Understanding demulsification in pickering emulsions

Pickering乳液是由固体颗粒而不是分子表面活性剂稳定的两种不相容流体的分散体，可以是水包油(O/W)或油包水(W/O)。它们具有超稳定性，这是因为它们在具有高脱附能的流体-流体界面上有很强的吸附作用。这些乳液可以由微凝胶颗粒稳定，微凝胶颗粒可以是由聚(N-异丙基丙烯酰胺)(PNIPAM)等合成聚合物制成的，也可以是由各种动植物蛋白质物理交联制成的生物兼容微凝胶。由于它们的超稳定性，这些颗粒通常很难被解吸，除非施加外部刺激或力量。然而，破乳对于从内部(乳化)液滴释放活性物质或疏水营养物质是很重要的，这在文献中引起的关注相对较少。本综述的目的是了解影响Pickering乳状液破乳的因素，特别是使用合成的聚合物颗粒来制备Pickering乳状液，以指导由生物相容颗粒稳定的乳状液的未来破乳。根据脱离能理论，力随位移的变化在很大程度上取决于流体之间的界面张力、润湿性和颗粒尺寸[1]。初步研究结果表明，尽管颗粒在表面上的粘附力很强，在许多情况下似乎是不可逆的，但通过施加合适的外场，仍有可能导致这些颗粒的解吸或脱离。文献中曾使用一系列物理条件(pH、温度、热、剪切)或与添加剂(表面活性剂、酶、盐、细菌)混合来破乳Pickering乳状液。在这一领域有大量关于合成颗粒的研究，然而，关于触发破乳的机制仍然存在不确定性。相比之下，对生物相容颗粒破乳的研究非常有限，更好地了解影响破乳的因素有助于释放Pickering乳剂的全部潜力，以及它在化工、食品和制药行业的许多应用，如有效成分的释放。

Pickering emulsions are dispersions of two immiscible fluids of any type either oil-in-water (o/w), water-in-oil (w/o) stabilized by solid particles instead of molecular surfactants. They possess ultrastability due to strong particle adsorption at the fluid-fluid interface with high desorption energies. These emulsions can be stabilized by microgel particles, that could be either made of synthetic polymers like poly-(N-isopropylacrylamide) (pNIPAM) or biocompatible microgels made by the physical crosslinking of various animal and plant proteins. Because of their ultra-stability, often these particles are difficult to desorb unless external stimuli or forces are applied. Nevertheless, demulsification is important to allow release of actives or hydrophobic nutrients from the internal (emulsified) droplets, which has attracted relatively little attention in literature.The aim of this review is to understand the factors affecting demulsification in Pickering emulsions, particularly fabricated using synthetic polymeric particles to inform future demulsification in emulsions stabilized by biocompatible particles. From the detachment energy theories, the variation of the force with displacement is largely governed by the interfacial tensions between the fluids, the wettability and the particle size [1]. Preliminary findings suggest that even though the adhesion of particles to a surface can be strong and appear irreversible in many situations, it is possible to cause desorption or detachment of these particles through the application of a suitable external field. A range of physical conditions (pH, temperature, heat, shearing) or mixing with additives (surfactants, enzymes, salts, bacteria) have been used in literature to demulsify Pickering emulsions. There is significant research in this area on synthetic particles, however, there are still uncertainties regarding the mechanisms that trigger demulsification. In contrast, there is very limited research on demulsification of biocompatible particles.Having a better understanding of the factors that influence demulsification can help unlock the full potential of Pickering emulsions and its many applications such as the release of active ingredients in the chemical, food, and pharmaceutical industries.