²³Na Nuclear Quadrupolar Relaxation as a Probe of the Microstructure and Dynamics of Aqueous Clay Dispersions:  An Application to Laponite Gels

Abstract

23Na relaxation studies of aqueous clay dispersions showed influence of the order/disorder transition of clay gels and the quadrupolar relaxation of neutralizing sodium counterions. Detection of the fast and slow components of the 23Na transverse magnetization gives access to the microstructural and dynamical properties of colloidal dispersions. Numerical simulations of sodium diffusion, using a simplified model of clay dispersion, explain the pathology of the 23Na relaxation rates by the high sensitivity of the quadrupolar coupling to the orientational correlations of charged interfaces. Frequency variation of the NQR relaxation rates of confined counterions therefore appears as a new tool for the investigation of the spatial correlations of charged interfaces within disordered porous media.

Multi-Quanta Spin-Locking Nuclear Magnetic Resonance Relaxation Measurements: An Analysis of the Long-Time Dynamical Properties of Ions and Water Molecules Confined within Dense Clay Sediments

“…7 Li NMR spectra ( Figure 5a) recorded for lithium counterions confined within Laponite sediment [64] exhibit the same behavior except that the previous doublet is replaced by a triplet, as expected for 3/2 spin nuclei [63]. As determined by numerical simulations [65], the principal axis of the tensor describing the EFG felt by the neutralizing counterions is oriented parallel to the clay director. As a consequence, the residual quadrupolar coupling is also monitored by the film orientation into the static magnetic field.…”

A Multinuclear Magnetic Resonance Study of Synthetic Clays Suspended in Water and in Dodecyldimethylamine Oxide Solutions

“…Thus, these data do not provide any support for these two possible mechanisms. However, we have reproduced (next section) the recently published temperature effect on the 23 Na relaxation rates of Na + -laponite suspensions (23). …”

“…A sodium salt in formamide was used as an appropriate external reference to measure 23 Na NMR visibility (19). 7 Li double-quantum filtered spectra were obtained according to the published pulse sequence (20) (τ = 20 µs; t = 200 µs).…”

“…To complement the above data, 23 Na (I = ) NMR measurements were also performed. Sodium counterions of the investigated clays dispersed in water are only partly NMR visible.…”

“…The extreme narrowing condition (ωτ c 1) does not prevail anymore, and the mobility dependence on the longitudinal relaxation time is opposite (24) to that reported for 13 C relaxation data. The 23 Na longitudinal relaxation rate can be expressed as…”