Contrast-Variation Time-Resolved Small-Angle Neutron Scattering Analysis of Oil-Exchange Kinetics Between Oil-in-Water Emulsions Stabilized by Anionic Surfactants.

Contrast-variation time-resolved small-angle neutron scattering (CV-SANS) was used to examine oil-exchange kinetics between identical mixtures of hydrogenated/deuterated hexadecane emulsion systems. Oil-exchange rates were estimated by transforming recorded scattering profiles to a relaxation function and by fitting to exponential decay models. We find that the oil-exchange process was accelerated when the droplets were stabilized by anionic surfactants even at concentrations well below the surfactant critical micelle concentration. Moreover, the exchange rate was not significantly accelerated when surfactant micelles were present. This suggests that micellar-mediated transport mechanisms do not play the dominant role in these systems. Screening electrostatic repulsion by increasing the ionic strength of the medium also had a negligible effect on oil-exchange kinetics. In contrast, the use of oils with shorter alkane chain lengths (e.g., dodecane), having a higher solubility in water, significantly accelerated rates of oil transport between droplets. Oil-transport rates for hexadecane were also found to increase with temperature and to follow Arrhenius behavior. These results were rationalized as an increase in the droplet-collision frequency due to Brownian motion that results in direct oil transport without irreversible coalescence. Thus, primary mechanisms for oil exchange in insoluble anionic surfactant-stabilized emulsion systems are hypothesized to be through direct emulsion contact, reversible coalescence, and/or direct oil permeation through thin liquid films. CV-SANS is also demonstrated as a powerful technique for the study of transport kinetics in all kinds of emulsion systems.