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J. Phys. Chem. C 2007, 111, 3066-3070
Effect of Water-Soluble Solutes on Sonoluminescence under Dual-Frequency Sonication Adam Brotchie, Muthupandian Ashokkumar, and Franz Grieser* Particulate Fluids Processing Centre, School of Chemistry, UniVersity of Melbourne, VIC 3010, Australia ReceiVed: NoVember 14, 2006; In Final Form: December 20, 2006
Sonoluminescence (SL) originating from a dual-frequency ultrasound system, in water and in the presence of certain water-soluble solutes, was investigated. The ultrasound arrangement consisted of a 20-kHz horn, vertically opposing a high-frequency transducer operating at 355 or 1056 kHz. It was found that the power setting of the high frequency sensitively determined the SL yield of the dual-frequency system. A significant synergy effect was observed at low power, while at high power the dual-frequency mode attenuated the SL signal. Additives representing three different classes of solutes, namely, surfactants, alcohols, and polymers, were studied, with respect to their effect on the intensity of SL. The results obtained reinforce those from previous single-frequency studies, while simultaneously providing some insight into cavitation under dualfrequency conditions. It was demonstrated that the synergy effect of dual-frequency ultrasound on SL observed in water could be further enhanced in the presence of these solutes.
Introduction The passage of ultrasound through a liquid induces certain physical and chemical processes, largely through acoustic cavitation.1 This involves the growth and rapid collapse of micrometer-sized gas bubbles. Upon bubble collapse, at or near its resonance size,2 the near adiabatic process results in localized heating,3 and consequently, through thermal mechanisms, the emission of light ensues, termed sonoluminescence (SL). The intensity of SL is related to both the maximum temperature of the bubble reached during collapse and also the number of active bubbles that exist in the reactor. As the efficiency of most sonochemical processes largely depends on a combination of these two factors, SL can serve as an effective indicator of cavitation activity of an ultrasound system. The effect of various parameters and conditions on the SL intensity has been extensively investigated under singlefrequency ultrasound irradiation. Recently, several research groups have begun to explore the possibilities associated with the combination of two or more ultrasound sources. Since the first multiple-frequency work of Dmitrieva and Margulis4 in 1985, a wide range of multiple-frequency studies has been conducted concerning sonochemistry,4-10 numerical modeling of bubble dynamics,11-17 and sonoluminescence.15,18-21 The combination of multiple frequencies, under appropriate operating conditions, is believed to enhance the overall cavitation activity of the system by affecting the cavitation bubble temperature and/or the active cavitation bubble population. The effect of various solutes, e.g., alcohols, surfactants, polymers, and other organic solutes, has been previously investigated for single-frequency sonication.22-27 It has been reported that certain solutes can greatly influence the SL generated in aqueous solutions, making their presence an important consideration in experimental designs. The mechanism by which solutes may affect sonoluminescence intensity relates to both inter- and intrabubble effects. First, bubble coalescence creates large bubbles that either move to acoustic pressure * To whom correspondence should be addressed. E-mail: franz@ unimelb.edu.au.
