Sonochemistry and Sonoluminescence under Dual-Frequency

The sonoluminescence emission and sonochemical efficiency of a typical laboratory scale, high-frequency standing wave reactor (355 kHz), stimulated by...
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J. Phys. Chem. C 2008, 112, 10247–10250

10247

Sonochemistry and Sonoluminescence under Dual-Frequency Ultrasound Irradiation in the Presence of Water-Soluble Solutes Adam Brotchie, Franz Grieser, and Muthupandian Ashokkumar* Particulate Fluids Processing Centre, School of Chemistry, UniVersity of Melbourne, VIC 3010, Australia ReceiVed: February 28, 2008; ReVised Manuscript ReceiVed: April 15, 2008

The sonoluminescence emission and sonochemical efficiency of a typical laboratory scale, high-frequency standing wave reactor (355 kHz), stimulated by low-frequency pulses (20 kHz) has been investigated in the presence and absence of some water-soluble solutes, namely, propanol and polyethylene oxide. It has been found that, although dual-frequency sonication causes a decrease in the integrated sonoluminescence intensity and sonochemical efficiency in water, in the presence of solutes, a significant enhancement in activity could be attained. This enhancement effect is ascribed, in part, to changes in the extent of bubble coalescence brought about by the water-soluble solutes. 1. Introduction Multiple-frequency ultrasound reactors are increasingly being employed to improve the efficiency of a range of different sonochemical processes. In 1979, Khavskii1 showed that a synergistic enhancement of some ultrasonic processes could be achieved through the combination of two frequencies, one at 1 MHz, the other at 44 kHz. A considerable body of work has subsequently been produced with multiple-frequency irradiation being applied to a large range of processes ranging from pollutant degradation,2–4 tumor ablation,5,6 catalysis,7 and metal leaching.8 All of these studies have yielded positive results, with a significant increase in efficiency obtained in the multiplefrequency mode. With a more fundamental focus, experimental work investigating sonoluminescence emission9–16 and imaging of the cavitation zone,17,18 as well as theoretical studies modeling the cavitation zone19,20 and bubble dynamics2,10,21–24 under the influence of a dual-frequency sound field, have been conducted. The outcome of the theoretical work is the result that it is possible to attain a greater active volume of cavitation in a multiple-frequency reactor. Also, under certain conditions the bubble collapse ratios, Rmax/Rmin and Rmax/Ro, can be increased, resulting in elevated peak collapse temperatures. However, it has also been shown that, under certain experimental conditions, multiple-frequency irradiation can have a destructive effect on acoustic cavitation.15,17,24 Suzuki et al.17 reported a frequency-dependent effect when using a combination of a low-frequency (20 kHz) and high-frequency sources. They observed a synergistic effect when the higher frequency was either below 100 kHz or greater than 500 kHz. In the range of 200-500 kHz a suppression of sonochemistry was observed and was attributed to the destruction of the high-frequency standing wave in the reactor. We have previously reported that the sonoluminescence (SL) signal in water could be synergistically enhanced at low power from the combination of high (355 kHz) and low (20 kHz) frequency sound fields.16 It was shown that the SL enhancement could be significantly elevated in the presence of various solutes, compared to that observed in pure water. The acoustic power * Corresponding author e-mail: [email protected].

levels chosen in the previous study16 were such that no SL was observed when the low and high frequencies were individually operated. In the present study, the influence of water-soluble solutes is examined with respect to the rates of selected sonochemical processes (hydrogen peroxide formation and gold(III) reduction), in addition to SL emission, under single- and dual-frequency sonication conditions. The acoustic power levels chosen in this study were at a level where small amounts of SL and sonochemical activity could be observed when low and high frequencies were individually operated. Under the experimental conditions used, significant enhancement in the sonochemical activity and SL intensity have been observed in the presence of water-soluble solutes. 2. Experimental Section All chemicals were used as received. Ammonium molybdate was obtained from BDH Ltd. Potassium hydrogen phthalate was supplied by Ajax Chemicals Pty Ltd. Polyethylene oxide (PEO) (MW ) 100 000 g mol-1), potassium iodide, sodium bromide, and gold chloride trihydrate were obtained from Sigma-Aldrich. Propanol used was analytical reagent grade, obtained from Merck. All solutions were made using Milli-Q filtered water on the day of the experiment. An Allied Signal transducer (55 mm diameter) powered by an ELAC RF-generator, was operated at 355 kHz and was fitted with a customized Pyrex reaction cell (220 mL) with a watercooling jacket to ensure a near constant solution temperature (20 ( 3 °C) during sonication. A 35 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 plate. The power of the 355 kHz unit was 6.2 W, and that of the 20 kHz was 11.2 W, as determined calorimetrically. The 20 kHz signal was pulsed with a 4 ms pulse width and 12 ms pulse separation. These conditions were used under both single- and dual-frequency sonication. The yields reported have not been corrected for the “dead time” between pulses (in the case of the 20 kHz, and 20 + 355 kHz modes) and refer simply to the total time of sonication. The SL emission signal from the ultrasound cell was measured in an enclosed, light-insulated cabinet. A Hamamatsu

