J. Phys. Chem. C 2008, 112, 8343–8348
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Sonochemistry and Sonoluminescence under Simultaneous High- and Low-Frequency Irradiation Adam Brotchie, Muthupandian Ashokkumar,* and Franz Grieser* Particulate Fluids Processing Centre, School of Chemistry, UniVersity of Melbourne, VIC 3010, Australia ReceiVed: January 24, 2008; ReVised Manuscript ReceiVed: March 13, 2008
Sonoluminescence emission, sonochemiluminescence from luminol and the sonochemical yield of hydrogen peroxide were studied under a dual frequency field combining low-frequency (20 kHz) and high-frequency (355 kHz) ultrasound. Under certain conditions it was found that a synergistic enhancement of both sonoluminescence and sonochemical processes could be attained. Ultrasound parameters including pulsing conditions and sonication time were found to sensitively influence the efficiency of the dual-frequency mode operation. Possible mechanisms responsible for the dual-frequency effects are discussed. 1. Introduction The passing of ultrasound through a liquid causes micronsized gas bubbles to oscillate and, under appropriate conditions, undergo a large change in volume and collapse in a near adiabatic process. This is commonly referred to as acoustic caVitation, and the concentration of energy upon collapse results in extreme localized conditions including high temperatures (∼5000 K) and pressures (∼100 atm) inside the cavity.1,2 Under such conditions both inside and in the fluid surrounding cavitation bubbles, a wide range of chemical and physical processes may occur. During the heating of the cavity, homolysis/pyrolysis reactions of the gaseous contents occur, resulting in radical formation. In aqueous systems, water vapor is cleaved into H and OH radicals, and with other species present, various other radicals may form. Much of the sonochemistry that ensues results from the reactivity of these radicals. Some specific radical mediated applications of ultrasound include organic and inorganic colloid synthesis, pollutant degradation and polymerization.3,4 The motion of bubbles during oscillation and collapse causes significant fluid flow effects, such as microjetting and strong shear forces that can also be exploited in processes such as emulsification and cleaning. In addition to sonochemical processes, bubble collapse is accompanied by a short burst of light, referred to as sonoluminescence (SL), which is known to be thermal in origin,5,6 due to the extreme temperatures generated in the center of the bubble during the final stages of collapse. This should not be confused with another type of light emission, sonochemiluminescence (SCL), which results from the reaction of radicals with certain chemicals, such as luminol. Both SL and SCL provide a convenient and effective means of probing an ultrasound system and quantifying its cavitation activity.7 For both fundamental research and industrial applications of ultrasound, there is a desire to improve the efficiency and efficacy of the system used. A relatively recent approach, which is gaining interest, is to improve efficiency through the employment of a multiple-frequency sound field. Khavskii8 in 1979 demonstrated that by using a 44 kHz and 1 MHz dual-frequency reactor a synergistic enhancement in physicochemical processes, such as the pitting of aluminum foil and the stability of * Authors for correspondence. E-mail:
[email protected], masho@ unimelb.edu.au.
