Formulation Optimization of Highly Stable and Responsive Pickering Emulsions

Recent advantages in soft matter technologies are awaking interest in Pickering emulsions as an attractive alternative to conventional surfactant-stabilized emulsions, attempting to overcome the environmental and health issues associated with surfactants. Even if Pickering emulsions generally display high stability, optimizing the formulation and eventually scaling up the process requires careful tailoring of numerous parameters: oil/water composition, phase volume ratio, pH, oil phase viscosity, emulsification technique, and nature and surface modification of emulsifier particles. However, this complexity is also a valuable asset to tailoring the physicochemical properties of the system. The wide range of newly considered emulsifiers, such as oxide semiconductors, carbonaceous particles and naturally derived nanoparticles, open the door to the application of Pickering emulsions in various fields (such as medicine, beauty industry, food formulation, sensing) owing to the assorted chemistry of the different particles. Moreover, the responsiveness of selected particle emulsifiers to stimuli (i.e. change of pH or light irradiation) can be exploited to destabilize the emulsion and release an active species previously loaded inside the dispersed phase. Our research group previously focused on the development of ZnO-stabilized Pickering emulsions obtained by in situ functionalization, which is a highly stable system suitable for the triggered release of active ingredients using stimuli such as UV lamp irradiation, acids addition or CO2 bubbling. Here we report on extending this one-step emulsion preparation procedure to other emulsifier materials and on the scaling up of the process. A systematic investigation was performed on parameters such as the quantity and type of powder (TiO2, Al2O3 and carbonaceous materials), phase ratio (from 1:9 to 1:9 oil:water ratio), emulsification technique (sonication, rotor stator homogenization) and final pH (from 5 to 9). Due to the high number of variables involved, a chemometric tool, Design of Experiment, was employed to obtain a time-efficient tuning of the emulsification conditions. Emulsions were characterized by their droplet size, actual phase volume, emulsion stability index and encapsulation efficiency. These results were complemented by an in-depth characterization of the structural, morphological and surface properties of the tested emulsifier materials. Results show that both O/W and W/O emulsions with a broad range of phase volumes could be obtained by a suitable choice of emulsification technique. The emulsification strategy also plays a key role on the average droplet size, with a notable effect of emulsion pH on the actual disperse phase amount and size distribution, which are strongly related to the droplet surface charge. Our work offers fundamental insights into the interplay between materials chemistry and emulsification procedure in developing highly stable and stimuli-responsive Pickering emulsions.