Silicon micromixers with infrared detection for studies of liquid-phase reactions

We present an integrated microchemical system that combines micromixing, a reaction channel, an IR detection region, and temperature control for monitoring and kinetic studies of liquid-phase reactions. The microdevices exploit the transparency of silicon to infrared radiation in most of the wavelength region of interest (4000-800 cm(-1)), the precise definition of microfluidic channels by deep reactive ion-etching, the high thermal conductivity of silicon, and the fusion bonding of silicon for fixed-path-length transmission cells. Two devices are considered, a simple T-shaped mixer and an efficient mixer with interleaving channels for rapid mixing. The first device is used to characterize IR transmission characteristics in silicon-based microreactors and to demonstrate the feasibility of monitoring exothermic reactions, the hydrolysis of propionyl chloride under isothermal conditions. The mixing characteristics of the second microreactor are evaluated experimentally by an acid-base reaction and predicted by computational fluid dynamics simulations. Typical mixing times are 25 ms. The alkaline hydrolysis of methyl formate, a reaction following second-order kinetics with a half-life of 70 ms, exemplifies the use of the microreactor in determining rate constants. The results demonstrate the main advantages of the integrated microchemical systems in reaction monitoring: faster mixing times, temperature control, in situ detection, and elimination of sample postprocessing.