nodes28 or rise to the solution surface, may be affected by the presence of solutes. In addition, the formation of dense bubble clouds, which can shield the interior placed bubbles from the acoustic field, may be influenced. Changes in both coalescence and clustering translate to changes in the active (meaning SL producing) bubble population. Second, volatile solutes may evaporate into bubbles, thereby reducing the energy transferred to heating the bubble during collapse, and consequently decreasing the SL intensity. In the present work, the effect of acoustic power on dualfrequency SL in water is investigated and then subsequently extended to aqueous solutions of various solutes. Through the implementation of a multiple-frequency ultrasound reactor with certain solutes, the chemical reaction efficiency may be greatly enhanced. To the best of our knowledge, this is the first such study concerning the effects of solutes on cavitation, specifically, SL, under multiple-frequency conditions. Experimental Section Sodium dodecyl sulfate (SDS), special purity grade, and alcohols, ethanol and butanol, analytical reagent grade, were supplied by BDH. Sodium chloride was supplied by Ajax Chemicals. Polyethylene oxide (PEO) with weight average molecular weight ≈ 200 000 g/mol was supplied by Aldrich. All solutes were used as received, and solutions were prepared using Milli-Q purified water. A Branson B-30 Cell Disrupter was modified to enable 20kHz ultrasound to be delivered in a pulsed mode through a Branson 45-mm diameter titanium alloy horn. The pulse width and time between pulses were 3.5 and 12 ms, respectively. These conditions were found to be the most useful for examining the SL emission behavior under the conditions selected. Continuousmode high-frequency ultrasound was generated by an ELAC RF generator and delivered through an Allied Signal transducer (plate diameter of 55 mm) with a Pyrex cell attached. The horn was inserted into the cell from above, vertically opposing the high-frequency transducer, with a separation distance of about 6.5 cm. It was found through an initial investigation using distilled water that the optimum power for dual-frequency
10.1021/jp067524r CCC: $37.00 © 2007 American Chemical Society Published on Web 01/27/2007
SL under Dual-Frequency Sonication
J. Phys. Chem. C, Vol. 111, No. 7, 2007 3067
Figure 2. SL pulse profiles from solutions of different solutes at low concentration under dual-frequency sonication under 20/355 (left) and 20/1056 kHz (right) conditions. Output power: 355 kHz/1056 kHz ≈ 4.5 W.
Figure 1. SL signals from 20, 355, and 20/355 kHz when the 355kHz output power is (a) 4.5 ( 0.5 and (b) 18 ( 3 W.
sonication for this setup was 6 ( 1 W for 20 kHz and 4-5 W for the high frequency, as determined by calorimetry. These conditions were used for all experiments, unless otherwise specified. A Hamamatsu photomultiplier tube, placed approximately halfway between the two transducers, was used to detect the SL emission, which was subsequently displayed on a Tektronix digital oscilloscope. The SL signal recorded for each of the experiments examining the effects of the solutes was taken as the ratio of the difference between the dual-frequency signal and the sum of the individual frequency signals to the dualfrequency signal in water under the same acoustic power, i.e., [Idual solute - (I20-kHz solute + I355/1056-kHz solute)]/Idual water. Results Parts a and b of Figure 1show the SL signals in water under 20 and 355 kHz, operating both independently and simultaneously. In Figure 1a, the 355 kHz power is 4.5 ( 0.5 W, and in Figure 1b, it is 18 ( 3 W. The 20-kHz power was kept constant. It can be seen that when the 355-kHz power is relatively low, at 4.5 ( 0.5 W, the dual-frequency mode SL is enhanced synergistically with respect to the individual SL signals. In fact, both the individual frequencies generate almost no detectable SL when acting alone. When the power of the 355 kHz is increased (Figure 1b), the opposite effect is observed. A large individual SL signal from the 355 kHz is attenuated, by about 80%, for the duration of the 20-kHz pulse. The same trend can also be observed for the 20/1056-kHz combination (data not shown). A power dependence of the dual-frequency synergy effect has been reported by Dezhkunov,21 although under somewhat different conditions to those of this study. The resultant SL emission pulse profiles from the superimposition of the 20-kHz pulse and constant high-frequency irradiation is presented in Figure 2 for the different solute systems at given concentrations, under both dual-frequency combinations, 20/355 and 20/1056 kHz. The high-frequency power was kept low (4-5W), such that the synergistic effect was maximized. It is clear, from this figure, that the SL intensity is enhanced relative to water, in the presence of these solutes at low concentration. It is also evident from the pulse profiles that the solutes have an influence on whether a steady-state level of SL generating
Figure 3. Relative SL intensity as a function of SDS concentration in the absence and presence of sodium chloride under 20/355 and 20/ 1056 kHz conditions. Output power: 355 kHz/1056 kHz ≈ 4.5 W.
