, biomaterials, swollen polymers or membranes, oriented phases, geology or pedology...

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