NMR diffusion studies of translational properties of oil inside core-shell latex particles

The diffusion behavior of core-shell latex particles with a liquid core of hexadecane and a solid polystyrene shell in water solution has been studied using the pulsed field gradient spin-echo (PFG-SE) NMR technique. The apparent diffusion coefficient and the root mean square displacement of oil were strongly dependent on the diffusion time Delta. With increasing diffusion time, the obstructing effect from the particle wall caused a decrease in the apparent oil diffusion coefficient. The root mean square displacement of oil inside the particle core was constant for all diffusion times and was used for the calculation of the particle radius. The volume fraction dependence of the apparent diffusion coefficient was found to be roughly consistent with the hard-sphere model. The diffraction pattern in the echo decay predicted from the q-space formalism for molecules diffusing inside a spherical cavity was almost completely smeared out due to polydispersity and wall relaxation effects. It was observed that 10-20% (w/w) of the particle shell consisted of hexadecane. This fact imparted a slow component to the echo decay, since the exchange time between oil in the shell and oil in the cavity was slow, which further contributed to the smearing out of the diffraction pattern. It was concluded that by using the core-shell concept very good signal-to-noise is obtained in the PFG-SE experiment, thus making possible studies of translational properties of colloidal particles in different environments to an extent that previously has been very difficult to perform.