2H NMR Spectroscopy and Multiquantum Relaxometry as a Probe of the Magnetic-Field-Induced Ordering of Clay Nanoplatelets within Aqueous Dispersions

2H NMR spectroscopy and relaxometry were used to investigate the orientation of nontronite clay nanoplatelets induced by the static magnetic field within dilute aqueous dispersions by exploiting the residual quadrupolar splitting resulting from the specific orientation of heavy water molecules physisorbed at the clay surface. A careful analysis of the variation of the residual 2H splitting as a function of clay concentration and magnetic field strength was required to extract the intrinsic clay ordering induced by the magnetic field. The variation of clay ordering as a function of clay concentration clearly indicated two concentration regimes, corresponding to free and collective orientations of the clay platelets, respectively. Multiquantum NMR relaxation measurements were further used to identify the main NMR relaxation mechanism whose temperature variation is compatible with a fast exchange, at the NMR time scale, between free and physisorbed water molecules.