16860
J. Phys. Chem. B 2005, 109, 16860-16865
Effect of Surfactants on Inertial Cavitation Activity in a Pulsed Acoustic Field Judy Lee,† Sandra Kentish,† Thomas J. Matula,§ and Muthupandian Ashokkumar*,‡ Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, School of Chemistry, UniVersity of Melbourne, 3010 ParkVille, Victoria, Australia, and Applied Physics Laboratory, UniVersity of Washington, 1013 NE 40th Street, Seattle, Washington 98015 ReceiVed: June 20, 2005; In Final Form: July 12, 2005
It has previously been reported that the addition of low concentrations of ionic surfactants enhances the steady-state sonoluminescence (SL) intensity relative to water (Ashokkumar; et al. J. Phys. Chem. B 1997, 101, 10845). In the current study, both sonoluminescence and passive cavitation detection (PCD) were used to examine the acoustic cavitation field generated at different acoustic pulse lengths in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). A decrease in the SL intensity was observed in the presence of low concentrations of SDS and short acoustic pulse lengths. Under these conditions, the inhibition of bubble coalescence by SDS leads to a population of smaller bubbles, which dissolve during the pulse “off time”. As the concentration of surfactant was increased at this pulse length, an increase in the acoustic cavitation activity was observed. This increase is partly attributed to enhanced growth rate of the bubbles by rectified diffusion. Conversely, at long pulse lengths acoustic cavitation activity was enhanced at low SDS concentrations as a larger number of the smaller bubbles could survive the pulse “off time”. The effect of reduced acoustic shielding and an increase in the “active” bubble population due to electrostatic repulsion between bubbles are also significant in this case. Finally, as the surfactant concentration was increased further, the effect of electrostatic induced impedance shielding or reclustering dominates, resulting in a decrease in the SL intensity.
Introduction Acoustic cavitation describes the activity of bubbles in a liquid in the presence of an acoustic field. Upon the collapse of these bubbles, high temperatures, pressures, and other forces, such as microjetting, turbulence, acoustic streaming, etc., are generated.1-4 These physical effects are implicated as mechanisms for ultrasonic cleaning,5 damage to hydraulic mechanical systems,5 and beneficial or detrimental bioeffects in medical ultrasound applications.6-11 The extreme conditions within these cavitating bubbles also make them ideal microreactors for chemical reactions.12-15 It is therefore important to understand how acoustic cavitation fields develop in an acoustic field, how they interact, and how to control them for various applications. In many ultrasonic applications, surface active materials are usually present in the solutions. The surface active properties of surfactants can be expected to affect the bubble-bubble interactions within an acoustic cavitation field. Therefore, an understanding of the effect of surfactants on the acoustic cavitation field is important. In this study, sodium dodecyl sulfate (SDS), an anionic surfactant, is used. It is a commonly used surfactant, and its surface properties are well characterized. SDS, along with other surface active solutes, has been used in a number of other multibubble and single bubble sonoluminescence (SL) experiments,16-19 and it appears that in most cases the effect of surfactants on SL seems to be independent of the nature of the surfactant. For bubbles undergoing inertial cavitation (IC) activity (violent collapses), useful probes of the acoustic cavitation field * To whom correspondence should be addressed. E-mail: masho@ unimelb.edu.au. Fax: +61 3 9347 5180. † Department of Chemical and Biomolecular Engineering. ‡ School of Chemistry, University of Melbourne. § Applied Physics Laboratory, University of Washington.
include SL16-18,20-24 and passive cavitation detection (PCD).25-29 SL refers to the light emission generated within a violently collapsing bubble.3,4 PCD refers to the detection of sound emitted from violently collapsing bubbles. With this second method, a quantitative measure of the IC activity, that is, the measure of the number of bubbles undergoing inertial collapse, can be experimentally measured. This is defined as the IC dose.28,29 Ashokkumar et al.16 and Tronson et al.17 have used SL to monitor the effect of ionic surfactants on acoustic cavitation activity. They have shown that the addition of low concentrations of SDS enhances SL intensity in water. A strong correlation between this increase in SL intensity and the acoustic cavitation noise has also been reported by Segebarth et al.18 They examined the effect of SDS on the width of the second harmonics of the acoustical signal. The enhancement of the SL intensity with SDS was found to correspond to a decrease in the width of the second harmonics. This decrease in the width suggests a narrower size distribution, which they attributed to the inhibition in bubble coalescence by SDS. The prevention of smaller bubbles from coalescing to form larger bubbles has the effect of increasing the number of bubbles available for IC activity, which supports the observed increase in SL intensity. While Segebarth et al.18 used a continuous ultrasound to generate acoustic cavitation activity, Ashokkumar et al.16 and Tronson et al.17 used a series of acoustic pulses of fixed length. However, the effect of acoustic pulse length on SL intensity for SDS/water system has not been investigated. The effect of pulse length on the IC activity of ultrasound contrast agents has been reported in the literature. Using the PCD method, Chen et al.29 reported a pulse length dependence of IC activity in the presence of gas-based ultrasound contrast agents. They found that an increase in acoustic cavitation activity is associated with
10.1021/jp0533271 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/11/2005
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Figure 1. Passive cavitation detection (PCD) system.
