The effect of frequency on sonochemical reactions

Ultrasonic irradiation, or sonication, is a unique method of activating and accelerating chemical reactions in a liquid medium through the action of cavitation. Typically, ultrasonic irradiation can increase the rates of reactions, decrease the required reaction temperature and improve product yields. Historically, the effect of frequency on sonochemical reactions has been considered to be negligible. It was thought that when the ultrasonic power is sufficient to produce cavitation, it does not matter what frequency is used. We focussed our study on the effect of frequency on different sonochemical reactions in order to test this assumption. The frequencies generally used in sonochemistry are in the range of 20-100 kHz. Only a very few groups have used ultrasound with frequencies outside this range. Since cavitation is responsible for sonochemical effects, there is however no reason to restrict our studies to this normal frequency range. Our work focused on two different frequencies far from each other, 20 and 900 kHz. The 900 kHz apparatus was specially designed and constructed using a commercially available power supply. The effects of temperature, intensity, and gas were investigated at these two different frequencies. It was found that the behaviour with 900 kHz irradiation can be very different from that with 20 kHz. For example, the sonication of potassium iodide solution under the same conditions was more effective with 900 kHz than 20 kHz. Sonication of chlorobenzene in water with 900 kHz led to a yellow solution with a high rate while with 20 kHz this reaction was very slow. However, the sonication of liquid carbon disulfide with 20 kHz led to a heterogeneous system, while sonication with 900 kHz had no apparent effect. Our results suggest that the surface area of the sonicator is very important. There seems to be a dependence of the reaction rate on irradiation frequency arising from the temporal and spatial characteristics of the cavitation bubbles produced. The bubble size and collapse time are significantly different at the two frequencies. This may have an important role in determining the different behaviours observed. A sonochemical reaction can occur in the gas phase inside the bubble, at the interface, or at some distance away from the cavitating gas bubble. This may be another important consideration in explaining the difference in results with frequency.