Mass transfer microdroplets

The effects of mercury film electrode morphology in the anodic stripping SWV of electrochemically reversible and quasi-reversible processes were investigated experimentally [47-51], Mercury electroplated onto solid electrodes can take the form of either a uniform thin film or an assembly of microdroplets, which depends on the substrate [51 ]. At low sqtrare-wave frequencies the relationship between the net peak crrrrent and the frequency can be described by the theory developed for the thin-film electrode because the diffusion layers at the snrface of microdroplets are overlapped and the mass transfer can be approximated by the planar diffusion model [47,48],... 

IV. Mass transfer across a microdroplet/water interface 194... 

B. Mass transfer of FeCp-X+ from a single microdroplet to water 195... 

C. Mass transfer of FeCp-X from water to a single microdroplet diffusion-limited case 197... 

In this review, we describe a laser trapping-spectroscopy-electrochemistry technique as a novel methodology for studying single microdroplets in solution and, demonstrate recent progress in the research on electron transfer and mass transfer across a microdroplet/solution interface in special reference to a droplet size dependence of the process. 

In the previous section, we demonstrated the micrometer droplet size dependence of the ET rate across a microdroplet/water interface. Beside ET reactions, interfacial mass transfer (MT) processes are also expected to depend on the droplet size. MT of ions across a polarized liquid/liquid interface have been studied by various electrochemical techniques [9-15,87], However, the techniques are disadvantageous to obtain an inside look at MT across a microspherical liquid/liquid interface, since the shape of the spherical interface varies by the change in an interfacial tension during electrochemical measurements. Direct measurements of single droplets possessing a nonpolarized liquid/liquid interface are necessary to elucidate the interfacial MT processes. On the basis of the laser trapping-electrochemistry technique, we discuss MT processes of ferrocene derivatives (FeCp-X) across a micro-oil-droplet/water interface in detail and demonstrate a droplet size dependence of the MT rate. 

A. Electrochemically Induced Mass Transfer of Ferrocene Derivatives Across a Microdroplet/Water Interface... 

MASS TRANSFER ACROSS A MICRODROPLET/WATER INTERFACE... 

B. Mass Transfer of FeCp-X + from a Single Microdroplet to Water... 

Mass Transfer and Reaction Rate in the Nano-Region of Microdroplet/Solution Interfaces... 

MICRODROPLET SIZE EFFECT ON MASS TRANSFER AND REACTION RATE... 

The mass transfer and chemical reaction between a single microdroplet and the surrounding solution phase can be induced by the laser and microcapillary manipulation and microelectrochemistry system. An example of the electrochemically induced mass transfer across a single microdroplet/solution interface is shown in Figure 9.3. If... 

FIGURE 9.3. General scheme of electrochemically induced mass transfer across a single microdroplet/ solution interface. 

Mass transfer from the water phase to the microdroplet under stationary conditions quickly reaches the steady-state because of spherical diffusion. When the extraction of X is diffusion-limited in the water phase. Equation (3) is obtained using the diffusion coefficient of X in the water phase (Z w) [18]. 

For ion-pair extraction, a cation is extracted with an anion into oil. In this case, individual ions or the ion pair species transfer across a microdroplet/water interface and the extraction rate is expected to depend on the Galvani potential between the microdroplet and water, the ion transfer potentials across the liquid/liquid interface, the association constant of the ions in the solution and so forth [46-54]. Therefore, the mass transfer processes are complicated even in the absence of adsorption of an ion at the microdroplet/water interface. In this section, the kinetic analysis of a simple ion-pair extraction without adsorption is described and the extraction mechanism is discussed on the basis of the single microdroplet technique. 

We showed direct measurements of the mass transfer rate of a simple ion-pair extraction in the single microdroplet/water system and discussed the transfer mechanism on the basis of the kinetic data. 

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