Diffusion Measurements by NMR Techniques

A new view of diffusion in nanoporous materials

“…It is in contact with the atmosphere of the desired guest species, the pressure of which may be regulated from zero to the saturation pressure of the liquid. Information about the guest molecules is provided by the proton magnetic resonance signal of the sample generated by appropriately chosen sequences of rf pulses [26,27,46]. This includes in particular the determination of the total amount of molecules within the sample (resulting from the intensity of the signal following a so-called 90°pulse, the "free induction", top centre of Fig.…”

Benefit of Microscopic Diffusion Measurement for the Characterization of Nanoporous Materials

“…(4), PFG NMR is capable of confirming the occurrence of normal diffusion with direct experimental evidence. In order to translate this theory to the primary PFG NMR data, one has to ensure that (i) the semi-logarithmic plot of the signal intensity vs. the squared gradient intensity yields a straight line, and that (ii) the slope of its decay increases in proportion with the observation time [45]. It is important to emphasize that this assessment clearly only refers to the space scale of the observation.…”

“…Hydrogen is contained in most molecules of technical relevance and offers the best measuring conditions with respect to both signal intensity and spatial resolution [45]. The space and time scales typically covered in proton PFG NMR are in the range of micrometers (lm) for x and milliseconds (ms) to seconds (s) for t. Both the observation times and the minimum displacements still observable are controlled by the nuclear magnetic relaxation times of the considered spins.…”

“…In addition to a constant magnetic field, by applying a highly inhomogeneous magnetic field, i.e., "gradient pulses", over two short intervals of time, the diffusion path length covered by each individual molecule in the time interval t between these two gradient pulses is recorded by the difference in the respective resonance frequencies. PFG NMR may thus be shown to directly yield the probability distribution P(x,t) that, during time t, an arbitrarily selected molecule within the sample is shifted over a distance x in the direction of the applied field gradient [45]. The function P(x,t) is referred to as the mean propagator and results in the Fourier transform of the primary experimental data, i.e., the attenua-tion of the NMR signal intensity under the influence of the applied gradient pulses [51,52].…”

“…As a general tendency, the diffusivities are found to be larger for "more microscopic" measuring techniques. Thus, the QENS diffusivities (which refer to displacements over nanometers [26]) tend to slightly exceed the PFG NMR diffusivities (referring to displacements over typically micrometers [45]), while the techniques following molecular uptake on the individual crystallite, i.e., mesoscopic measurement [18,19], or on the whole bed reveal the smallest diffusivities. Such findings have to be correlated with the existence of a hierarchy of transport resistances acting in addition to the genuine pore structure: If a technique is able to resolve molecular displacements much smaller than the separation between these additional resistances, the vast majority of the observed molecules and hence, the resulting diffusivities, are unaffected by these resistances.…”