Measurement of NMR diffusion coefficients to characterize complex materials at the nanometric scales

Gradient Field Nuclear Magnetic Resonance (PFG-NMR) allows to measure the diffusion coefficients of molecular and macromolecular species. The probed scales correspond to the nanometric and micrometric domains found in ‘nano’ materials. Researchers from LPS, CEA and LCMCP present in a pedagogical and technical paper the theoretical principles, practical aspects and use of data obtained by PFG-NMR to describe soft ‘nano’ materials: i) size, shape, complexation and association of nanoparticles, ii) dynamic behavior of polymeric or nanocomposite materials, iii) properties of phases confined in a heterogeneous medium such as porous, mesophase or gel.

Nuclear magnetic resonance (NMR) provides a wealth of information about molecular structure (chemical shifts, couplings, etc.) and dynamics (relaxation times, etc.). The development of NMR techniques using magnetic field gradients (PFG-NMR) also provides access to spatial information: magnetic resonance imaging and the translational mobility of molecules. In a recent article in Techniques de l’Ingénieur, researchers from LPS, CEA and LCMCP provide an overview of this technique and the possibilities it offers in the field of “nano” materials. It is intended for a wide audience of students, technicians, engineers, and researchers who want to know more about the capabilities of this spectroscopic method.

In the first part, the principles of NMR spectroscopy are described. In particular, the contribution of field gradients to the encoding of the magnetic field map is discussed. The inhomogeneity of the magnetic field is used to measure the translational displacements of the species present (molecules, ions, nanoparticles, etc.) and to deduce the associated self-diffusion coefficients.

Figure 1. a) NMR spectrometer at LPS, b) PFG-NMR “stimulated echo” pulse sequence used to measure diffusion coefficients c) experimental curve (highly concentrated hemoglobin solution).

Two main areas of  soft ‘nano’ science can benefit from gradient field NMR:

1) Solution characterization: This technique can be used to determine the size or molecular mass of entities in solution. The method is suitable for the characterization of species in the size range from a few Angstroms to several hundred nanometers (corresponding to molecular masses between 102 and 106 g/mol). It can be used not only to determine the the shape or conformation of objects, but also to: (i) spectrally separate information about the different entities present; (ii) and/or explicit 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 made directly in the medium of interest (solvent, pH, ionic strength, temperature…) and have applications in the fields of chemistry, materials, colloidal chemistry, synthetic and natural polymers, agro-food, petroleum industry, formulating and more.

Figure 2. Two examples of the use of self-diffusion coefficient measurements in the “nano” domain a) Determination of the hydrodynamic diameter of stabilized CdSe nanoparticles b) Porous medium: Influence of diffusion time and geometry of the confining medium on the space probed by a molecule.

2) Characterization of divided matter (porous, emulsions, mesophases…): This technique allows 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…… By analyzing these data, we can determine the transport properties in these reduced dimensional spaces as well as the interactions between confined molecules and the confining space (or its surface). These measurements have applications in catalysis, energy materials, filtration materials, soft and biological matter, biomaterials, swollen polymers or membranes, oriented phases, geology or pedology…

These non-destructive measurements can be extended to in-situ studies (chromatography columns, electrochemical devices, rheometers, natural reservoirs (water, oil), biomimetic systems, plants, etc.) and allow us to study matter under a wide range of physicochemical conditions.

Coefficients de diffusion RMN pour décrire les matériaux complexes
P. Judeinstein, F. Ribot, P. Wzietek, M. Zeghal
Techniques de l’Ingénieur, R 1307 (2024)
DOI : 10.51257/a-v1-r1307

Mehdi Zeghal
Pawel Wzietek
Patrick Judeinstein