an increase in either the pulse length or pulse repetition frequency (PRF). This increase in the IC dose was explained in terms of a “cascade” effect where bubbles generated in long acoustic pulses survive through the shorter pulse intervals and thus act as further cavitation nuclei for the subsequent pulse. Henglein et al.23,30 have similarly investigated the effect of pulsing on the initial SL (chemiluminescence) growth and the steady-state intensity reached for luminol solutions. The number of acoustic pulses required to reach a stable SL intensity level is affected by the pulse length and the PRF. In this paper, the effect of pulsed ultrasound on acoustic cavitation activity in surfactant solutions is considered. In particular, the effects of ultrasound pulse length, frequency, and power on acoustic cavitation activity are examined and related to bubble behavior. Acoustic cavitation activity is measured through both IC dose and SL intensity. Experimental Details SDS special purity grade and sodium chloride (99.9%) were used as purchased from BDH. Deionized water with a conductivity of less than 10-6 S cm-1 at 20 °C was left overnight to allow for equilibrium air-saturation to be reached before use. The temperature was not controlled but monitored. The temperature did not increase by more than 2 °C during the course of the experiment. The two different acoustic cavitation systems for measuring IC dose and SL data are as follows. Passive Cavitation Detection (PCD). The experimental arrangement, similar to that used by Chen et al.28 and Porter et al.31, is shown in Figure 1. It consisted of a 12.5 cm × 12.5 cm × 16.5 cm acrylic tank with a 1 MHz high-intensity focused ultrasound (HIFU) transducer located such that the focal zone intersects with the focal zone of a 5 MHz hydrophone located in quadrature with the HIFU transducer. The 1 MHz transducer has an aperture of 70 mm and a focal length of 62.6 mm. The 5 MHz transducer has an aperture of 12 mm and a focal length of 46 mm. The HIFU transducer was controlled by a function generator (33120A Hewlett-Packard) whose signal was amplified with a power amplifier (400B, ENI Amplifier) set at the maximum output. The 5 MHz hydrophone acted as a passive detector for any acoustic signals. These signals were collected by an oscilloscope (LeCroy 9384L) that was set to trigger by the HIFU function generator. For each experiment, a volume of 1.1 L of solution was used. This volume ensured that the
solution fully covered the HIFU transducer and the hydrophone. These solutions were made using deionized, filtered water. Bubbles were nucleated using the 1 MHz HIFU transducer by setting the pressure amplitude sufficiently high and the pulse length at 100 cycles. The pressure amplitude was adjusted by adjusting the voltage output from the function generator to the HIFU transducer. These conditions gave intense inertial cavitation activity in water, whose acoustic emissions were easily measurable by the hydrophone. A window of 100 µs with a 70 µs delay (to account for the delay caused by the traveling time of the sound wave from the HIFU transducer to the hydrophone) captured the acoustic signals generated by the acoustic cavitation activity. The sequence mode on the digital oscilloscope was employed to capture 200 signals (or events) triggered by the HIFU. However, the oscilloscope was set to sample every 10th triggered event rather than every successive event. There does not appear to be any periodicity in the inertial cavitation activities; therefore triggering every 10th event does not bias the observed results. Every 10th event was necessary because at certain surfactant concentrations the inertial cavitation activity was very low and 200 successive events were not sufficient to capture the full inertial cavitation activity without compromising the sampling resolution of 20 ns per point. The IC dose was calculated in a similar method as that adopted by Chen et al.28 The signals captured were converted to the frequency domain using fast Fourier transforms (FFT). The broadband noise between the harmonic peaks signifies the level of IC activity,28,29 and the amplitude of this broadband noise at a frequency between 2.6 and 2.8 MHz was measured for all 200 events. The amplitudes were then plotted as a function of time, and the IC dose, in units of V s, was calculated as the area under the amplitude-time curve. Further details can be found in the papers by Chen et al.28,29 Sonoluminescence. The setup used by Tronson et al.17 was adopted in this investigation for measuring the SL intensity as illustrated in Figure 2. A cylindrical Pyrex cell, filled with 50 mL of solution, was mounted over a 35 mm, 515 kHz flat plate unfocused transducer. The transducer was connected to a 515 kHz Undatim Ultrasonics D-reactor generator with an adjustable power setting between 0 and 100 W. The generator was modified in-house to allow for pulsed energy delivery, where both pulse length and pulse repetition frequency (PRF) could be varied. For this work, the off time of the acoustic pulse train was fixed at 12 ms and the on time varied between 0.5 and 4 ms (258-2060 cycles). This gave a PRF of between 80 and
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Figure 2. Sonoluminescence experimental setup.
