The structural and dynamical properties of water molecules confined within dense clay sediment are investigated by 2H NMR spectroscopy and multiquanta relaxometry. The relative contribution of both quadrupolar and paramagnetic NMR relaxation mechanisms is evaluated by carefully analyzing the variation of 2H multiquanta NMR relaxation rates as a function of the orientation of the clay sediment within the static magnetic field. The same analysis is successfully applied to 2H multiquanta NMR spin-locking relaxation measurements, significantly increasing the probed dynamical range. That procedure leads to an accurate determination of the average residence time of the water molecule confined within the interlamellar space of the clay lamellae.

We have used a multiscale statistical analysis to interpret the mobility of water molecules diffusing within nematic aqueous dispersions of charged anisotropic nanocomposites (synthetic Laponite clays). The nematic ordering of dense aqueous suspensions (29-52% w/w) prepared by uniaxial compression is detected by analyzing the splitting of the nuclear magnetic resonance line of the quadrupolar counterions ( 7 Li and 23 Na) neutralizing the negative charges of the clay. The tensor describing the water self-diffusion is measured by 1 H pulsed gradient spin-echo (PGSE) NMR spectroscopy. It exhibits a large anisotropy of water mobility in these nematic dispersions. The macroscopic mobility of the water molecules is obtained from numerical simulations of Brownian dynamics (BD), by integrating the water trajectories over a time scale of 1 µs. The local mobility of the water molecules in the vicinity of the surface of the Laponite particles is deduced from preliminary molecular dynamics (MD) simulations of the trajectories of the water molecules confined between two clay fragments by integrating their trajectories over a time scale of a few picoseconds. The equilibrium density and initial configuration of these confined water molecules are deduced from grand canonical Monte Carlo (GCMC) simulations, by using a new clay/water force field determined from semi-empirical periodic (MINDO) quantum calculations coupled to perturbation theory for dispersion forces. This multiscale statistical analysis of the water mobility bridges the gap between the time scale (nanoseconds) accessible by MD simulations and the time scale (microseconds) accessible by BD, leading to macroscopic behavior comparable with experimental data. (I) IntroductionClays are charged anisotropic colloids used in many industrial applications (waste management and storage, heterogeneous catalysis, drilling, ionic exchange, etc.) exploiting their various physico-chemical properties (large affinity for water molecules and polar solvents, high specific surface and surface charge density, gelling, thixotropy, surface acidity, etc.). By contrast with natural clays which contains numerous impurities, Laponite is a synthetic clay frequently used for scientific investigations 1-13 because of its high chemical purity. Thus we have used this synthetic clay in order to validate our experimental procedures before investigating the behavior of the natural clay materials which are used for the industrial applications. Laponite clay results from the sandwiching of one layer of magnesium oxides, with an octahedral geometry, between two layers of silicium oxides, with an tetrahedral geometry. In dilute aqueous regime, Laponite clays behave as isolated rigid disks (diameter, 200-300 Å; thickness, 10 Å; and density, 2.7). 8,14,15 Because of the substitution of some Mg 2+ cations from their octahedral network by Li + cations, each Laponite disk bears about a thousand of negative electric charges neutralized by hydrated sodium counterions.The purpose of this study is to determine t...

2 H NMR spectroscopy, relaxometry, and two-time correlation measurements are used to investigate the structural and dynamical properties of water molecules confined within the multiscale porous network of dense clay sediment. The residual quadrupolar splitting detected by 2 H NMR spectroscopy is the fingerprint of the specific orientation of the water molecules pertaining to the first hydration shell of clay lamellae. Multiquantum 2 H NMR relaxation measurements are used to quantify the distribution of the clay platelet orientations within the dense sediment. The average residence time of the water molecules confined within the clay interlamellar space is determined by exploiting 2 H multiquantum NMR relaxation measurements under spin-locking conditions. Finally, long-time scale diffusion of the water molecules within the multiscale porous network of the clay sediment is quantified by measuring the attenuation of the 2 H NMR two-time stimulated echo.

A detailed line shape analysis of 23 Na nuclear magnetic resonance spectra within dense suspensions (12% vol/vol) of Laponite clay exhibits a macroscopic ordering of these charged anisotropic colloids within a nematic phase. The angular variation of the order parameter limits to 20% of the maximum amount of disorder in these dense suspensions. By contrast, dilute Laponite suspensions (1-4% vol/vol) remain isotropic while the variation of 23 Na relaxation rates over a broad range of frequencies indicates a local ordering of the clay platelets within microdomains of the same spatial extent than the particle diameter (300 Å).