Microdroplet size effect on mass transfer and reaction rate

MICRODROPLET SIZE EFFECT ON MASS TRANSFER AND REACTION RATE 

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

For a simple extraction of a solute (X) from a water phase into a single spherical micro-oil-droplet, the time (t) dependence of the amount of the solute transferred across the microdroplet/water interface (m) is obtained by Equation (1) [41]. 

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 chemical and physical processes across microdroplet/solution interfaces, obs having dimensions of s or dm mor s is often proportional to r ( = 0, 1 or 2). A linear relationship between obs and has been reported for the extraction of a neutral compound such as ferrocene derivatives from water into a micro-oil-droplet without adsorption at the microdroplet/water interface [18,19] and for a photographic dye formation reaction in an oil-in-water emulsion [23]. The proportionality of a kobs versus r plot has been reported for a relatively slow process such as a photographic dye formation reaction [23,29,42], electron transfer [43-45] and adsorption at the micro-oil-droplet/water interface [19,20]. For the chemical reaction with the rate-determining step in a solution phase or a microdroplet in a microdroplet/solution system, fcobs is independent of r[23]. Based on the droplet size dependence of the reaction rate, the rate-determining step of the overall reaction processes across a microdroplet/solution interface is analysed and the reaction mechanism can be discussed in detail. 

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