Interfacial solute flux promotes emulsification at the water|oil interface

Emulsions are critical across a broad spectrum of industries. Unfortunately, emulsification requires a significant driving force for droplet dispersion. Here, we demonstrate a mechanism of spontaneous droplet formation (emulsification), where the interfacial solute flux promotes droplet formation at the liquid-liquid interface when a phase transfer agent is present. We have termed this phenomenon fluxification. For example, when HAuCl₄ is dissolved in an aqueous phase and [NBu₄][ClO₄] is dissolved in an oil phase, emulsion droplets (both water-in-oil and oil-in-water) can be observed at the interface for various oil phases (1,2-dichloroethane, dichloromethane, chloroform, and nitrobenzene). Emulsification occurs when AuCl₄^- interacts with NBu₄⁺, a well-known phase-transfer agent, and transfers into the oil phase while ClO₄^- transfers into the aqueous phase to maintain electroneutrality. The phase transfer of SCN^- and Fe(CN)₆^3- also produce droplets. We propose a microscopic mechanism of droplet formation and discuss design principles by tuning experimental parameters.