62.5 Hz. For SL measurements at 1056 kHz, an Allied Signal (ELAC Nautic) RF generator and a transducer with a plate diameter of 54.5 mm were used. A Hamamatsu end-on photomultiplier tube (model no. R64704) was used to record the SL intensity, and the output from the photomultiplier tube was fed to a digital oscilloscope (Tektronix, model no. TDS 320). The oscilloscope, triggered by the ultrasonic generator, recorded the sequence of signals at a set time interval. The data were then displayed on the oscilloscope and saved on a computer. The cell and the photomultiplier tube were both housed in a dark enclosure to minimize background light. The steady-state SL intensity was recorded by first presonicating the solution for 2 min. This presonication allowed reproducible results to be obtained. The solution was then allowed to stand for 5 min for any bubbles formed within the 2 min to dissolve away. To obtain the maximum steady-state SL intensity, the solution was then sonicated for at least 1 min until a steady SL intensity was reached. The averaging function of the oscilloscope was then used to record an average of at least 200 pulses. Results and Discussion The IC dose as a function of SDS concentration at two PRFs is plotted in Figure 3, which shows that the acoustic cavitation activity in water is generally higher at a PRF of 60 Hz than at 30 Hz, consistent with the work previously reported by Chen et al.29 Of greater interest is that both sets of results show a decrease in acoustic cavitation activity upon addition of small quantities of SDS. The extent of this decrease is influenced by the PRF with a much sharper decrease evident at 30 Hz than at 60 Hz. The acoustic cavitation activity passes through a minimum at around 0.5 mM and then increases again. The location of the minimum appears to be independent of the PRF (Figure 3). With further increases in surfactant concentrations the acoustic cavitation activity increases again, although this effect is less obvious for a PRF of 60 Hz. At this higher PRF,
Figure 3. IC dose as a function of SDS concentrations at pulse repetition frequencies (PRF) of 60 and 30 Hz. The dashed and dotted lines represent the IC dose for water at 60 and 30 Hz, respectively. These measurements were conducted using a 1 MHz HIFU transducer set at a pulse length of 100 cycles.
the IC dose appears to plateau at the same level as that of water. However, for a PRF of 30 Hz the plateau reached is much higher than that of water. These results seem to contradict the SL results of Ashokkumar et al.16 As mentioned earlier, both IC dose and SL report on the extent of acoustic cavitation activity. While the data shown in Figure 3 show a decrease in acoustic cavitation activity at low SDS concentrations, Ashokkumar et al.16 have shown an increase in SL at low SDS concentrations. This apparent discrepancy may lie in the differences in the experimental conditions used, that is, frequency and pulse length. To identify if a change in the ultrasonic frequency was responsible for the observed difference between the IC dose data and SL data,16 SL experiments were performed at 1056 kHz and compared with 515 kHz SL data. As can be seen from Figure 4, the increase in frequency does not cause any significant change in SL behavior. There is still an increase in SL intensity at low SDS concentrations. Indeed, the effect of SDS on SL
Surfactants on Inertial Cavitation Activity
Figure 4. Relative sonoluminescence intensity as a function of SDS concentrations for two frequencies of 515 kHz (at acoustic power of 12.3 and 25.0 W) and 1056 kHz (acoustic power of 25.0 W). The dashed line represents the water value.
