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Frequency Effects during Acoustic Cavitation in Surfactant Solutions Shuhui Wu, Thomas Leong, Sandra Kentish,* and Muthupandian Ashokkumar* Particulate Fluids Processing Centre, School of Chemistry and Department of Chemical and Biomolecular Engineering, UniVersity of Melbourne, VIC 3010, Australia ReceiVed: August 29, 2009; ReVised Manuscript ReceiVed: October 20, 2009
The acoustic cavitation-induced events, multibubble sonoluminescence (MBSL) and initial growth of MBSL have been studied in surfactant solutions and correlated with bubble coalescence data at three different ultrasound frequencies. For an ionic surfactant, both the number of ultrasonic pulses required to reach a steady state MBSL intensity (Ncrit) and the magnitude of this intensity increases to a maximum as the surfactant concentration increases and then falls again. The total bubble volume generated for a fixed sonication time, which is indirectly related to bubble coalescence, similarly falls as surfactant concentration increases and then rises again. These effects are caused by a combination of electrostatic and coalescence factors at relatively low surfactant concentrations and the screening of the electrostatic factor as surfactant concentration increases further. The peak in coalescence inhibition occurs almost at the same surfactant concentrations as the acoustic frequency is increased; however, the concentrations at which peaks in MBSL and Ncrit occur vary at different frequencies. These results have been discussed in terms of coalescence, electrostatic interactions, rectified diffusion growth, and the adsorption kinetics of the surfactants. 1. Introduction Acoustic cavitation is a phenomenon that has been well researched for over a century. Under the influence of an acoustic field, bubbles are generated from existing gas nuclei in liquids. These bubbles oscillate in a nonlinear manner, and under specific experimental conditions violently collapse to generate high temperatures and pressures, which can lead to light emission referred to as sonoluminescence (SL). A number of reports have found that surface-active solutes present in the solution can influence bubble behavior and specifically the bubble growth rate, size distribution, SL intensity, and bubble cloud structure.1–7 Among these effects, the effect of surface-active solutes on bubble coalescence is particularly important.5,7 It is well-known that within a narrow range of surfactant concentrations, bubble coalescence decreases dramatically.5,7 Ashokkumar et al.2 and Segebarth et al.8 showed that coalescence is inhibited when the repulsive forces induced by the electrostatic charge of the surfactant exceed the Bjerknes attractive forces between bubbles. Further, this bubble repulsion leads to an expanded bubble cluster that can absorb more acoustic energy. At higher concentrations of ionic surfactants, coalescence can increase again and the bubble cluster can contract, as the higher ionic strength reduces the impact of charge repulsion between surfactant molecules. Our group has been investigating the effects of surface-active additives on acoustic processes for a long time,1–7,9–11 and methods to monitor these effects in an acoustic field have been developed. One technique to quantify the extent of bubble coalescence in the absence and presence of surface active solutes is the capillary cell method.6 This method measures the change in total bubble volume following sonication (∆VT). When coalescence is inhibited, a smaller total volume is obtained, as larger bubbles that can persist through the sonication process do not form as easily. Another approach is to study the initial * To whom correspondence should be addressed. E-mail: sandraek@ unimelb.edu.au (S.K.);
[email protected] (M.A.).
