Microscale and macroscale diffusion of water in colloidal gels. A pulsed field gradient and NMR imaging investigation

Magnetic resonance imaging (MRI) has been used to determine quantitatively one-dimensional proton concentration profiles in clay gels prepared from wa ter/heavy water mixtures, taking into account the variations of spin-spin r elaxation rate and other effects on the signal intensity. The method has be en used to determine the diffusion coefficient of water in Laponite gels at macroscale, by following the diffusion of water from a volume of gel prepa red with water into a volume of gel prepared with heavy water: The results have been compared with the self-diffusion coefficients at microscale obtai ned by pulsed gradient spin-echo (PGSE) NMR. Both results are in agreement in spite of the complex structure of the gel, but the reduction in diffusio n coefficient as compared to bulk water is larger than predicted from effec tive medium theories.