10.1021/jp801763v CCC: $40.75  2008 American Chemical Society Published on Web 06/18/2008

10248 J. Phys. Chem. C, Vol. 112, No. 27, 2008

Figure 1. SL profiles in water and aqueous solution of 1-propanol (100 mM) and polyethylene oxide (1%) under pulsed 20 kHz, continuous mode 355 kHz, and dual-frequency (20 + 355 kHz) irradiation.

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. SL profiles were acquired within the first minute of sonication unless otherwise stated. The amount of hydrogen peroxide formed during sonication was determined spectrophotometrically, using a Varian Spectrophotometer, according to a technique described by Alegria et al.25 Fresh reagent solutions were prepared on the day of the experiment. Aqueous solutions of 0.2 mM gold chloride trihydrate in aqueous alcohol solution were sonicated under an air atmosphere. Immediately following sonication, as described by Okitsu et al.,26 saturated NaBr was added to the sample, and the resulting gold bromide band was measured spectrophotometrically after 10 min. 3. Results The SL profiles obtained in water, propanol (100 mM), and PEO (1%) solutions under single- and dual-frequency sonication are combined and are shown in Figure 1. For the pure water system, it can be seen that the SL intensity is enhanced during the low frequency (LF) pulse in dual-frequency mode, whereas the emission signal is attenuated, relative to that from the individual high frequency (HF) signal, in the time period between LF pulses. In the propanol solution, the individual HF emission signal is significantly reduced relative to the signal observed in water, and the individual LF signal is unaffected (within experimental error). In contrast, the dual-frequency SL signal is considerably enhanced in the presence of propanol. In the polymer solution, the HF signal is also reduced relative to the signal obtained in water, whereas the LF and dual-frequency emission signals are both significantly enhanced. The ratio of the net SL (integrated over one LF pulse cycle, i.e., ∼16 ms) obtained under dual-frequency operation relative to that of the algebraic sum of the two single frequencies is presented as a function of sonication time in Figure 2. It can be observed that, in water, the relative SL is initially close to 1 but decreases steadily with exposure time. As a consequence of the addition of the solutes used, the initial relative SL can be significantly enhanced: by a factor of about 5 in the case of propanol and by about 2 for PEO. In the case of the polymer, the enhancement factor is consistent over the 30 min sonication period. For the propanol, this ratio decreases in time but still remains greater than 1, reflecting a net enhancement, sustained with time.

Brotchie et al.

Figure 2. Relative dual-frequency integrated SL emission (ratio of intensity obtained in dual-frequency mode to the algebraic sum of the individual frequency intensities) with time in water, 1-propanol (100 mM), and PEO solution (1%).

Figure 3. Hydrogen peroxide formation as a function of time in a PEO (1%) solution under single-frequency (algebraic sum of yields obtained under single, continuous mode 355 kHz, and pulsed 20 kHz sonication) and dual-frequency (20 + 355 kHz) sonication.

The generation of hydrogen peroxide in PEO solutions is shown in Figure 3 under single- and dual-frequency sonication. There is a clear enhancement in the rate of peroxide formation in the dual-frequency mode, which is consistent with the enhancement also observed in SL emission intensity. It should be noted that, under identical conditions in the absence of the polymer, the yield of hydrogen peroxide was significantly suppressed in the dual-frequency mode (results not shown). Due to the ability of alcohol molecules to effectively scavenge the primary radicals produced from the homolysis of water molecules, measuring the production of hydrogen peroxide is not necessarily indicative of the chemical activity in such systems. Therefore, to quantify the sonochemical activity in the presence of alcohol, the reduction of gold(III) chloride was employed as a model reaction. This reaction proceeds mainly via relatively stable secondary alcohol radicals rather than comparatively short-lived primary radicals (H · and · OH).27 The reduction of gold(III) is shown in Figure 4 as a function of sonication time under single- and dual-frequency operation in 100 mM propanol. It can be seen that the rate of reduction is enhanced several-fold in the dual-frequency mode. 4. Discussion The SL profiles shown in Figure 1 reveal several features: (i) the addition of propanol did not affect the LF signal of water

