Electrochemical processes at a flowing organic solvent∣aqueous electrolyte phase boundary

Abstract

A microfluidic double channel device was employed to study reactions at a flowing liquid∣liquid interface in contact with a gold electrode. The rectangular flow cell was calibrated for both single phase liquid flow and biphasic liquid∣liquid flow for the case of the immiscible N-octyl-2-pyrrolidone (NOP)∣aqueous electrolyte system. The influence of flow direction and speed and liquid viscosity on the position of the phase boundary was examined. The Ru(NH 3 ) 6 3 + / 2 + redox system was employed in aqueous solution to calibrate the flow cell in the absence and in the presence of the organic NOP phase. A significant “undercutting” of the organic phase into the aqueous phase was observed in particular for shorter gold band electrodes. The triple phase boundary reaction zone was visualized with a colour reaction based on the oxidation of N-benzylaniline. An approximate expression can be given to predict the mass transport controlled limiting currents even under two-phase flow conditions. Next, n-butylferrocene in NOP (without intentionally added electrolyte) was employed as the organic redox system with 0.1 M NaClO 4 as the adjacent aqueous electrolyte phase. Under these conditions the electrochemical reaction only proceeded at the organic liquid∣aqueous liquid∣solid electrode triple phase boundary reaction zone and significant currents were observed. In contrast to the processes at conventional liquid∣electrode interfaces, these currents decreased with an increasing flow rate. The level of conversion at the triple phase boundary reaction zone can be further enhanced (i) at sufficiently slow flow rates and (ii) at larger electrodes. Bulk electrosynthetic processes are feasible, but the reactor design has to be further improved.