Water self-diffusion within nematic dispersions of nanocomposites:: A multiscale analysis of 1H pulsed gradient spin-echo NMR measurements

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 (Li-7 and Na-23) neutralizing the negative charges of the clay. The tensor describing the water self-diffusion is measured by H-1 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 (131)), by integrating the water trajectories over a time scale of I us. 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 BID, leading to macroscopic behavior comparable with experimental data.