Electro-guided self-propelled ionic liquid droplets as vessels for chemical reactions

摘要

Utilizing surfactants and surface tension effects, smart droplets have been previously reported that can solve complex mazes1, are capable of photo chemo-propulsion2 or can be guided or repelled by light3. In contrast, herein we present self-propelled ionic liquid droplets composed solely of an ionic liquid (IL), namely trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14][Cl]), that are guided to specific destinations inside open fluidic networks through electro-stimulation. The movement of these droplets is controlled by creating conditions which result in a biased release of the cationic surfactant [P6,6,6,6,14]+, component of the IL. Once released the surfactant lowers the surface tension of the aqueous solution. Liquid flows from areas of low surface tension to high surface tension, phenomena known as the marangoni effect. The rate of [P6,6,6,14]+ release depends on the solubility of the closely associated Cl- anion, as the formation of free [P6,6,6,14]+ (the active surfactant at the air-water boundary) in the aqueous phase depends on the local Cl- concentration at the IL-aqueous boundary. In this work, Cl- gradients required for droplet movement are generated electro-chemically. The chip and the embedded titanium mesh electrodes used in this work were 3D printed using an Objet350 Connex and a Realizer SLM-50 metal printer, respectively. The [P6,6,6,14][Cl] droplet can be moved from the cathode (-) to the anode (+) by applying an external electric field. Once the voltage (1-9 V) has been applied to the solution (NaCl 10-3 M), the mobile cations migrate towards the cathode (Na+ ions towards the starting position) and the mobile anions migrate toward the anode (Cl- ions towards the destination), creating a Cl- gradient for the droplet to follow. The use of electro-generation of ion-gradients in the external aqueous environment allows for on-demand unidirectional droplet movement. Since the droplets are composed solely of an IL, which has no vapour pressure and high thermal stability, they are ideal to act as micro-reactors for chemical reactions. For example, the introduction of chromoionophores into the droplet allows for the droplet to possibly act as a dynamic sensing unit. In this fashion the droplets can be used to detect ions present in the fluidic network as they travel along the air/liquid interface.