The sonochemistry of aqueous solutions

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The ultrasonic irradiation of organic compounds in aqueous solution is studied under various physical conditions. The observed sonochemical rate constants can be analyzed to yield information about the nature of the bubbles which exist during acoustic cavitation and to determine which parameters are significant for enhancing cavitation chemistry. Aqueous solutions of p-nitrophenylacetate (PNPA), p-nitrophenol (PNP), and carbon tetrachloride (CCl4) were sonicated in various reactor configurations and with different saturating gases. The production of hydroxyl radical ([...]) and hydrogen peroxide (H2O2) was quantified at four different ultrasonic frequencies. The most significant features of the sonolytic hydrolysis of PNPA at 20 kHz are that the observed first order rate constants are independent of pH in the range of 3 to 8, but significantly influenced by the saturating gas (Kr, Ar, or He). Furthermore, the apparent activation parameters for hydrolysis of PNPA ([...], [...], [...]) are significantly different in sonicated systems than in solutions under ambient conditions. These changes are attributed to changes in the physical nature of the solvent when a layer of transient supercritical water is formed around the cavitation bubbles. The psuedo-first order degradation rate constants for the transformation of PNP in a near-field acoustical processor (NAP) increases with increasing power-to-volume ratio over the range of 0.98-7.27 W/[...]. An increase in the power-to-area ratio results in an increasingly fast degradation rate up to 1.2 W/[...] and a slight decrease thereafter. The nature of the background gas influences the degradation rate constant and product distribution. The fastest rate is observed when a mixture of Ar and O2 is used to saturate the solution. The sonolytic degradation of CCl4 is investigated in Ar- and O3-saturated aqueous solutions. The chlorine mass balance is typically >70%, mostly in the form of chloride ion (Cl-). The reactive intermediate, dichlorocarbene, is identified and quantified by means of trapping with 2,3-dimethyl-2-butene. Low concentrations (0.01-0.1 [...]) of hexachloroethane and tetrachloroethylene are detected. Ultrasonic irradiation of a mixture of PNP and CCl4 results in the acceleration of the sonochemical degradation rate of p-NP. Both the ultrasonic frequency and the saturating gas influence the production of [...] and H2O2 during sonolysis of buffered, aqueous solutions. Ultrasonic frequencies of 20,40,80 and 500 kHz are used, and the background gas in solution are Kr, Ar, He or O2. Both species are found at the highest rates during sonolysis of a Kr-saturated solution at 500 kHz, whereas the lowest rates of production are observed during sonolysis of a He-saturated solution at 20 kHz. A range of rate constants for the production of [...] and H2O2 are observed at each frequency because each of the four saturating gases has different physical properties (e.g., thermal conductivity.) However, the range narrows at the higher ultrasonic frequencies (80 and 500 kHz) compared to that observed at the lower frequencies (20 and 40 kHz)