Sonochemistry and Sonoluminescence

Figure 4. Reduction of gold chloride in PrOH (100 mM) as a function of sonication time under single-frequency (algebraic sum of amount reduced under single, continuous mode 355 kHz, and pulsed 20 kHz sonication) and dual-frequency (20 + 355 kHz) sonication.

when operated alone, (ii) the addition of PEO increased the LF signal of water when operated alone, (iii) the addition of both propanol and PEO decreased the HF signal when operated alone, (iv) an enhancement in intensity for the duration of the LF pulse in dual-frequency mode in the absence and presence of the solutes, and (v) an attenuation of the HF signal between LF pulses in dual-frequency mode in propanol and PEO solutions compared to that observed in water. It has been shown in previous studies that propanol does not affect the LF SL signal due to the “transient” nature of the cavitation.28 On the basis of this fact, the observation of a constant SL signal in water and propanol solution is expected behavior. Regarding the second feature listed above, it should be pointed out that, under different experimental conditions such as under high or moderate acoustic power levels where the cavitation bubble density is relatively high, it is well-established that bubble coalescence, driven by Bjerknes forces, reduces the number of active bubbles available in the system. Inhibition of bubble coalescence through the action of water-soluble solutes can, therefore, vastly improve the cavitation efficiency of these systems. We have previously reported that the presence of polymer solutes, such as PEO, significantly enhances the SL intensity by inhibiting bubble coalescence.16 The increase in SL intensity observed at LF and dual-frequency in the presence of the polymer (Figure 1) can be directly attributed to the inhibition of bubble coalescence. The process responsible for the reduction in the individual HF signal intensity in the presence of propanol and PEO is different for the two solutes. Propanol, like other volatile solutes, evaporates into the bubble core, and the subsequently generated decomposition products (e.g., hydrocarbons) accumulate inside the “stable” cavitation bubbles, therefore reducing the peak bubble temperature reached. As a result of this, the intensity of SL emission and amount of radicals generated are reduced.29,30 This quenching process has been found to be prevalent in HF and negligible in LF sound fields,28 which is precisely what is observed in the present study. In the case of PEO, the reduction in the signal cannot be due to the above-mentioned mechanism since PEO is a nonvolatile solute. However, the observed effect can be explained by taking into account of the coalescence effect. Under HF action, “stable” cavitation bubbles may reach their resonance size via either rectified diffusion or a bubble coalescence pathway in water. In the presence of PEO, the inhibition of bubble coalescence

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10249 may not allow the bubbles to reach their resonance size by this pathway at the low ultrasound powers used, and this may result in a decrease in the number of active bubbles. Note that, under LF operation, the growth of cavitation bubbles by the coalescence pathway does not play a major role due to the “transient” nature of the cavitation bubbles produced. The observation that the SL signal is enhanced in dualfrequency mode relative to that observed in SF modes in water has been reported in our previous report.16 It has been suggested that the bubble nuclei generated by HF is acted upon by the LF field, leading to an increase in the number of active bubbles for the duration of the LF pulse that results in an increase in the SL intensity. Bubble dynamics under dual-frequency conditions has been discussed in our recent publication.24 In that study,24 numerical simulations under single- and dual-frequency conditions have been conducted in order to account for the changes observed in multibubble sonoluminescence (MBSL) intensity. However, we did not match the driving frequencies. The current manuscript reports on the effect of water-soluble solutes on SL observed under dual-frequency conditions. In multibubble systems, the presence of low levels of solutes does not affect the bubble dynamics and hence the MBSL intensity. For example, the MBSL intensity of a solution containing 25-100 mM of nonvolatile solutes is approximately the same as that observed in water.31,32 On the basis of several of our studies,33 it can be concluded that the bubble dynamics is not affected by the presence of solutes. In the absence of changes to the bubble dynamics by the water-soluble solutes, the increase in the SL intensity, in the presence of both propanol and PEO, relative to that observed in pure water for the duration of the LF pulse can be linked to the inhibition of bubble coalescence caused by the water-soluble solutes. As mentioned above, both propanol and PEO inhibit bubble coalescence, which results in an increase in the number of bubble nuclei that could be acted upon by the LF field. This ultimately leads to an increase in the number of active bubbles and results in an increase in the SL signal. Although the SL intensity in the different solutions under single- and dual-frequency operation are discussed above for very short sonication times (typically less than 1 min), it would be constructive to consider the SL intensity over longer sonication times for comparison with sonochemical data. For this purpose, the SL signal was integrated over an entire pulse cycle, and the ratio of the integrated SL values from dualfrequency to that of the algebraic sum of the individual frequencies is presented in Figure 2 as a function of sonication time. It should also be noted that the data presented is a ratio of the dual-frequency to single-frequency emission intensities. In water, the decrease in intensity ratio observed with increasing sonication time results mainly from an increase in the singlefrequency (355 kHz) signal. In the presence of solutes, this enhancement of the HF signal is not observed, and the SL appears relatively constant over the 30 min sonication period. In water, the algebraic sum is always less than 1 (Figure 2). The difference becomes very clear at longer sonication time (>10 min). This indicates that the net cavitation activity in water is reduced in the dual-frequency mode, especially at longer sonication times. However, the results of the present study show that, despite this reduction in cavitation activity in pure water, a synergistic enhancement effect can be achieved in the presence of watersoluble solutes at low acoustic power levels. The dual-frequency emission is also significantly enhanced relative to water in the presence of both solutes (propanol and PEO) to such an extent