emulsions, could be achieved. Dmitrieva and Margulis9 in 1985, however, using a similar frequency combination of 20 kHz and 1 MHz, examined three sonochemical processes and observed suppression in the reaction rates in two of the systems using the dual-frequency mode. In subsequent years, multiple-frequency systems have been explored in various reactor types, with both a fundamental and an applied focus. Suzuki et al.10 observed frequency dependent sonochemical reaction efficiencies when various high-frequency fields were combined with a 20 kHz horn, finding that a synergistic enhancement of sonochemical yield occurred at frequencies below 100 kHz or above 500 kHz. At frequencies between these limits, a suppression of sonochemistry was reported. They also showed that the enhancement was due largely to the generation of a greater number of active bubbles whereas the attenuation effect (observed between 100 kHz and 500 kHz) arose from the destruction of the high-frequency standing wave in the dual-frequency sound field. Other studies combining vastly different frequencies have observed an enhancement in SL11–15 and in the rates of sonochemical processes.16–19 Ciuti et al.13 and Dezhkunov15 studied the short-time action of low-frequency (LF) stimulation on a pulsed high-frequency (HF) field, observing enhancement not only during the LF stimulation but also upon its termination. Importantly, they also noted that, at high acoustic power (of the LF), a cancelation effect of the SL occurs, attributed to an “oversaturation” of bubbles in the cavitation zone. More specific applications have also been investigated under a multiplefrequency field, such as metal leaching,20 tumor ablation,21,22 pollutant degradation23–25 and catalyst synthesis,26 all of which show an enhanced efficiency under the conditions reported. Various explanations have been proposed to explain the synergy of a multiple-frequency sound field, including cavitation occurring over a wider range of bubble sizes, reduction in quasistatic pressure, breaking up of bubble clouds, generation of new nuclei,11 reduction of the cavitation threshold,27 increased active cavitation volume fraction28,29 and increased severity of bubble collapse.29–31 Much work has been conducted in single-bubble systems.32–38 Holzfuss et al.32 and Ketterling and Apfel33 studied the enhancement of SBSL by the second harmonic frequency. The found experimentally that the SL intensity increased by about 300% with the presence of the harmonic frequency and
10.1021/jp8006987 CCC: $40.75 2008 American Chemical Society Published on Web 05/03/2008
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Figure 1. Photographs of sonochemiluminescence from luminol solution in the HF sound field (left) and dual-frequency sound field (right) with both ultrasound units operating in continuous mode.
attributed this to a change in bubble dynamics, specifically an increased Rmax/Ro ratio (resulting in a greater maximum temperature). Moraga et al.35 used the 10th harmonic to enhance the SBSL at a fundamental driving frequency of 23.4 kHz and also observed an enhancement of almost 300% under optimal conditions. Interestingly, the enhancement observed did not correspond to an increased maximum radius prior to bubble collapse. The authors argue that a more violent collapse under dual-frequency driving collapse is responsible for the enhancement observed. Lu et al.38 provide a theoretical work for the optimization of multiple-frequency driving of a single bubble to obtain the greatest peak collapse temperature. We have previously reported that, for a system combining 20 kHz and 355 kHz ultrasound, a synergistic enhancement in SL could be attained at low operating power, whereas a decrease in SL occurs at higher acoustic power.39 We also examined in that study the effect of certain solutes on the SL enhancement at low power. In the present study we extend this investigation to sonochemical reactions occurring over what is considered to be a typical sonochemical reaction period (30 min), drawing a comparison between SL, SCL and sonochemical yields. The outcome of this study sets the basis for a more comprehensive understanding of the effect solutes have on the efficiency of this and other dual-frequency systems, allowing for an optimization of operating conditions, in order to achieve the greatest sonochemical efficiency for a chosen system. It should be noted that the objective of this study is the optimization of radical mediated processes, and therefore the results do not necessarily apply to processes involving the mechanical action of ultrasound (i.e., cleaning, emulsion polymerization, etc.). 2. Experimental Details All chemicals were used as received. 3-Aminophthalhydrazide (luminol) (97%) was supplied by Sigma-Aldrich. Sodium hydroxide (99%) and ammonium molybdate were obtained from BDH. Potassium iodide and sodium bromide were obtained from Chem-supply. Potassium hydrogen phthalate was supplied by Ajax Chemicals Pty Ltd. All solutions were made using Milli-Q filtered water, and left exposed to the atmosphere overnight to ensure air saturation. An Allied Signal transducer (55 mm diameter), operated at 355 kHz, was fitted with a customized Pyrex reaction cell (with the transducer fitted at the bottom). The volume of solution used was 220 mL, and the cell was fitted with a water-cooling jacket to ensure constant temperature (21 ( 3 °C) during sonication.
