Evolving micro-structures in drying detergent pastes quantified using NMR

Magnetic resonance methods have been used to probe the evolution of the internal micro-structure of an industrially important detergent mixture, as a function of total water content. Measurements of the apparent diffusion coefficient of the water content were obtained as a function of diffusion observation time. These data were interpreted to render the surface-to-volume ratio and tortuosity of the pore space in which the water resided. Pore dimensions were found to decrease as moisture content was reduced, partially as a consequence of solute deposition. Deposition of solute material was confirmed through the application of NMR T(1) relaxation measurements. These were analysed using regularisation techniques to yield T(1) population distributions. Average pore sizes as a function of water content were extracted from this data and were in good agreement with the results of the diffusion analysis.