23Na 2D 3QMAS NMR and 29Si, 27Al MAS NMR investigation of Laponite and synthetic saponites of variable interlayer charge

Si-29, Al-27 MAS NMR is used to characterize Laponite RD and synthetic saponites of variable interlayer charge. The Si/Al ratios are in good agreement with the Calculated charge from chemical analysis except for the lowest-charged saponite. In contrast to the Si-29 MAS NMR spectra in which resolved signals are detected, the Al-27 MAS NMR spectra show one signal whose linewidth increases with the clay charge. The water content of the clay samples was obtained from H-1 MAS NMR. The 2D MQMAS NMR technique is required to obtain a high-resolution Spectrum Of nuclei With strong quadrupolar interaction. This method was applied to the Na-23 nucleus of Clay counterions and to the Al-27 Structural nucleus. One well-defined Na-23 NMR signal is observed for all the clays studied except the highest-charged saponite. Possible explanations for this different behaviour are discussed. The Calculated isotropic chemical shift evolves progressively with the clay charge whereas the deduced quadrupolar interaction does not change significantly. The Al-27 2D 3QMAS technique was not able to resolve more than one signal.