Dynamic NMR Relaxometry as a Simple Tool for Measuring Liquid Transfers and Characterizing Surface and Structure Evolution in Porous Media

Abstract

Porous media containing voids which can be filled with gas and/or liquids are ubiquitous in our everyday life: soils, wood, bricks, concrete, sponges, and textiles. It is of major interest to identify how a liquid, pushing another fluid or transporting particles, ions, or nutriments, can penetrate or be extracted from the porous medium. High-resolution X-ray microtomography, neutron imaging, and magnetic resonance imaging are techniques allowing us to obtain, in a nondestructive way, a view of the internal processes in nontransparent porous media. Here we review the possibilities of a simple though powerful technique which provides various direct quantitative information on the liquid distribution inside the porous structure and its variations over time due to fluid transport and/or phase changes. It relies on the analysis of the details of the NMR (nuclear magnetic resonance) relaxation of the proton spins of the liquid molecules and its evolution during some process such as the imbibition, drying, or phase change of the sample. This rather cheap technique then allows us to distinguish how the liquid is distributed in the different pore sizes or pore types and how this evolves over time; since the NMR relaxation time depends on the fraction of time spent by the molecule along the solid surface, this technique can also be used to determine the specific surface of some pore classes in the material. The principles of the technique and its contribution to the physical understanding of the processes are illustrated through examples: imbibition, drying or fluid transfers in a nanoporous silica glass, large pores dispersed in a fine polymeric porous matrix, a pile of cellulose fibers partially saturated with bound water, a softwood, and a simple porous inclusion in a cement paste. We thus show the efficiency of the technique to quantify the transfers with a good temporal resolution.

Critical Role of Boundary Conditions in Sorption Kinetics Measurements

“…The same regime change will occur with a given system when the air flux intensity is increased from a very low to a high value or by increasing the sample thickness. This is in agreement with the observations of the water distributions over time during the drying of bead packings with pore sizes ranging from 1 mm to a few nanometers 5,72 or clayey materials. 74 Note that an additional complexity with such systems is that, in contrast with hygroscopic systems, at some stage of drying, an apparently dry region of significant thickness can develop below the free surface, while there is still water below this region.…”

“…This absorbed water is the so-called “bound water”, − which can amount to 30% of the dry mass. In contrast with molecules or particles adsorbing onto the surface of various materials, this bound water essentially forms by entering the amorphous regions between cellulose microfibrils, , thanks to the high sorption affinity of the cellulose chains for water molecules, in particular near their polar OH groups. − This bound water is thus confined in nanopores that it has itself created, which is at the origin of the swelling or shrinkage of such materials.…”

Mechanism of Hydration Induced Stiffening and Subsequent Plasticization of Polyamide Aerogel

“…Therefore, the water molecules localized in the different regions of a nanostructured material and participating in different chemical interactions with the solid backbone can be classified and characterized based on their respective T 2 values. [ 47,72 ]…”

“…Therefore, the water molecules localized in the different regions of a nanostructured material and participating in different chemical interactions with the solid backbone can be classified and characterized based on their respective T 2 values. [47,72] Naturally, water molecules bound by strong secondary chemical bonds (e.g., H-bonds) to the PA molecules in the primary hydration sphere of the aerogel backbone have characteristically low T 2 . Water molecules in the secondary hydration sphere or the ones forming puddles or droplets in the focal points of the PAA backbone have significantly higher relaxation times.…”

Characterization of functionalized chromatographic silica materials : Coupling water adsorption and intrusion with NMR-relaxometry

“…The relaxation time ratio has been used in the petrochemical industry to analyze the wetting behavior of oil in sedimentary rocks, e.g. [23,24]. In addition, it was demonstrated that the T1/T2-ratio of catalysts in contact with different liquids can be associated with the effective interaction strength between these liquids and the catalyst surface [25][26][27].…”