When operating in continuous wave mode, an ELAC rfgenerator was used, and in pulsed mode, a Hameg function generator (model HM8131-2) was triggered to pulse by a Datapulse (100A) pulse generator. The pulsed electrical output was amplified by a T&C Power Conversion, Inc. Amplifier (AG series). A 45 mm diameter, 20 kHz titanium-alloy horn, powered by a Branson B-30 cell disrupter, was inserted vertically into the reaction cell, a nominal distance of 60 mm from the HF transducer. The 355 kHz power was 6.2 W when operated in continuous mode, and 4.4 W when operated in pulsed mode. The 20 kHz power was 11.2 W in both pulsed and continuous mode operation. The power of the 20 kHz was pulsed with a 4 ms pulse width and 12 ms pulse separation, and the 355 kHz unit was pulsed with a 5 ms pulse width and 17 ms separation. The pulse parameters (on and off times) chosen were found to give the greatest enhancement factor during the pulse. At this time scale, the effect of pulse parameters is not strong enough to significantly affect the trends reported in this study. The acoustic power was determined calorimetrically. To measure emission signals of both SL and SCL from luminol (independently measured in different solutions), the ultrasound cell was placed in an enclosed light-insulated cabinet. A Hamamatsu photomultiplier tube (PMT) was placed close to the face of the reaction cell roughly halfway between the two ultrasound emitting surfaces. A Canberra high voltage supply was used to amplify the PMT signal, which was subsequently displayed and measured on a LeCroy oscilloscope. Unless otherwise specified, SL and SCL profiles were acquired within the first minute of sonication. Hydrogen peroxide formed during sonication was determined spectrophotometrically, using a Varian spectrophotometer and employing a molybdate catalyzed tri-iodate reaction, as described by Alegria et al.40 Samples were withdrawn from the sonicating solution at regular intervals and allowed to react with the reagent for 10 min prior to analysis. Fresh reagent solutions were prepared on the day of the experiment. 3. Results 3.1. Continuous Operation. Figure 1 shows photographs of the SCL emission resulting from the continuous high-frequency (355 kHz) and dual-frequency sound fields. Under HF sonication, the emission occurs through reactions taking place at antinodal regions of the standing wave. In dual-frequency mode, the standing wave structure is destroyed and the emission is reduced to a narrow band in the middle of the reactor. The net
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Figure 3. Sonochemical formation of hydrogen peroxide with time under both single- and dual-frequency sonication with the HF unit operating in continuous mode and the LF unit pulsed.
Figure 2. SL (a) and SCL (b) profiles obtained in water with the LF unit operating in pulsed mode and the HF unit in continuous mode.
emission of SL and SCL was measured and found to decrease under the conditions used (data not shown). 3.2. Pulsed LF. The SL and SCL profiles resulting from the action of dual-frequency irradiation, with the LF (20 kHz) unit operated in pulsed mode, are shown in Figure 2. It can be seen that, for the duration of the LF pulse, there is a synergistic enhancement of the emission (of both SL and SCL) under dualfrequency operation. Between pulses, the signal is attenuated, relative to the individual HF signal as a result of the action of the LF. The emission was integrated over one pulse cycle (0–16 ms) to obtain a net intensity, and the ratio of integrated dual: single-frequency emission is inset in the plot. For SL emission, this value is 0.8, indicating a net attenuation in dual-frequency mode, and for SCL it is 1.1, reflecting a slight enhancement. The sonochemical formation of hydrogen peroxide was measured over the period of 1 h under the pulse conditions described above, the results of which are presented in Figure 3. This shows the formation of hydrogen peroxide as a function of time for the combined single frequency mode (i.e., the algebraic sum of yields of both individual frequency modes) and for the dual-frequency mode. It can be seen that with increasing sonication time there is an increasing difference between yields obtained under single- and dual-frequency conditions. As the sonication time increases, the singlefrequency yield (mainly contributed by the HF source) becomes increasingly more efficient compared to the dual-frequency mode. As the sonochemical efficiency of the dual-frequency mode was observed to decrease with increasing sonication time, the
Figure 4. (a) Integrated SL intensity and relative dual-frequency integrated intensity as a function of sonication time with the HF unit operating in continuous mode and the LF unit pulsed and (b) SL signals obtained at 30 min sonication time.
SL emission was measured at constant temperature over a period of 30 min. The experiment was truncated at 30 min as beyond this time the emission intensities remained relatively constant. Figure 4 (a) shows the integrated SL emissions for the sum of the two individual frequency modes and the dual-frequency mode as a function of sonication time. Also shown is the relative dual-frequency emission (Idual/Isingle) measurement. At shorter
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Figure 6. Sonochemical production of hydrogen peroxide under singleand dual-frequency sonication with the HF unit operating in pulsed mode and the LF unit in continuous mode.
Figure 5. SL (a) and SCL (b) profiles with the HF unit operating in pulsed mode and the LF unit in continuous mode.
times (
