Longitudinal relaxation rate measurements for different signals unveiled by nutation spectroscopy: Application to the characterization of two arrangements experienced by water in a clay network

• Published in: Magnetic resonance in chemistry Vol. 60; no. 1; pp. 113 - 120
• Main Authors: Ferrari, Maude, Moyne, Christian, Canet, Daniel
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01-01-2022
• Subjects: Clay; Complex systems; Evolution; Kaolinite; NMR; Nuclear magnetic resonance; Nutation; organized clay platelets; Platelets; Radio frequency; Recovery; Relaxation time; Saturation; saturation recovery for nutation lines; water in clays; organized clay platelets; NMR; water in clays; saturation recovery for nutation lines; 1H
• ISSN: 0749-1581; 1097-458X
• DOI: 10.1002/mrc.5191

Nutation consists in monitoring the motion of nuclear magnetization under the application of a radiofrequency (rf) field. With an appropriate amplitude of the rf field, the nutation frequency depends on the NMR relaxation times. This property offers the possibility of differentiating species having the same Larmor frequency but differing by their relaxation times. This may occur for the composite proton NMR signal of water in complex systems. Separate nutation signals are thus observed with the possibility of measuring their longitudinal relaxation times by simply applying a saturation hard pulse, followed by an evolution interval, prior to the nutation sequence. This novel experiment has been used for studying the two sites existing for water in two kaolinite samples (one hydrated after stabilization of several months and the other nonhydrated). It turns out that water in these two sites differs essentially by its transverse relaxation time. Moreover, recovery is surprisingly biexponential for these two signals. A proper analysis of results obtained by this saturation‐recovery nutation experiment provides not only the specific longitudinal relaxation rates of water in these two sites but also information about the averaging which occurs at long evolution times. This is discussed with regard of the structure and organization of the clay network. In particular, from relaxation rates at short evolution times, it is shown that this network is mainly constituted of ordered platelets, with a relatively weak proportion of randomly distributed platelets.
The nutation spectroscopy involves two lines for water in a kaolinite sample. One line occurs at zero frequency corresponds to clusters of organized platelets and the other at a usual nutation frequency corresponds to clusters of randomly distributed platelets. The saturation recovery has been applied to these two lines and reveals some degree of organization.