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CHEMISTRY INDUCED BY HYDRODYNAMIC CAVITATION

Cavitation (the formation, growth, and implosive collapse of gas or vapor-filled bubbles in liquids) can have substantial chemical and physical effects. While the chemical effects of acoustic cavitation (i.e., sonochemistry and sonoluminescence) have been extensively investigated during recent years,1-5 little is known about the chemical consequences of hydrodynamic cavitation created during turbulent flow of liquids. Hydrodynamic cavitation is observed when large pressure differentials are generated within a moving liquid and is accompanied by a number of physical effects, erosion being most notable from a technological viewpoint.6,7 In contrast, reports of hydrodynamically induced chemistry or luminescence and direct comparisons to sonochemistry or sonoluminescence have been extremely limited.8,9 In aqueous liquids, acoustic cavitation leads to the formation of reactive species such as OH•, H•, and H2O2. These shortlived species are capable of effecting secondary oxidation and reduction reactions. For example, iodide can be sonochemically oxidized to triiodide by OH• radicals or H2O2 produced during cavitation. From aqueous solutions containing chlorocarbons, Cl• and Cl2 are also liberated in high yields and this increases rates of iodide oxidation.10 The rate of triiodide formation is easily monitored spectrophotometrically. For many years, this so-called Weissler reaction has remained the standard dosimeter for sonochemical reactions. The recent advent of commercially available high-pressure jet fluidizers capable of pressure drops as high as 2 kbar and jet velocities approaching 200 m/s has led to numerous applications in the physical processing of liquids, for emulsification, cell disruption, etc. Chemical consequences of such processing, however, have received little examination. One important exception comes fromW. R. Moser and co-workers,11 who have shown that such a device can be utilized to prepare nanostructured catalytic materials. Moser speculated that the unusual properties of his catalysts resulted from hydrodynamic cavitation within the fluidizer.11 We describe here conclusive experimental evidence for chemical reactions caused by hydrodynamic cavitation within a jet fluidizer. In a typical run,12a 60 mL of 1.0 M KI in purified water saturated with carbon tetrachloride was introduced at a constant flow rate into the Microfluidizer with a liquid pressure of 1.24 kbar. The reaction solution temperature increased 10 to 12 °C within 90 s and stabilized at the temperatures reported herein. Aliquots (4 mL) of the processed solution were periodically extracted from the reaction system by airtight syringes, analyzed spectrophotometrically, and returned to the reservoir after analysis. The rate of I3 formation was calculated from the change in absorbance at 353 nm ( ) 26 400 M-1 cm-1)12b as a function of reaction time. Initial studies conducted with Arsparged water gave relatively low rates of I3 production; saturation of the Ar sparged H2O with CCl4 resulted in a 20fold increase in I3 production, as has been typically observed for ultrasonic cavitation.10 This is attributed to ready formation of Cl• and Cl2 from CCl4 under cavitation conditions. The effect of upstream liquid pressure on the rate of I3 production was investigated over the range 100-1500 bar. The reaction rate increases linearly with liquid pressure (Figure 1), but with a threshold pressure of 150 bar. Below 150 bar of hydrostatic pressure, no chemical reactions were observed; this probably represents the minimum jet velocity necessary to induce cavitation. The resistance of a turbulent flow to cavitation is given by its cavitation number σ, as defined in eq 1:6