Variable frequency sonochemistry
Wayment, Darcey G.
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The effect the frequency of ultrasound exerts on chemical systems has been widely ignored. This may be due, in part, to the difficulty in obtaining commercial ultrasonic transducer systems of other frequencies and to the belief that there is no frequency effect. However, the physics of the cavitation process implies that a frequency effect should be expected for systems undergoing cavitation at constant power and temperature. On passage of ultrasonic waves through a liquid medium, the liquid is exposed to alternating compression and rarefaction cycles. During the rarefaction cycle, a void is formed containing the liquid vapor, dissolved gases and volatile solutes. During the compression cycle, the cavity is compressed resulting in high temperatures and pressures within the bubble. Three sites for chemical reactions exist during a cavitational event They are the gas phase at the center of the collapsing cavity, the interfacial region - a thin shell of superheated liquid surrounding the vapor phase, and the bulk liquid solution. Chemical reactions are initiated principally by pyrolysis within the gas phase or at the bubble interface and by the attack of free radicals (also generated in the gas phase) at the interface or in the cool bulk solution. The frequency of the irradiating ultrasound will effect the physical properties of the collapsing bubbles, and hence the chemistry that occurs thereafter. An ultrasonic generator capable of operating at a range of frequencies from 20 kHz to 500 kHz has been built and used to investigate the effect of ultrasonic frequency at constant power, on three different aqueous chemical systems, these include potassium iodide oxidation, Rhodamine 6G (a laser dye. named also as Rh-590 in some catalogs) degradation and Alachlor (a herbicide) degradation in aqueous media. These three systems were chosen because it was believed that their chemical interactions occur in three reaction zones of the cavitation bubble, depending upon the substance.