10250 J. Phys. Chem. C, Vol. 112, No. 27, 2008 that the relatively intensity is synergistically enhanced, as is evident in Figure 2. The relative signal in the PEO solution is always greater than 1, and the signal remains approximately constant over a longer period of sonication time. In propanol solution, the signal decreases with time and reaches a value closer to that of PEO and remains approximately constant after 20 min sonication. The mechanisms responsible for the enhancement most likely involve the inhibition of bubble coalescence by both the alcohol and the polymer. The reason for the difference between propanol and PEO effects (the enhancement observed with propanol is about 5 times greater at the initial stages of sonication; the signal decreases with time for about 20 min of sonication) is not clear and is the subject of further investigation. Although the results presented in Figures 1 and 2 deal with SL, the effect of the water soluble solutes on the sonochemical activity is presented in Figures 3 and 4. The data presented in Figure 3 shows an enhanced rate of formation of hydrogen peroxide. This observation is consistent with the enhancement of the SL emission in dual-frequency mode in the presence of PEO. Although the relative SL values in Figure 2 are constant with time for PEO, the rate of generation of H2O2 at 355 kHz becomes noticeably faster after 20 min of sonication. The enhancement in SL emission in the presence of propanol is also consistent with observed trends in the sonochemical reduction of gold(III) chloride (Figure 4). In this reaction, the alcohol acts to greatly accelerate the reduction process of gold(III), which in the absence of alcohol proceeds at a negligible rate under these experimental conditions. It is clear that the efficiency of this process can be further improved by operating in the dual-frequency mode, with a 3-4-fold increase observed in the reduction rate in aqueous solution containing 100 mM propanol. The synergistic effect observed in both SL and sonochemistry in the presence of alcohol suggests that some property of the system is changed such that dual-frequency operation is more favorable to sonochemistry than single-frequency operation. Two possible reasons that can be suggested are: either a greater number of photons and radicals are being produced within individual cavitation bubbles, or a greater number of active bubbles are generated in the system. For the former to be true, a higher bubble temperature is required. Although theoretical studies of single-bubble dynamics predict elevated peak temperatures in the dual-frequency field under the power and frequency conditions of this experiment, preliminary experimentally determined bubble temperatures have failed to verify this. However, this possibility cannot be completely discounted. The second possible explanation may be more likely - considering that both volatile (alcohol) and nonvolatile solutes (polymer) cause a synergistic enhancement in activity in the dual-frequency mode, it is reasonable to infer that that bubble-coalescence plays an integral role in this effect. 5. Conclusions It has been found that the pulsed action of a low frequency (20 kHz) source on continuous high frequency (355 kHz) cavitation has a destructive effect on SL and sonochemical efficiency in water. In the presence of coalescence-inhibiting solutes (propanol and polyethyleneoxide) however, the SL intensity and sonochemical rates of hydrogen peroxide formation

