(40c) Amphiphilic Janus Particles As Solid Surfactants

Emulsions are multiphasic fluid mixtures in which one phase (e.g., oil) is dispersed in another (e.g., water). The stability of emulsions and emulsion-based products is crucial for their use in a variety of applications. Conventional emulsions stabilized by surfactants typically suffer from poor stability; that is, these emulsions eventually undergo phase separation via coarsening and coalescence. One alternative to surfactants is amphiphilic Janus particles, which have been proposed as excellent solid surfactants for emulsion stabilization. In fact, theoretical work shows that thermodynamically stable emulsions can be generated with amphiphilic Janus particles as stabilizing agents. An emerging idea in this area is that the shape of Janus particles, in addition to their amphiphilic nature, has a significant influence on their behavior and functionality as surface active agents. Our work is motivated by amphiphilic diblock copolymers such as poly(styrene)-block-poly(acrylic acid) that show pH-responsive surfactant properties.  Such stimuli-responsive amphiphilic molecules can stabilize different types of emulsions or even induce phase inversion emulsification in response to changes in local environment.  We synthesize a new class of stimuli-responsive Janus particles that change their shape and in turn amphiphilicity in response to changes in the solution pH.  We show that these shape-changing Janus particles can stabilize emulsions of different types depending on the solutions pH and that the emulsions stabilized by these Janus particles can undergo phase inversion. We also demonstrate that the interaction between emulsion droplets can be tuned by controlling the solution pH.  Our results present a new class of polymer particles that will further widen the functionality and properties of Janus particles as solid surfactants.