Binding and Organization of Interfacial Assemblies of Nanoparticle-Surfactants at Liquid-Liquid Interfaces

The ability to lock-in non-equilibrium shapes in all liquid constructs or “structuring liquids” has steered extraordinary technological advancements such as printing liquids-in-liquids, energy storage devices, all liquid microfluidics, biphasic reaction vessels, liquid electronics. However, a limitation in the understanding of the details of the underlying physics using direct visualization has been a formidable challenge. In this work, we unravel the mechanistic details of the assemblies of nanoparticle-surfactants (NPSs) formed by harnessing the complementary chemical functionalities at the interface of two immiscible liquids by employing in-situ laser scanning confocal microscopy (LSCM) and in-situ atomic force microscopy (AFM) in the liquid phase. While experiments at low nanoparticle and ligand concentrations indicate a monolayer of NPSs at the interface and a monotonic reduction of interfacial tension, an unexpected increase in the interfacial tension and a phase inversion in the emulsion is observed for high nanoparticle and ligand concentrations. This is accompanied by formation of small drops by spontaneous emulsification at the interface. By employing LSCM and an in-house customized liquid flow cell for AFM, we explore this interesting phenomenon and the mechanisms thereof at the interface and in the bulk. Time resolved in-situ liquid phase AFM renders deeper insights into the kinetics of adsorption, structure of the assembly and reorganization of NPSs at the interface, enabling design of all liquid advanced functional materials.