Amphiphilic Janus Particles at Interfaces

Abstract

Janus particles (JPs) can be generally defined as asymmetric particles with at least two surface regions or bulk composition differing in their physicochemical properties. JPs can adopt different shapes, for example, perfectly spherical with two hemispheres having different surface properties, or more commonly snowman or dumbbell shapes. JPs can be amphiphilic if the surface polarity contrast between the two surface regions (Janus lobes) is sufficiently large. Due to their amphiphilicity JPs become interfacially active, resembling molecular surfactants, can partition at interfaces and can emulsify and stabilise Pickering emulsions. JPs can be prepared in wide range of sizes and different bulk or surface properties can be ``loaded'' through selective modification on each of the Janus lobes and sometimes can be combined in surprising ways. The application potential of the JPs grew significantly in the past decade and JPs become an established platform for creating new multifunctional materials. Janus particles have a relative recent history and came recently into existence through the work of Casagrande, Veysi and de Gennes in the late 1980s. The first synthesis of micron-sized JPs was attributed to the former two authors, while de Gennes baptised them after the Roman god Janus. This work is intended to be a general introduction into the rapidly growing and already mature field of JPs, by giving a short historical account and an overview of the available synthetic methods as well as their application potential arising mainly through JPs ability to adsorb at air--water and oil--water interfaces.