Probing the composition and molecular mobility in thin spherical films using NMR diffusion measurements
- 1. University of Glasgow, Institute of Neuroscience and Psychology, Glasgow, United Kingdom
- 2. Trier University of Applied Sciences, Institute for Micro-Process-Engineering and Particle Technology, Birkenfeld, Germany
- 3. University of Leeds, School of Chemical and Process Engineering, Leeds, United Kingdom
- 4. University of Birmingham, School of Chemistry, Birmingham, United Kingdom
We investigated the composition and molecular mobility within thin spherical liquid films using nuclear magnetic resonance (NMR) diffusion measurements. These films were formed either on the surface of pores inside a sponge at low saturation or by adsorbed water on the surface of urea prills (Fig.1a). Using pulsed field gradient (PFG) NMR experiments, the molecular mobility within these liquid films was determined through analysis of the conditional probability density for displacement (propagator). Molecular diffusion coefficients were determined for films in the sponge and prill systems by fitting the experimental propagators using a model for diffusion on an array of isotropically distributed infinite planes [1] (Fig.1b). By comparing these diffusion coefficients with bulk diffusion coefficients for a range of urea solutions, it was possible to optimize the PFG experimental parameters to enable accurate determination of molecular diffusion in these spherical liquid films. Determination of the diffusion coefficients for a range of urea solutions in the sponge (Fig.1c) enabled identification of the composition of the film that formed on the surface of the urea prills. Analysis of these data showed that the liquid layers are composed of saturated urea solution covering the surface of the prills, with an estimated layer thickness of the order of 10-5 meters. The shape of the propagators indicated the adsorbed water was uniformly distributed over the surface of the urea prills, rather than primarily in the meniscus between particles. This agrees with dye visualisation experiments on a pair of urea prills during caking. This work provides the first quantitative measurements of diffusion in thin spherical films, which is a key parameter for determining what controls the presence and rate of bonding between adjacent particle surfaces.
