Multiple nanoemulsions

The structural and chemical uniformity of traditional emulsions facilitates their production; however, this simplicity also limits the scope of their applications. Multiphase or 'multiple' emulsions have increased complexity and potentially broader utility than conventional emulsions, owing to their internalized phases, enabling chemical compartmentalization, control of active-ingredient release and complex particle templates. However, multiple-emulsion droplets prepared with conventional methods are too large for emerging applications. As a result, there has been increased research interest in the development of multiple nanoemulsions, whereby the nanoscale droplet size overcomes many of the limitations of more common, micrometre-scale multiple emulsions. Although numerous successful demonstrations of multiple-nanoemulsion production have emerged, there remain considerable challenges to improve stability, control over internal structure and characterization of multiphase nanodroplets. Overcoming these challenges provides unique opportunities to exploit the hierarchical structure, enhanced colloidal stability and multiphase chemical solubilization for applications, such as delivery vehicles, chemical sinks and particle templates, with functions that are otherwise inaccessible. In this Review, we summarize the techniques used to generate and characterize multiple nanoemulsions, theories to understand their formation and stability, and discuss current and future applications that are, or may be, enabled by their unique structures and chemistries. The preparation of multiphase nanodroplets has enabled the fabrication of emulsions with novel structures and functions for nanomaterials design and formulation. In this Review, the strategies and challenges to produce, stabilize and characterize such multiple nanoemulsions are discussed, as well as the applications they enable.