Understanding demulsification in pickering emulsions

皮克林乳状液是两种不混溶流体的分散体，它们可以是水包油（o/w），也可以是油包水（w/o），由固体颗粒而不是分子表面活性剂稳定。由于颗粒在流体-流体界面处的强吸附和高解吸能，它们具有超稳定性。这些乳液可以通过微凝胶颗粒来稳定，微凝胶颗粒可以由合成聚合物如聚n -异丙基丙烯酰胺（pNIPAM）制成，也可以通过各种动植物蛋白质的物理交联制成生物相容性微凝胶。由于它们的超稳定性，通常这些颗粒很难解吸，除非施加外部刺激或力。然而，破乳对于允许活性物质或疏水营养物质从内部（乳化）液滴中释放是很重要的，这在文献中引起的关注相对较少。这篇综述的目的是了解影响皮克林乳剂破乳的因素，特别是用合成聚合物颗粒制备的皮克林乳剂，为未来用生物相容性颗粒稳定的乳剂破乳提供信息。从分离能理论来看，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、润湿性和粒径[1]。初步研究结果表明，尽管颗粒与表面的附着力很强，在许多情况下似乎是不可逆的，但通过施加适当的外场，有可能导致这些颗粒的解吸或脱离。一系列物理条件（pH值、温度、热量、剪切）或与添加剂（表面活性剂、酶、盐、细菌）混合已被用于破乳皮克林乳剂。在这一领域，合成颗粒的破乳研究已经取得了重大进展，但破乳机理仍存在不确定性。相比之下，生物相容性颗粒的破乳研究非常有限。更好地了解影响破乳的因素可以帮助释放皮克林乳剂的全部潜力及其许多应用，例如化学，食品和制药工业中活性成分的释放。

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.