Core-shell microcapsules from clay-based Pickering emulsions
Core-shell microcapsules are typically made by preparing an emulsion and forming a polymer shell around the droplets. Conventionally, the emulsion is stabilised by surfactants or polymers. Alternatively, emulsions can be stabilised by fine particles; we discuss capsules made from ultrafine kaolin stabilised Pickering emulsions via interfacial polymerisation.
This approach has a number of potential benefits. The capsule size can be controlled by the dose of particles used, and Ostwald ripening is prevented. Particles become incorporated into the polymer shell of the capsule, potentially offering different mechanical properties and reduced permeability. The capsule surface is rough, allowing a powdered product to be made easily without adhesion of capsules to each other. The particles scatter light and offer some protection from photodegradation.
Microcapsules made from emulsions are used in a wide range of applications in which an active ingredient needs to be protected from its environment and released under controlled conditions. Examples include the encapsulation of pharmaceuticals (1), fragrances in household products (2), insecticides in agrochemical formulations (3), dyes in textiles (4), phase change materials in construction (5) and flavours and nutrients in food (6). Conventional techniques use surfactants or water-soluble polymers (protective colloids) to stabilise the emulsion droplets. A polymer shell is then formed around the droplets by, for example, phase separation, complex coacervation or interfacial polymerisation (7). Interfacial polymerisation usually involves an addition, condensation or crosslinking reaction, in which one reactant is soluble in the oil phase and the other in the water phase. The first reactant is dissolved in the dispersed phase (typically the oil) prior to emulsification, and the second reactant is added to the continuous phase after emulsification. Reaction occurs at the interface and forms the capsule shell.