Figure 5. Relative sonoluminescence intensity as a function of SDS concentrations at various pulse lengths. The frequency and power were fixed at 515 kHz and 12.3 W, respectively. The PRF ranges from 62.5 to 76.9. The dashed line represents the water value.
intensity at these two completely different frequencies seems to be similar. This led us to study the effect of varying the acoustic pulse length on the SL trend in SDS solutions. As shown in Figure 5, at long pulse lengths (1236 and 2060 cycles) the addition of SDS causes the SL intensity to increase to a maximum followed by a gradual decrease at high concentrations. This behavior is consistent with previous work.16-18,32 However, at the shorter pulse lengths of 515 and 824 cycles, a reverse effect is observed. The SL initially decreases with surfactant concentration, before increasing again. It should be noted that the zero point shown in Figure 5 may not indicate that there is no emission of light but that the light emitted is below the sensitivity of the photomultiplier tube. Further evidence for a pulse length effect is shown in Figure 6. Consider the comparison between water and 1.5 mM SDS. At very short pulse lengths, the SL intensity is quite low and difficult to detect for both solutions. As the pulse length increases, the SL intensity from water begins to increase after about 200 cycles, reaching a saturation level after about 1000 acoustic cycles. With the SDS solution however, the rise in SL intensity does not begin until the pulse length reaches about 700 acoustic cycles and then the intensity rises significantly. The addition of 10 mM SDS or 0.1 M NaCl results in SL intensities the same as that for water. For clarity, the 10 mM data is not shown in Figure 6. To confirm that the acoustic power is not responsible for the observed difference between the IC dose and SL data,16 SL experiments were performed at a low number of acoustic cycles
J. Phys. Chem. B, Vol. 109, No. 35, 2005 16863
Figure 6. Relative sonoluminescence intensity as a function of pulse length for different solutions. The frequency, PRF, and power were fixed at 515 kHz, 62.5 Hz, and 12.3 W, respectively.
Figure 7. Effect of SDS concentration and power on the relative sonoluminescence intensity. A 515 kHz transducer set at a pulse length of 250 cycles and a PRF of 80 Hz was used. The dashed line represents the water value.
with varying acoustic power. The results provided in Figure 7 show that at low SDS concentrations, increasing power has little effect on the relative SL intensity. However, at SDS concentrations above 1 mM, the relative SL intensity increases with an increase in acoustic power. This is consistent with literature data where an increase in SL intensity was observed with an increase in the acoustic power.22,33 The data in Figure 7 also shows that for power levels higher than 80 W the maximum SL intensity rises above that of water. At SDS concentrations above 5 mM the SL intensity declines again. The results shown in Figures 3-7 clearly demonstrate that the effect of surfactants on inertial cavitation activity (both IC dose and SL) depends on the acoustic pulse length. The key effects observed are (i) a decrease in acoustic cavitation activity at low SDS concentrations and short acoustic pulse lengths, (ii) an increase in acoustic cavitation activity at low SDS concentrations and long acoustic pulse lengths, (iii) an increase in SL between 1.5 and 5 mM SDS and short pulse lengths, and (iv) a decrease in acoustic cavitation activity at high SDS concentrations independent of the acoustic pulse length. It is well established that surface active solutes adsorb to the air/water interface of bubbles and decrease surface tension.34 Different surface active solutes have different surface activity, indicated by the equilibrium critical micelle concentration (cmc).34 For SDS, the equilibrium cmc is approximately 8 mM.35 However, it has been reported in the literature that there appears to be no correlation between the equilibrium surface tension and that of the SL results for ionic surfactants.16 This suggests that the equilibrium surface tension is not responsible for the observed SL results. The adsorption of surface active solutes
