Discovery of Non-Classical Self-Assembly in Janus Particle-Based Surfactants and the Field-Triggered Breakdown of Surface Activity and Amphiphilic Properties

The use of colloidal particles as models to understand processes on a smaller size-scale is a highly valuable approach. Compared to molecules, particles are less defined but their architecture can be more complex, so is their long-range interaction. Sometimes one can observe phenomena which are unknown or much more difficult to realize on the molecular level. The current paper focuses on particle-based surfactants and reports about numerous unexpected properties. The main goal is the creation of an amphiphilic system with responsiveness in surface activity and associated self-organization phenomena depending on the application of an external trigger, preferably a physical field. A key step is the creation of a Janus-type particle characterized by two types of dipoles (electric and magnetic) which geometrically stand orthogonal to each other. In a field, one can control which contribution and which direction dominates the inter-particle interactions and as a result one can change the properties of the system drastically. The features of ferrite-core organosilica-shell particles with grain-like morphology modified by click chemistry is studied in response to spatially isotropic and anisotropic triggers. A highly unusual aggregation-dissolution-reaggregation sequence was discovered. Using a magnetic field, one can even switch off the amphiphilic properties and use this for the field-triggered breaking of multiphase systems such as emulsions.