bubbles has been reached during the 20-kHz pulse interval. It can be seen that in the case of water, under both frequency combinations, the SL signal is still rising at the point the pulse is turned off (corresponding to the point where there is a rapid decline in the SL signal), indicating that a steady-state signal has not been reached. In the solute systems SDS, PEO, and butanol, under 20/355 kHz the SL signal can be seen to plateau, reflecting a steady-state condition. However, in the case of SDS + NaCl and also in all 5 solutions under 20/1056 kHz, no steady-state signal is reached within the 3.5-ms pulse. The SL intensity with SDS in the presence and absence of electrolyte, sodium chloride, is shown in Figure 3. With increasing SDS concentration, the SL intensity can be seen to initially increase, then at a higher concentration, above ∼1 mM, decrease. Furthermore, in the presence of the electrolyte, the SL intensity is reduced to a level only about 1.5 times that of water. These trends are largely consistent with those previously obtained under single-frequency sonication.22,29 Figure 4 shows the dual-frequency SL intensity in the presence of polymer, PEO. It can be seen that the intensity, over the range of concentrations studied, which was partially restricted by the solubility of the polymer, increases steadily for both frequency combinations. The relative increase in SL, however, is more pronounced for the 20/355-kHz system. The effect of alcohol on SL was studied with both ethanol and n-butanol, shown in parts a and b of Figure 5, respectively. At low concentration, both alcohols, under both frequency combinations, cause a large enhancement in SL as compared to water. As the bulk alcohol concentration increases, the SL intensity reaches a maximum, then subsequently declines, eventually reaching a value below that of water. The trend, while
3068 J. Phys. Chem. C, Vol. 111, No. 7, 2007
Figure 4. Relative SL intensity as a function of PEO concentration for 20/355 and 20/1056 kHz conditions. Output power: 355 kHz/1056 kHz ≈ 4.5 W.
Figure 5. Relative SL intensity as a function of (a) ethanol and (b) n-butanol concentrations under 20/355 and 20/1056 kHz conditions. Output power: 355 kHz/1056 kHz ≈ 4.5 W.
the same for both alcohols, takes effect at lower concentration for the longer alkyl chain length alcohol, n-butanol. Discussion The data shown in Figure 1 clearly indicate that, for the particular dual-frequency arrangement used for these experiments, the power output of the high frequency is crucial, with low power giving a synergistic enhancement in SL, while high power causes an attenuation effect. As previously mentioned, an increase in SL intensity can be attributed to an increase in the core temperature reached within the bubbles and/or the number of bubbles collapsing to generate SL. Several theoretical studies have indicated that a single bubble collapsing in a dualfrequency sound field will reach a much greater maximum collapse temperature, Tmax, relative to the single-frequency system, as a result of an increase in the ratio R/Ro, during the expansion phase prior to collapse, where R is the bubble radius and Ro is the equilibrium bubble radius.13,15,16 A numerical
Brotchie et al. model using the experimental parameters of the present experiment also predicts an increase in Tmax, at both high and low acoustic power.30 This data has not been included here as it is only strictly applicable to a single-bubble system with a phase difference of zero between the two frequencies. It is relevant to note that Lohse and co-workers31 theoretically predicted a harmonic enhancement of SBSL in a dual-frequency system. Their calculations showed that the maximum bubble core temperatures could be increased in dual frequency systems under specific experimental conditions. If it is to be assumed that the bubble core temperature does not decrease during dual-frequency sonication (note that Lohse and co-workers have predicted that the maximum bubble temperature may decrease for certain phase differences between two applied frequencies;31 however, a zero phase difference, assumed in this study, does not decrease the bubble temperature under the dual frequency sonication used in this study), then at least part of the observed trends with power must be governed by changes in the active bubble population. At this stage, it is difficult to isolate the factor that contributes most to the enhancement at low power. While the collapse temperature is predicted to rise, it is equally possible that the effects observed arise from an increase in bubble population. It should be noted that the acoustic powers used in the lower power system are very low, only slightly above the cavitation threshold. Gilles et al.32 have noted that the experimental cavitation threshold under dual-frequency