16864 J. Phys. Chem. B, Vol. 109, No. 35, 2005 at the interface can also lead to the inhibition of bubble coalescence.36-39 For charged surfactants, electrostatic repulsion between the headgroups on the surfactants also prevents the bubbles from coalescing. It is this electrostatic repulsion that is the dominant cause of reduced coalescence.18 Lee et al.40 have reported that different ionic surfactants of different surface activity show a similar degree of bubble coalescence in the presence of ultrasound. This coalescence inhibition will result in an increase in the number of bubbles but a reduction in their average size. It is likely to also result in a narrower size distribution than might be expected when bubble coalescence occurs.18 Within the ultrasonic field bubbles will grow by coalescence or rectified diffusion until they reach the resonance or active size (Rr), a size at which inertial cavitation occurs most violently.3,4 It should be noted that this resonance size is not a single size but a range of bubble radii.41 As explained by Grieser and Ashokkumar16,32 smaller bubbles will require a greater number of acoustic cycles to reach Rr. When the pulse length is short, many bubbles will not reach Rr during a single pulse. Further, if they are less than a certain size (Rd), then they will dissolve away during the “off time” of the pulse cycle. In the presence of low concentrations of SDS, the proportion of bubbles below Rd is greater than that in water, resulting in a lower population of bubbles reaching Rr. This decrease in the active bubble population is the reason for the low IC dose and SL intensity shown for low acoustic cycles and low SDS concentrations. Increasing the pulse length increases the proportion of bubbles that exceed Rd at the end of the pulse. These bubbles can survive the pulse “off time” and then further act as cavitation nuclei for SL in the subsequent pulse. This effect has been reported in the literature22,29,42 and has been referred to as a cascade effect.29 Thus, while smaller bubbles are generated in lower concentrations of SDS than in water, the number of bubble nuclei that survive through the pulse “off time” increases at long pulse lengths. This causes a net increase in acoustic cavitation activity, as shown in Figures 5 and 6. This cascade effect also explains the higher acoustic cavitation intensity shown in Figure 3 at a PRF of 60 Hz relative to a PRF of 30 Hz. The narrower size distribution for the 1.5 mM SDS system can also explain the steeper rise in SL intensity as a function of pulse length illustrated in Figure 6. The steady-state acoustic cavitation intensity for 1.5 mM SDS shown in Figures 5 and 6 is substantially higher than that of pure water. While this high intensity can partially be explained by the bubble population effects described above, this increased intensity has also been shown in previous reports to be the result of a more open bubble cluster.16,17,32 The electrostatic repulsion between bubbles causes them to separate, and this increases the penetration of the ultrasonic field into the cluster. Such an effect is likely to contribute to the high SL intensity observed at such intermediate SDS concentrations. Although Segebarth et al.18 suggested that it is unlikely that the increase in the SL is caused by the impedance shielding effect, Tronson et al.17 argued that electrostatic effects within the streamers may lead to different spatial structures of the bubble cloud, that is, a more “open” bubble cloud. The term streamers refers to the specific paths traveled by bubbles moving from a cavitation source toward a bubble cluster.43 At low acoustic cycles, addition of SDS in the concentration range 1.5 -5.0 mM increases the SL intensity (Figures 5 and 7). It has been shown by Crum44 and Lee et al.45 that surfactants can also enhance the rate of growth via rectified diffusion. This
Lee et al. effect may be dominating over the electrostatic effects observed at long pulse lengths. As SDS concentration increases, this enhanced bubble growth causes bubble sizes at the end of each pulse to be greater than Rd and thus allows these bubbles to persist through the “off time”. Increasing the acoustic power would also further enhance the rate of growth via rectified diffusion.44 This would explain the increase in the SL above that of water at acoustic powers above 80 W (Figure 7). It is also well established in the literature that the addition of salt will dramatically reduce electrostatic repulsion. This will cause a return to either higher coalescence rates,46 more closed bubble clusters, or a combined effect. Both phenomena will cause a decrease in SL intensity, which is reflected in our results. The addition of 0.1 M NaCl clearly returns bubble behavior to that of water (see Figure 6). This result reinforces the argument that it is electrostatic effects that dominate bubble behavior when charged surfactants are present. It has further been reported that when surface concentrations of SDS have reached saturation, further addition of SDS