Fourier transform-pulsed field gradient and 1D NMR imaging studies of molecular and ionic diffusion in aqueous dispersions

Abstract

The diffusion behavior of ions, e.g., MFP 2− (as sodium monofluorophosphate), Na +, and water molecules, in concentrated aqueous silica dispersions (pastes) has been examined by pulse field gradient (PFG)-spin echo (SE)-Fourier transform (FT) and simple one-dimensional magnetic resonance imaging (1D MRI) techniques, using a 300-MHz NMR spectrometer. Time-dependent 1D MRI concentration profiles for ions diffusing across ion-rich and ion-free paste boundaries are found to be in good agreement with those predicted from expressions based on standard Fickian laws of diffusion, using D self (self-diffusion coefficient) data determined by the FT-PFG-SE method. Accordingly, the D self value of 1 × 10 −6 cm 2 s −1 for MFP 2− ions is demonstrated to be representative of diffusion over realistic distances (0.5–1 cm), i.e., relevant to the transport of such species in practical systems, e.g., toothpaste. Initial evaluation of the resolution and multinuclear capabilities of the NMR methods is discussed via results on suitable models and solid-liquid “phantoms.”