irradiation is lower, providing an avenue for an increase in the bubble population. Under higher power, well above the cavitation threshold, when the bubble density is much greater, the change in the bubble population during dualfrequency sonication may be unfavorable. Dezhkunov21 has reported similar findings under the reversed conditions, with high-intensity low frequency and low-intensity high-frequency ultrasound. The attenuation effect observed was argued to result from the bubble density exceeding an optimum level for SL generation. A high-density bubble field may enhance processes detrimental to SL, such as bubble coalescence, and may also cause an acoustic impedance effect.28 The results observed for the case of SDS, shown in Figure 3, can be explained relatively simply. SDS, by nature, is a surface-active species, which fully dissociates in water to negatively charged surfactant and Na+ ions. From studies conducted under single-bubble conditions, any intrabubble effects due to SDS may be neglected.33 Therefore, the results may be explained purely with respect to interbubble effects. At low concentration, the SDS molecules adsorbed to the bubble gas/water interface create a negative interface and an electrostatic barrier to bubble coalescence and/or close bubble clustering, thus inhibiting processes by which the number of active bubbles can be expected to decrease. An increase in the number of bubbles capable of SL emission in this case translates to a greater net SL signal. The electrostatic barrier may be overcome by electrostatic screening provided by either the presence of the electrolyte, sodium chloride, or by SDS itself in higher concentration. Such behavior is typical for SDS in multibubble systems.22,23,29 The residual enhancement of the SL signal (about 1.5 times that of water) in the high ionic strength solutions indicates that electrostatics alone are not solely responsible for this effect. It is known that nonionic, surface-active solutes can retard bubble coalescence through nonelectrostatic mechanisms. Bulky head groups may impart steric hindrance between bubbles, while solutes adsorbed at the bubble/solution interface will slow the rate of film draining due to a concentration gradient of the solute as the film thins prior to coalescence.34 However,
SL under Dual-Frequency Sonication single-frequency studies indicated that for the case of SDS, these nonelectrostatic effects were ineffectual in causing an increase in SL, with the SL from SDS and salt solution returning to the same level as in pure water.29 It can be concluded that, in the present system, these nonelectrostatic mechanisms are, comparatively, of greater importance. It can also be observed that under 20/355-kHz conditions, the SL emission intensity decreases at high SDS concentration to the same level as observed with NaCl present, while under 20/1056 kHz, this limit is not reached. The SL intensity in the presence of PEO, which is shown in Figure 4, can be seen to steadily increase with increasing PEO concentration. As with the case of SDS, it has been shown, in a single-bubble system, that the presence of polymer does not affect single-bubble dynamics or SL,33 indicating that the effects observed in the multibubble system are interbubble in nature. It is proposed that the presence of polymer in bulk solution leads to less dense bubble clustering and reduced bubble coalescence, thereby increasing the active bubble population. As PEO is only slightly surface-active, this mechanism can be assumed to predominantly take place in the bulk solution. In this system, the frequency combination 20/355 kHz shows a greater relative SL increase as compared to 20/ 1056 kHz. The cause of this frequency effect is not clearly understood. The effect of alcohol on SL, shown in parts a and b of Figure 5 is seen to enhance the dual-frequency SL signal, relative to water at low concentration and quench it at higher concentration. In this study, we have used both ethanol and n-butanol in order to observe any influence of alkyl chain length, and in effect the hydrophobicity of the solute. From Figure 5, it can be seen that although frequency influences the maximum SL signal attained, there is little difference between the two alcohols, with respect to the maximum SL intensity reached. As expected, due to its greater surface activity, the effects of butanol occur at lower concentrations compared with ethanol. As seen with PEO, the relative enhancement for 20/355 kHz is greater than for 20/ 1056 kHz. One possibility that is supported by the pulse profiles shown in Figure 2, is that the nuclei from 1056 kHz, owing to their smaller size, are not permitted sufficient time during the 3.5-s 20-kHz pulse to reach a steady state intensity. However, considering that this was not apparent in the case of PEO, it