Kinetic analysis of fast mass transfer across oil/water interface using single microdroplet injection and potential step electrolysis.

study of the mass transfer (MT) across an oil/water interface is important for the fundamental understanding of liquid/liquid extraction, biological membranes, and so on. The kinetic analysis of the fast MT of neutral species across an oil/water interface is difficult since the MT occurs during the construction of the oil/water interface. Several efforts have been devoted to the direct kinetic analyses of the fast MT of neutral species. 1–6 In mm-sized oil/water interface systems, however, the analysis of the overall extraction rate including a fast interfacial process is frequently complicated by nonsteady-state linear diffusion from the bulk solution phase to the interface. In µm-sized droplet/solution interface systems, on the other hand, the MT from the bulk solution phase to the microdroplet surface is steady-state spherical diffusion and is relatively fast. 7 Therefore, the kinetic analysis of the fast interfacial MT can occur in single-microdroplet/solution systems. In this study, we propose single microdroplet injection and potential step electrolysis of the droplet to initiate the MT from the solution to the droplet. We applied the technique to the fast interfacial MT. Electrochemical measurements were performed without supporting electrolytes to demonstrate the interfacial MT as a model of actual liquid/liquid extraction systems. Experimental A single tributyl phosphate (TBP) droplet was injected (10 –11 dm 3) into an aqueous (ferrocenylmethyl) trimethylammonium bromide (FcN + Br – ; FcN + concentration in water ([FcN + (w)]) = 0.5 mM, 1 M = 1 mol/dm 3) and NaPF6 (PF6 – concentration in water ([PF6 – (w)]) = 0.5 or 1 mM) solution (7 cm 3) and put into contact with a gold disk microelectrode (25 µm in diameter), fabricated in a glass capillary (∼70 µm in diameter), using an injection-manipulation system (Narishige Co., Ltd., MN-151, MMW-200/IM-16) under an optical microscope (Nikon Co., Ltd., SMZ-U). As the counter and reference electrodes, Pt wire and Ag/AgCl/NaCl (sat.) electrodes were used. The electrochemical responses of the single droplet were measured by an electrochemical analyzer (BAS Inc., BAS100B/W). All measurements were performed at room temperature (20 – 22˚C). A single TBP droplet was injected and covered on the microelectrode surface. The contact angle between the glass insulator and TBP in water was ∼90˚. A cyclic voltammogram (CV) of a single TBP droplet with dx = 20 µm and dy = 33 µm is shown in Fig. 1, where dx and dy are the distance from the electrode to the droplet …