Two-phase flow electrosynthesis: Comparing N-octyl-2-pyrrolidone-aqueous and acetonitrile-aqueous three-phase boundary reactions

A microfluidic double channel device is employed to study reactions at flowing liquid-liquid junctions in contact with a boron-doped diamond (BDD) working electrode. The rectangular flow cell is calibrated for both single-phase liquid flow and biphasic liquid-liquid flow for the case of (i) the immiscible N-octyl-2-pyrrolidone (NOP)-aqueous electrolyte system and (ii) the immiscible acetonitrile-aqueous electrolyte system. The influence of flow speed and liquid viscosity on the position of the phase boundary and mass transport-control led limiting currents are examined. In contrast to the NOP-aqueous electrolyte case, the acetonitrile-aqueous electrolyte system is shown to behave close to ideal without 'undercutting' of the organic phase under the! aqueous phase. The limiting current for three-phase boundary reactions is only weakly dependent on flow rate but directly proportional to the concentration and the diffusion coefficient in the organic phase. Acetonitrile as a commonly employed synthetic solvent is shown here to allow effective three-phase boundary processes to occur due to a lower viscosity enabling faster diffusion. N-butylferrocene is shown to be oxidised at the acetonitrile-aqueous electrolyte interface about 12 times faster when compared with the same process at the NOP-aqueous electrolyte interface. Conditions suitable for clean two-phase electrosynthetic processes without intentionally added supporting electrolyte in the organic phase are proposed. Copyright (c) 2008 John Wiley & Sons, Ltd.