Core-shell vs. particle stabilization of oil-in-water emulsions using crystallized lipids - design and stability against breakdown

Pickering emulsions are valued for their stability against coalescence and phase separation. Here, we generated oil-in-water emulsions stabilized with crystalline CITREM, a citrated monoester of stearic acid, to investigate the influence of interfacial layer morphology on emulsion formation and response to environmental stressors, namely NaCl or ethanol. Emulsions were prepared via 1) direct interfacial solidification of CITREM at the oil-water interface, resulting in a crystalline core-shell droplet morphology, and 2) using pre-made CITREM particles that adsorbed to the oil-water interface, resulting in a discrete particle layer. The otherwise kinetically-stable emulsions were tested with up to 30 mM NaCl and/or up to 60 wt% ethanol added to the continuous phase to alter its charge environment or solvent quality. Addition of NaCl ≤ 30mM resulted in aggregation and coalescence of the particle-stabilized emulsions whereas the core-shell emulsion showed no signs of flocculation or change in droplet size, indicating that it resisted emulsion breakdown in spite of the reduction in charge repulsion. Introduction of EtOH at 60% led to coalescence of the particle-stabilized emulsion, whereas the shell-stabilized emulsion flocculated, but saw no changes in droplet size. The combined effect of both charge screening and changes in solvent quality via addition of 30 mM NaCl and 60% EtOH quickly destabilized the particle-stabilized emulsions whereas the core-shell emulsion was more kinetically stable slowly destabilized over time. This study suggested that the core-shell morphology acted as a barrier largely insensitive to changes in electrostatic repulsion. Moreover, it resisted changes in continuous phase polarity, distinguishing it from the traditional particle-stabilized emulsion which are more sensitive. These findings contribute to the understanding of Pickering emulsions and their potential applications in diverse industrial and environmental settings.