Emulsion Microstructure and Dynamics
Authors:   , Malassagne-Bulgarelli Nelly 
Published: November 2021

<p>Emulsions are kinetically stabilised mixtures of two immiscible fluids (e.g. oil and water). They are encountered in many industrial applications including cosmetics, food, road, drug delivery and paint technology. Despite their wide spread use, the formulation of emulsions remains largely empirical. The nature of the relationships between ingredients, composition, emulsification method and energy input, defining the microstructure (e.g. droplet size distribution and surfactant packing at the oil/water interface), the dynamics (e.g. interdroplet exchange) and the lifetime of emulsions, is still poorly understood. In particular, little work has focused on the mutual interactions between emulsifier and oil molecules and how these affect the properties of the interfacial domain and emulsion dynamics. The emulsion system oil/Triton X-100/water was investigated, where Triton X-100 is a commercially available non ionic surfactant and the oil is one of toluene, p-xylene or octane. The microstructure and the dynamics of these oil/Triton X-100/water emulsions were monitored upon varying oil type, oil concentration, emulsion age and ionic strength while maintaining the oil-to-surfactant weight ratio, temperature, energy input and emulsification method constant. For this purpose, laser scanning confocal microscopy, cryo scanning electron microscopy (cryo-SEM), pulsed field gradient NMR (PFG-NMR), macroscopic phase separation and light scattering techniques were used as experimental techniques. The occurrence of an oil exchange between oil droplets that is not coupled to droplet growth and emulsion destabilization is reported for the three oil systems: toluene, p-xylene or octane. The mixture of two separately stained emulsions, using green and red fluorescing dye molecules, leads to all droplets emitting yellow fluorescence under the confocal microscope within ∼10 min of mixing due to the interdroplet exchange of the two water insoluble dyes. Furthermore, the PFG-NMR data for both toluene and p-xylene systems indicate that, for long observation times, Δ, the echo attenuation of the oil signal decays as a single exponential upon increasing the diffusion parameters. In other words the individual motions of the droplets and oil molecules are described by a unique diffusion coefficient belying the system polydispersity and indicative of a dynamic process occurring on a time scale faster than the observation time. One way to explain this outcome is to consider a motional averaging of the oil diffusion arising from either oil permeation upon droplet collision or reversible coalescence of the droplets. These two mechanisms are supported by the extensive droplet contact observed by cryo-SEM. Such an oil transfer occurring in three distinct oil systems, independently of emulsion destabilization, has not been reported previously. Upon decreasing the NMR observation time below a specific value, Δswitch, a switch of the echo attenuation data was detected between a single exponential and a multiexponential decay, the latter indicative of the emulsion droplet size distribution. The time scale of the oil transfer, Δswitch, was probed upon varying oil type, oil concentration, emulsion age and ionic strength. In particular, the time scale of the oil exchange is an increasing function, spanning from ~300 ms to ~3 s, of droplet concentration in toluene emulsions despite the concomitant increase of the droplet collision frequency. Upon increasing the toluene content and decreasing the mean interdroplet spacing, the oil droplets are kinetically stabilized by the enhancement of the surfactant packing at the oil/water interface. In addition to the surfactant packing at the surface of the oil droplets, ionic strength and droplet size, the rate of oil exchange is controlled by the mutual interactions between oil and Triton X-100 molecules. The rate of oil transfer is a decreasing function from toluene to p-xylene to octane. The increase of the mean droplet size in the same order cannot solely account for the observed slowdown of the oil exchange. The macroscopic phase separation data indicate that the Triton X-100 layer is increasingly robust with respect to oil transfer from toluene to p-xylene to octane. This can be compared with the oil exchange process and explained in terms of oil penetration effects into the surfactant layer and energy cost for hole nucleation.</p>