NMR self-diffusion studies on PDMS oil-in-water emulsion

In this paper, standard spin-echo pulsed field gradient (SE-PFG) and stimulated spin-echo pulsed field gradient (STE-PFG) H-1 NMR methods have been used to study the translational diffusion of polymeric-oil-in-water (O/W) emulsions, made with poly(dimethylsiloxane) PDMS fluids and two nonionic surfactants polyoxethylene (4) lauryl ether (Brij-30) and polyoxethylene (23) lauryl ether (Brij-35p). A detailed analysis of the free (Gaussian) diffusion of the PDMS fluid, its restricted diffusion inside the emulsion droplets, and the Brownian diffusion of the droplets themselves are presented. A graphical representation, which relates the square root of the mean-squared displacement of the fluid and emulsion droplets with the experimental diffusion times and the radius of the droplets, helps distinguish the different diffusion regimes. The Brownian diffusion of the emulsion particles becomes the dominant diffusion process with dilution of the system and increase in the molecular weight (viscosity) of the PDMS. These different processes which contribute to diffusion in the system need to be considered if a correct size distribution of the emulsion droplets is to be determined. For a concentrated emulsion system, the condition for restricted diffusion is reached by increasing the diffusion time. Under this condition, the Murday and Cotts model for diffusion inside a spherical droplet can be used to determine the size distribution of the emulsion. In very dilute systems by increasing the diffusion time, the Brownian diffusion becomes the dominant process so that the Stokes-Einstein model can be applied to obtain the size distribution.