Brotchie et al. and gold chloride reduction were found to be synergistically enhanced in the dual-frequency mode. It was observed that the relative efficiency of the dual-frequency mode decreased with sonication time. It is concluded that the effect of coalescence inhibition is more pronounced under the dual-frequency sound field conditions, rendering it relatively more sonochemically efficient in the presence of the water-soluble solutes. Acknowledgment. The authors acknowledge the Australian Research Council’s award of a Discovery Project. A.B. acknowledges the Australian Postgraduate Award. References and Notes (1) Khavskii, N. N. SoV. Phys. Acoust. 1979, 25, 64. (2) Sivakumar, M.; Tatake, P.; Pandit, A. B. Chem. Eng. J. 2002, 85, 327. (3) Wang, S.; Huang, B.; Wang, Y.; Liao, L. Ultrason. Sonochem. 2006, 13, 506. (4) Zhao, D.; Xu, X.; Lei, L.; Wang, D. Chin. J. Chem. Eng. 2005, 13, 204. (5) He, P. Z.; Xia, R. M.; Duan, S. M.; Shou, W. D.; Qian, D. C. Ultrason. Sonochem. 2006, 13, 339. (6) Shang, Z.; Zhang, J.; Zhu, X.; Qi, H.; Liu, Q. Chin. J. Biomed. Eng, 2004, 23, 433. (7) Yu, F.; Ji, J.; Zheng, Y.; Liu, H. Petrochem. Technol. 2004, 33, 824. (8) Swamy, K. M.; Narayana, K. L. Ultrason. Sonochem. 2001, 8, 341. (9) Carpenedo, L.; Ciuti, P.; Francescutto, A.; Iernetti, G.; Johri, G. K. Acoustic Letters 1987, 10, 178. (10) Holzfuss, J.; Ruggerberg, M.; Mettin, R. Phys. ReV. Lett. 1998, 81, 1961. (11) Iernetti, G.; Ciuti, P.; Dezhkunov, N. V.; Reali, M.; Francescutto, A.; Johri, G. K. Ultrason. Sonochem. 1997, 4, 263. (12) Ciuti, P.; Dezhkunov, N. V.; Francescutto, A.; Calligaris, F.; Sturman, F. Ultrason. Sonochem. 2003, 10, 337. (13) Ciuti, P.; Dezhkunov, N. V.; Francescutto, A.; Kulak, A. I.; Iernetti, G. Ultrason. Sonochem. 2000, 7, 213. (14) Dezhkunov, N. V. Tech. Phys. Lett. 2001, 27, 491. (15) Dezhkunov, N. V. J. Eng. Phys. Thermophys. 2003, 76, 142. (16) Brotchie, A.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. C 2007, 111, 3066. (17) Suzuki, T.; Yasui, K.; Yasuda, K.; Iida, Y.; Tuziuti, T.; Nakamura, M. Res. Chem. Intermed. 2004, 30, 703. (18) Yasuda, K.; Torii, T.; Yasui, K.; Iida, Y.; Tuziuti, T.; Nakamura, M.; Asakura, Y. Ultrason. Sonochem. 2007, 14, 699. (19) Moholkar, V. S.; Rekveld, S.; Warmoeskerken, M. M. C. G. Ultrasonics 2000, 38, 666. (20) Servant, G.; Laborde, J. L.; Hita, A.; Caltagirone, J. P.; Gerard, A. Ultrason. Sonochem. 2003, 10, 347. (21) Ketterling, J. A.; Apfel, R. E. J. Acoust. Soc. Am. 2000, 107, 819. (22) Tatake, P.; Pandit, A. B. Chem. Eng. Sci. 2002, 57, 4987. (23) Prabhu, A. V.; Gogate, P. R.; Pandit, A. B. Chem. Eng. Sci. 2004, 59, 4991. (24) Kanthale, P.; Brotchie, A.; Ashokkumar, M.; Grieser, F. Ultrason. Sonochem. 2008, 15, 629. (25) Alegria, A. E.; Lion, Y.; Kondo, T.; Riesz, P. J. Phys. Chem. 1989, 93, 4908. (26) Okitsu, K.; Yue, A.; Tanabe, S.; Matsumoto, H.; Yobiko, Y.; Yoo, Y. Bull. Chem. Soc. Jpn. 2002, 75, 2289. (27) Caruso, R. A.; Ashokkumar, M.; Grieser, F. Langmuir 2002, 18, 7831. (28) Tronson, R.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2002, 106, 11064. (29) Ashokkumar, M.; Grieser, F. J. Am. Chem. Soc. 2005, 127, 5326. (30) Rae, J.; Ashokkumar, M.; Eulaerts, O.; von Sonntag, C.; Reisse, J.; Grieser, F. Ultrason. Sonochem. 2005, 12, 325. (31) Ashokkumar, M.; Mulvaney, P.; Grieser, F. J. Am. Chem. Soc. 1999, 121, 7355. (32) Ashokkumar, M.; Vinodgopal, K.; Grieser, F. J. Phys. Chem. B. 2000, 104, 6447. (33) Ashokkumar, M.; Grieser, F. ReV. Chem. Eng 1999, 15, 41.

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