Reversible Aggregation and Dispersion of Particles at a Liquid–Liquid Interface Using Space Charge Injection

Abstract

large interparticle spacing, akin to Wigner crystals, under the combined effect of the attractive and repulsive interparticle interactions. These forces are long-ranged compared to corresponding interactions in bulk media. The attractive interaction between interfacial particles are believed to originate from interface undulation around individual particles due to nanoscale surface roughness and heterogeneity, [14,15] whereas the long-ranged repulsion arises from the asymmetric surface charge distribution across the interface. [16][17][18] Changes in the structure of particle monolayers at liquid-liquid interfaces can be achieved by tailoring the interparticle interactions with different external stimuli. One well-established method for inducing aggregation of initially hexagonal monolayers, for example, is to add electrolytes and/or surfactants to the subphase, which decreases the magnitude of the repulsive interactions. [19,20] Also, changing particle wettability at the interface can affect the order-disorder transition of interfacial silica particles. [21,22] In addition, electric fields can also tune the interfacial structures by exerting a lateral electric force and a lateral capillary force resulting from a vertical electrical force. [23][24][25] External ions or ultraviolet light can also transform the structure of particle monolayers to fractal aggregates or mesostructures. [26] Despite these strategies, the transitions are typically unidirectional without reversibility, limiting their practical applications. Although a scheme has achieved reversible order-disorder transitions at soft (i.e., oil-gel) interfaces by altering the attractive interaction through addition or removal of the oil phase, [27] this method requires the addition of a gelling agent in the aqueous phase and also lacks tunability by relying on oil evaporation. Therefore, a strategy to induce reversible assembly of colloidal monolayers at a liquid-liquid interface without changing the chemical nature of the two phases would significantly advance our understanding of particle-laden fluid interface phenomena and facilitate the use of these interfacial monolayers in fundamental investigations.In this work, reversible aggregation-dispersion transition of interfacial colloid monolayers is induced by injecting space charge from a corona discharge, inspired by the principles of surface charge neutralization, [28,29] and electrostatic precipitation. [30] We show that an ordered colloidal monolayer of charged particles at an oil-water interface can be induced to Colloids at water-oil interfaces can form ordered monolayers when surface charge-induced repulsion overcomes capillary attraction. Such particle monolayers play an important role in the stabilization of emulsions and also can serve as an exquisite model system to study fundamental physical phenomena. However, it is challenging to dynamically control the relative magnitudes of repulsion and attraction between the particles, especially with reversibility, to induce reversible aggregation and...

SERS biosensors based on cucurbituril-mediated nanoaggregates for wastewater-based epidemiology

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