Real-time monitoring of room-temperature ionic liquid purity through optical diode-based sensing

Abstract Room-temperature ionic liquids (RTILs) are promising for use in many industries due to their unique properties, including wide electrochemical windows, low vapor pressures, high ionic conductivities, and chemical and thermal stability. All of these properties require high RTIL purity, and achieving this high purity is a major driver of RTIL manufacturing costs. Continuous flow processes to synthesize highly pure RTILs at a reduced cost have been developed, but due to exothermic synthesis reactions and temperature dependent reaction rates, these processes require real-time control. An ultraviolet LED based optical sensor has been designed to measure RTIL purity at millisecond sampling rates using a liquid flow cell. The sensor is demonstrated by measuring 1-methylimidazole (MIM) concentration in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). Sensor results are compared against spectroscopic measurements with good agreement.