will have a similar saltlike effect.16,17,32 The saturation of SDS under equilibrium conditions occurs at 8 mM (cmc). However, as mentioned earlier, the maximum effect due to electrostatic repulsion caused by the surfactant adsorption occurs well below the cmc. A further increase in SDS concentration results in the distribution of these charged molecules within the bulk solution and around the bubbles, which results in the reduction of the electrostatic repulsion between bubbles. As discussed above, a reduction in electrostatic repulsion will cause bubble behavior to revert toward the water case. This effect is indeed observed in our results with a decrease in the SL intensity at high concentrations of SDS at all pulse lengths. These results with salt also confirm that the SL behavior described earlier is not caused by surface tension. If this was the case, then these effects would persist when salt is added. Conclusions It has been shown that there is a strong correlation between the effect of SDS on both the SL intensity and the IC dose. Different effects govern the level of IC dose or SL observed and depend upon the pulse length. At short pulse lengths and low SDS concentrations, the SL intensity and IC dose initially decrease due to bubble coalescence inhibition. Further additions of SDS cause the acoustic cavitation intensity to increase, and this increase is attributed to enhanced rectified diffusion and a decrease in electrostatic repulsions. For longer acoustic cycles, the SL intensity is observed to increase at low SDS concentrations. This increase is partly attributed to the cascade effect, that is, an increase in the number of bubbles brought about by bubble coalescence inhibition that results in the survival of more bubbles between acoustic pulses. The increase can also be attributed to a more open bubble cluster, which will reduce the shielding of bubbles from the ultrasonic field. Addition of higher SDS concentrations and/or the addition of a salt such as, NaCl, reduce these electrostatic effects, returning the system closer to the behavior of pure water. Acknowledgment. The authors thank Prof. Franz Grieser for valuable discussions, Jarred Swalwell for his assistance with the electronic equipment, and Dorothy Lowell for her assistance in backing up the experimental data. Financial support from Kodak and the infrastructure support from the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council, are gratefully acknowledged. Judy Lee received a postgraduate stipend from the University of Mel-
Surfactants on Inertial Cavitation Activity bourne (UM). She also received a UM Postgraduate Overseas Research Experience Scholarship for travel to the University of Washington. References and Notes (1) Ohl, C. D.; Kurz, T.; Geisler, R.; Lindau, O.; Lauterborn, W. Philos. Trans. R. Soc. London, Ser. A 1999, 357, 269. (2) Matula, T. J. Philos. Trans. R. Soc. London, Ser. A 1999, 357, 225. (3) Leighton, T. G. The Acoustic bubble; Academic Press Limited: Cambridge, U.K., 1994. (4) Young, F. R. CaVitation; Imperial College Press: London, 1999. (5) Apfel, R. E. J. Acoust. Soc. Am. 1997, 101, 1227. (6) Coleman, A. J.; Saunders, A.; Crum, L. A.; Dyson, M. Ultrasound Med. Biol. 1987, 13, 69. (7) Williams, J. C., Jr.; Woodward, J. F.; Stonehill, M. A.; Evan, A. P.; McAteer, J. A. Ultrasound Med. Biol. 1999, 25, 1445. (8) Barnett, S. B.; Rott, H. D.; ter Haar, G. R. Ultrasound Med. Biol. 1997, 23, 805. (9) Debus, J.; Spoo, J.; Huber, P.; Peschke, P. Ultrasound Med. Biol. 1999, 25, 301. (10) Fry, F. J.; Sanghvi, N. T.; Foster, R. S.; Bihrle, R.; Hennige, C. Ultrasound Med. Biol. 1995, 21, 1227. (11) Crum, L. A.; Mao, Y. J. Acoust. Soc. Am. 1996, 99, 2898. (12) Lohse, D. Nature 2002, 418, 381. (13) Ashokkumar, M.; Grieser, F. ChemPhysChem 2004, 5, 439. (14) Suslick, K.; Didenko, Y.; Fang, M.; Hyeon, T.; Kolbeck, K. J.; McNamara, W. B., III; Mdleleni, M. M.; Wong, M. Philos. Trans. R. Soc. London, Ser. A 1999, 357, 335. (15) Suslick, K. Science 1990, 247, 1439. (16) Ashokkumar, M.; Hall, R.; Mulvaney, P.; Grieser, F. J. Phys. Chem. B 1997, 101, 10845. (17) Tronson, R.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2002, 106, 11064. (18) Segebarth, N.; Eulaerts, O.; Reisse, J.; Crum, L. A.; Matula, T. J. J. Phys. Chem. B 2002, 106, 9181. (19) Ashokkumar, M.; Guan, J.; Tronson, R.; Matula, T. J.; Nuske, J. W.; Grieser, F. Phys. ReV. E 2002, 65, 046310. (20) Griffing, V.; Sette, D. J. Chem. Phys. 1955, 23, 503. (21) Finch, R. D. Ultrasonics 1963, 1, 87. (22) Henglein, A.; Ulrich, R.; Lilie, J. J. Am. Chem. Soc. 1989, 111, 1974.
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