is likely that another mechanism is responsible for the differences observed. In their study using 358 kHz, Sunartio et al.35 observed both enhancement, at low concentration, and quenching at high concentration for a variety of alcohols. Price et al.36 for 20 kHz observed only enhancement, with a plateau in the SL intensity at high alcohol concentration. The difference in behavior at the different frequencies has been explained with to the respect to the frequency dependence of the nature of cavitation. Generally speaking, bubbles under the influence of low-frequency ultrasound (i.e., 20 kHz), undergo transient caVitation, collapsing after only a few acoustic cycles. At higher frequencies (i.e., 355 kHz), bubbles will exist under stable caVitation conditions, oscillating for many acoustic cycles prior to collapse.28,36,37 Alcohol can have two opposing effects on a cavitation bubble: the inhibition of coalescence and dense clustering through its adsorption at the bubble interface34,38 and the lowering of bubble collapse temperature, through its evaporation into the core of the cavitation bubble. The volatility of alcohol permits evaporation during the expansion of the oscillating bubble. Upon bubble collapse, energy will be consumed in the thermal decomposition of the alcohol. Furthermore, decomposition products may accumulate in the bubble, increasing the gas pressure and
J. Phys. Chem. C, Vol. 111, No. 7, 2007 3069 providing more material that can partake in endothermic processes, also acting to lower the maximum temperature reached. The inhibition of coalescence/dense clustering and reduction of collapse temperature serve to enhance and quench SL, respectively. In the case of a transient bubble, at low frequency, the quenching effect of alcohol is not prominent owing to the fact that the bubbles do not exist long enough to accumulate sufficient alcohol and its hydrophobic decomposition products. At higher frequency, stable bubbles exist long enough to permit the significant evaporation and pyrolysis of alcohol and the subsequent accumulation of decomposition products. Thus at high frequency, SL quenching competes with enhancement, becoming more dominant as the alcohol concentration increases. In the present experiment, combining high and low frequencies, it is proposed that the nuclei generated from the continuous high-frequency ultrasound are acted upon by the low-frequency pulse. The SL observed is thought to result from largely transient bubbles of which the nuclei are stable high-frequency bubbles, which have already accumulated alcohol prior to the lowfrequency pulse. In addition, the SL enhancement relative to pure water, obtained using ethanol and butanol, is significantly larger for this system compared to that observed in previous single frequency studies. A possible explanation for this lies in the acoustic power delivered. Sunartio et al.35 have demonstrated that the enhancement is most prevalent at lower acoustic powers. The relatively low power used in the dual-frequency system may favor this enhancement. Conclusions The SL resulting from a dual-frequency ultrasonic system has been found to be enhanced synergistically when the power of the higher frequency is low and attenuated when the power is high, relative to SL from the individual frequencies. The enhancement at low power is attributed to a greater bubble core temperature and possibly an increased bubble population when operating in the dual-frequency mode. The attenuation observed at higher power is proposed to result from a reduction in the active bubble population, due to a dense bubble field, facilitating processes such as bubble coalescence and/or acoustic shielding. The effect of dual-frequency ultrasound on the SL intensity of aqueous solutions containing low concentrations of different solutes has been investigated. All three classes of solutes used were shown to further enhance the SL intensity under the dualfrequency conditions that already provide a synergistic effect over single-frequency sonication. Acknowledgment. Financial support from an Australian Research Council Discovery Project grant is acknowledged. A.B. receives an Australian Postgraduate Award and a postgraduate stipend from the University of Melbourne, which are also gratefully acknowledged. References and Notes (1) Young, F. R. CaVitation; Imperial College Press: London, 1999. (2) Yasui, K. J. Acoust. Soc. Am. 2002, 112, 1405. (3) Ciawi, E.; Rae, J.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2006, 110, 13656. (4) Dmitrieva, A. F.; Margulis, M. A. Russ. J. Phys. Chem. 1985, 59, 1569. (5) Gogate, P. R.; Mujumdar, S.; Pandit, A. B. AdV. EnViron. Res. 2003, 7, 283. (6) Swamy, K. M.; Narayana, K. L. Ultrasonics Sonochem. 2001, 8, 341. (7) Hua, I.; Hochemer, R. H.; Hoffmann, M. R. EnViron. Sci. Technol. 1995, 29, 2790.
