Reaction rates in ultrasonic emulsions of dense carbon dioxide and water

Power ultrasound provides a surfactant‐free means of emulsifying dense near‐critical carbon dioxide and water. Droplet size distributions and volume fractions of the dispersed phases have been measured for acoustically formed emulsions. Hydrolysis rates were measured for a series of 7 benzoyl halides under both silent, phase‐separated conditions and for sonicated, emulsified conditions (30°C, 80 bar, 0.6 W/cm3, 20 kHz). Sonication always accelerated the overall hydrolysis rate, sometimes by as much as 200‐fold. Two physical models were developed to describe global kinetics: one for silent and one for sonicated conditions. The model for silent conditions agreed well with the available data set and suggested conditions under which reaction in the carbon dioxide phase (rather than the water phase) might become important. The model for sonicated conditions properly captured the trends in the data set and predicted the experimental results to within a factor of 2 in the worst case. Our analysis strongly suggests that ultrasound accelerates phase‐transfer reactions by increasing the water/carbon dioxide interfacial area. © 2005 American Institute of Chemical Engineers AIChE J, 2006