Long-term Retention of Small, Volatile Molecular Species within Metallic Microcapsules

The efficient encapsulation of active ingredients within formulated products and their controlled, targeted delivery to the sites of action, is very important to a range of industries such as pharmaceuticals, agrochemicals, home and personal care and cosmetics to name but a few. By successfully stopping the release of active chemicals and triggering their release where and when they are needed, product efficiency can be improved which reduces the required dose, lowering costs, environmental impact and/or side effects. Such active chemicals include pharmaceutical drugs, pesticides, fragrant and flavour oils, enzymes, vitamins. Encapsulation and full retention of small molecular weight actives is a particularly challenging task that remains unsolved by current technologies used in industry and academia. In particular, certain everyday product formulations provide difficult environments in which preventing active leakage through capsule walls is not feasible. For example, a continuous phase that can fully dissolve an encapsulated active will typically force full release over a fraction of the intended lifetime of a product. This is due to the inherent porosity of polymeric membranes typically used as capsule wall materials in current technologies. In this work, a method for preventing undesired loss of encapsulated actives under these extreme conditions using a simple three step process is developed. The developed methodology forms an impermeable metal film around polymer microcapsules, prevents loss of small, volatile oils within an ethanol continuous phase for at least 21 days while polymeric capsules lose their entire content in less than 30 min under the same conditions. Polymer shell−oil core microcapsules are produced using a well known cosolvent extraction method to precipitate a polymeric shell around the oil core. Subsequently, metallic catalytic nanoparticles are physically adsorbed onto the microcapsule polymeric shells. Finally, this nanoparticle coating is used to catalyse the growth of a secondary metallic film. Specifically, this work shows that it is possible to coat polymeric microcapsules containing a model oil system or typical fragrance oil with a continuous metal shell. It also shows that the coverage of nanoparticles on the capsule surface can be controlled, which is paramount for obtaining a continuous impermeable metal film. In addition, control over the metal shell thickness is demonstrated without altering the capability of the metal film to retain the encapsulated oils. In addition, a method to grow a continuous, non-porous metallic film directly onto nanoparticle stabilised Pickering emulsion droplets is demonstrated, negating the need for an underlying polymeric shell.