3070 J. Phys. Chem. C, Vol. 111, No. 7, 2007 (8) Zhu, C.; Feng, R.; Chen, Z.; Li, L.; Ruan, Y.; Wei, L. Journal of Nanjing UniVersity Natural Sciences Edition 1998, 34, 93. (9) Sivakumar, M.; Pandit, A. B. Ultrasonics Sonochem. 2001, 8, 233. (10) Gogate, P. R.; Shirgaonkar, I. Z.; Sivakumar, M.; Senthilkumar, P.; Vichare, N. P.; Pandit, A. B. AICHe J. 2001, 47, 2526. (11) Sivakumar, M.; Tatake, P.; Pandit, A. B. Chem. Eng. J. 2002, 85, 327. (12) Servant, G.; Laborde, J. L.; Hita, A.; Caltagirone, J. P.; Gerard, A. Ultrasonics Sonochem. 2003, 10, 347. (13) Tatake, P.; Pandit, A. B. Chem. Eng. Sci. 2002, 57, 4987. (14) Moholkar, V. S.; Rekveld, S.; Warmoeskerken, M. M. C. G. Ultrasonics 2000, 38, 666. (15) Holzfuss, J.; Ruggerberg, M.; Mettin, R. Phys. ReV. Lett. 1998, 81, 1961. (16) Prabhu, A. V.; Gogate, P. R.; Pandit, A. B. Chem. Eng. Sci. 2004, 59, 4991. (17) Ketterling, J. A.; Apfel, R. E. J. Acoust. Soc. Am. 2000, 107, 819. (18) Ciuti, P.; Dezhkunov, N. V.; Francescutto, A.; Kulak, A. I.; Iernetti, G. Ultrasonics Sonochem. 2000, 7, 213. (19) Carpenedo, L.; Ciuti, P.; Francescutto, A.; Iernetti, G.; Johri, G. K. Acoust. Lett. 1987, 10, 178. (20) Iernetti, G.; Ciuti, P.; Dezhkunov, N. V.; Reali, M.; Francescutto, A.; Johri, G. K. Ultrasonics Sonochem. 1997, 4, 263. (21) Dezhkunov, N. V. J. Eng. Phys. Thermophys. 2003, 76, 142. (22) Ashokkumar, M.; Hall, R.; Mulvaney, P.; Grieser, F. J. Phys. Chem. B 1997, 101, 10845. (23) Grieser, F.; Ashokkumar, M. AdV. Colloid Interface Sci. 2001, 8990, 423. (24) Tronson, R.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2003, 107, 7307.
Brotchie et al. (25) Toegel, R.; Hilgengeldt, S.; Lohse, D. Phys. ReV. Lett. 2000, 84, 2509. (26) Toegel, R.; Gompf, B.; Pecha, R.; Lohse, D. Phys. ReV. Lett. 2000, 85, 3165. (27) Brenner, M. P.; Hilgengeldt, S.; Lohse, D. ReV. Mod. Phys. 2002, 74, 425. (28) Leighton, T. G. The Acoustic Bubble; Academic Press Inc.: London, 1994. (29) Tronson, R.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2002, 106, 11064. (30) Kenthal, P.; Brotchie, A.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. C. Submitted. (31) Lu, X.; Prosperetti, A.; Toegel, R.; Lohse, D. Phys. ReV. E 2003, 67, 056310. (32) Gilles, B.; Bera, J. C.; Mestas, J. L.; Cathignol, D. Appl. Phys. Lett. 2006, 89, 941061. (33) Ashokkumar, M.; Guan, J.; Tronson, R.; Matula, T. J.; Nuske, J. W.; Grieser, F. Phys. ReV. E 2002, 65, 463101. (34) Oolman, T. O.; Blanch, H. W. Chem. Eng. Commun. 1986, 43, 237. (35) Sunartio, D.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2005, 109, 20044. (36) Price, G. J.; Ashokkumar, M.; Grieser, F. J. Am. Chem. Soc. 2004, 126, 2755. (37) Price, G. J.; Ashokkumar, M.; Hodnett, M.; Zequiri, B.; Grieser, F. J. Phys. Chem. B 2005, 109, 17799. (38) Lee, J.; Kentish, S. E.; Ashokkumar, M. J. Phys. Chem. B 2005, 109, 5095.
