Dynamics of oil transfer in oil-in-water emulsions

Abstract

We have investigated the dynamics of the dispersed phase of a kinetically stabilised complex fluid, here an oil-in-water emulsion (toluene–Triton X-100–water). The emulsions were investigated using light scattering, laser scanning confocal microscopy, cryo-scanning electron microscopy and pulsed field gradient NMR (PFG-NMR). The PFG-NMR attenuation data follow a single exponential decay as a function of gradient strength at long observation times. This indicates that a unique diffusion coefficient can describe the diffusion of the dispersed oil phase in the emulsion, belying the inherent polydispersity of the droplet distribution. Hence motional averaging of the oil molecule's diffusion occurs at a rate faster than the NMR observation time. The switch between motional averaging (single exponential decay) and a true representation of the droplet polydispersity (multiexponential decay for shorter observation times) could be detected. This gave the time constant for the motional averaging, measured to be several hundred milliseconds depending on the concentration and age (and therefore oil droplet size) of the emulsion. Transfer of highly hydrophobic dye molecules between oil droplets was evidenced using confocal microscopy. Such transfer occurred without a concomitant change in oil droplet size (size distributions were stable over times several orders of magnitude longer than the experimental time) thus indicating that this particular process of oil transfer between oil droplets occurs independent of ongoing macroscopic phase separation. The time constant for macroscopic phase separation is of the order of hours for the system investigated. Cryo-electron microscopy images gave evidence for direct contact between the oil droplets as a feature of the oil transfer investigated here. Hence oil transfer occurs via a transient hole mechanism or via continual reversible coalescence (fusion–fission mechanism).