ABSTRACT
As recalled in the “Introduction,” the quadrupolar splitting indicates a preferential orientation of the observed species. In highly dilute suspensions, small, unresolved splitting gives rise to non-Lorentzian line shape that can also occur in other situations. To display small quadrupolar splitting, double-quantum filtering (DQF) methods are required.[1] Using such techniques, we have shown preferential orientation of clay counterions in dilute suspensions of Li-saponite,[1] but not of Li-laponite.[8] Thus, lithium cation environment remains isotropic in laponite suspensions, and the clay presence decreases only the mobility of the cation surroundings compared to that of lithium cations in bulk water. The stronger clay-cation interaction in the tetrahedrally substituted saponite accounts for the observed ordering effect. Such an observation has been also made for sodium counterions of saponite from 23Na DQF experiments.[9]
The lithium quadrupolar coupling obtained by DQF methods has been used to monitor clay-surfactant interaction. Addition of (A) to Li-saponite suspensions results in the increase of the cation quadrupolar splitting. Thus, the electric field gradient generated by the alkali ion environment is enhanced (lower symmetry), leading to a stronger quadrupolar interaction. Accordingly, surfactant molecules interact with clay counterions.[8] This interaction remains competitive as far as the hydrated cation is not too strongly condensed to the solid surface as it appears with the increase of the clay charge. Thus, that explains how charge modulates the interaction. 23Na longitudinal relaxation can also complement 13C relaxation data. A decreasing interaction of (A) with the increase of the saponite charge has been found, corroborating the previous results.[8]
Local ordering is detected in dilute suspensions, as shown by quadrupolar splitting, but macroscopic alignment is expected in dense clay suspensions. In agreement with self-diffusion measurements,[4] a nematic phase has been revealed by 23Na NMR spectra, which exhibit a quadrupolar splitting dependent on the sample orientation with respect to the external magnetic field.[11]
With high-resolution liquid-state NMR spectrometer, a loss of 23Na NMR visibility can occur. In anisotropic medium such as laponite or saponite suspensions, the narrow central signal (40% of the total intensity) is only observed.[1] When the clay is suspended in a solution of (B), sodium are counterions both of the negatively charged clay (minor contribution ca. 5%) and the surfactant carboxylate.[9] Micellar systems are isotropic (no splitting), and the totally visible 23Na NMR signal has the same intensity as a sodium chloride solution of equal concentration. No significant change in the signal intensity of these two solutions has been observed in the presence of small amount of a high-charge saponite. This corroborates the absence of surfactant interaction with this clay. By contrast, a drop of ca. 60% of the signal intensity occurs with laponite or the lowest charged saponite. If sodium ion exchange between the clay surface region and bulk water is fast on the scale of the quadrupolar interaction, the spectrum must exhibit an averaged triplet pattern. When the satellite signals are outside the detection range, only the central band (40%) is observed, accounting for the loss of signal intensity.[9] A similar observation is made with sodium dodecylsulfate, showing that anionic micelles in bulk water enhance the counterion exchange rate compared to the same clay suspended in an equimolar sodium chloride solution.[9]
High-resolution NMR in the solid state has been mainly used to characterize the structure and dynamics of adsorbed, intercalated species, and polymer nanocomposites.
