On the Stability of Pickering and Classical Nanoemulsions: Theory and Experiments

Emulsification is a crucial technique for mixing immiscible liquids into droplets in various industries, such as food, cosmetics, biomedicine, agrochemistry, and petrochemistry. Quantitative analysis of the stability is pivotal before the utilization of these emulsions. Differences in X-ray attenuation for emulsion components and surface relaxation of the droplets may contribute to X-ray CT imaging and low-field NMR spectroscopy as viable techniques to quantify emulsion stability. In this study, Pickering (stabilized solely by nanoparticles) and Classical (stabilized solely by low molecular weight polymers) nanoemulsions were prepared with a high-energy method. NMR and X-ray CT were employed to constantly monitor the two types of nanoemulsions until phase separation. The creaming rates calculated from NMR match well with the results obtained from X-ray CT. Furthermore, we show that Stokes' law coupled with the classical Lifshitz-Slyozov-Wagner theory underestimates the creaming rate of the nanoemulsions compared to the experimental results from NMR and X-ray CT imaging. A new theory is proposed by fully incorporating the effects of Pickering nanoparticles, hydrocarbon types, volume fraction, size distribution, and flocculation on the droplet coarsening. The theoretical results agree well with the experimentally measured creaming rates. It reveals that the attachment of nanoparticles onto a droplet surface decreases the mass transfer for hydrocarbon molecules to move from the bulk aqueous phase into other droplets, thus slowing the Ostwald ripening. Therefore, Pickering nanoemulsions show a better stability behavior compared to Classical nanoemulsions. The impacts of hydrocarbon and emulsification energy on the stability of nanoemulsions are reported. These findings demonstrate that the stability of the nanoemulsions can be manipulated and optimized for a specific application, setting the stage for subsequent investigations of these nanodroplets.