An NMR investigation of water self-diffusion and relaxation rates in controlled ionic strength laponite sols and gels

H-1 relaxation and pulsed-gradient NMR techniques have been used to probe s olvent mobility and binding in aqueous suspensions of Laponite, a synthetic Hectorite clay. The ionic strength and the particle concentration were var ied in wide ranges in order to explore the phase diagram. Increasing either one of these parameters is known to induce a sot-soft solid rheological tr ansition. However, neither the proton relaxation rates, R-1 and R-2, nor th e water selfdiffusion coefficient, D, were found to be affected by this tra nsition. Only a continuous increase of R-1 and R-2 and a continuous decreas e of D were observed as the particle concentration was increased, whatever the ionic strength. The results are discussed in terms of obstruction to di ffusion and surface hydration, in the fast exchange regime. Specific effect s due to the particle anisotropy and surface heterogeneity are evidenced. ( C) 2001 Academic Press.