growth and steady state intensity of multibubble sonoluminescence (MBSL)9 using a pulsed ultrasound technique. The effect of surface active solutes on the number of pulses required to reach a steady state MBSL (Ncrit) and the steady state MBSL intensity can be monitored. Large values of Ncrit are indicative of strong coalescence inhibition as they imply that longer times are required for bubbles to reach a size range where inertial cavitation dominates. Similarly, higher steady state MBSL intensity is indicative of more active bubbles in the presence of certain surface active solutes. In our previous study, the effect of surface active solutes on the ∆VT at various frequencies was investigated.5 On the basis of these results it was postulated that the extent of bubble coalescence slightly decreased with an increase in ultrasound frequency. The correlation between these coalescence results, Ncrit, and steady state MBSL intensity was investigated at 515 kHz.9 When bubble coalescence decreased, both Ncrit and steady state MBSL intensity increased. However, the frequency effects were not investigated to fully correlate these three parameters (∆VT, Ncrit, and the steady state MBSL intensity). An understanding of the effect of surfactants on acoustic cavitation events at various frequencies is important since many sonochemical reactions and other ultrasound based applications are carried out in the presence of surface active solutes at different frequencies.12–20 Hence the aim of this study was to systematically investigate the effect of surface active solutes on ∆VT, Ncrit, and steady MBSL intensity at three different frequencies in order to expand the knowledge base in the acoustic cavitation research field. 2. Experimental Section The experimental setup used for measuring the MBSL intensity is described elsewhere.2 A cylindrical Ultraschalltechnik NS71/51 cell, filled with 130 mL of solution, was mounted over a plate (Meinhardt Ultraschalltechnik Technik Leipzig, Germany) transducer, operated at 575, 856, or 1136 kHz. The transducer was connected to a HAMEG HM8131-2 function
10.1021/jp9083458 2009 American Chemical Society Published on Web 11/06/2009
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Figure 1. Effect of SDS at 575 kHz (continuous or pulsed at 4 ms on 12 ms off): (a) ∆VT, (b) Ncrit, and (c) steady-state MBSL intensity.
generator with a power amplifier (Meinhardt Ultraschalltechnik Technik Leipzig, Germany) set at 29 W for continuous sonication. For pulsed sonication, we used the same power setting but the loading ratio depended upon the pulse on and off time in order to maintain the same power level. The function generator was triggered by a pulse generator, where pulse repetition frequency and pulse width could be varied. The pulse train was set as 4 ms on and 12 ms off unless otherwise specified. A Hamamatsu end-on photomultiplier tube (model no. E849-35) was used to record the SL intensity, and the output from the photomultiplier tube was fed to a digital oscilloscope (Lecroy Wavesurfer 452). The oscilloscope, triggered by the ultrasonic pulse generator, recorded the sequence of signals. The cell and the photomultiplier tube were both housed in a dark enclosure to minimize background light. The anionic surfactant, sodium dodecyl sulfate (SDS) special purity grade and sodium chloride (NaCl) (99.9%) were used as purchased from BDH. The zwitterionic surfactant, 3-(N,Ndimethyloctyl-ammonio) propanesulfonate (Zwittergent; zwitterionic surfactant) was supplied by Calbiochem. 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. After preparation, the surfactant solution was presonicated for 30 s to remove any pre-existing bubble nuclei. The solution was then allowed to stand for at least 5 min for any bubbles formed within the 30 s to dissolve away. Pulsed sonication was then applied. The Ncrit was recorded when a steady intensity of SL was reached.9 The steady-state SL intensity was averaged by 20 steady pulses. The final data was averaged by two measurements. The temperature was not controlled but monitored as 20 ( 2 °C. The temperature did not increase by more than 2 °C during the course of the experiments. The setup for the volume change experiment has been detailed elsewhere.7 The cylindrical cell was replaced by a Pyrex capillary cell with a base volume of approximately 110 mL and a long capillary. The function generator delivered a continuous wave in this experiment with the same voltage output setting
as the SL measurement. A 60 mL syringe with a 19-gauge needle was used to transfer the solution into the capillary cell carefully to avoid visible gas pockets in the cell. At the start of all experiments, the ultrasound generator was allowed to stabilize for 15 min. The solution was then sonicated for 15 s. During this time, the liquid interface was observed to rise up the capillary. The height of the interface after the ultrasound was turned off was recorded. 3. Results and Discussion Effect of SDS Concentration. The effects of SDS concentration on ∆VT, Ncrit, and steady state MBSL at 575 kHz have been shown in parts a, b, and c of Figure 1, respectively. All data are presented relative to that of pure water. A number of key observations can be made in Figure 1 as summarized below: (i) The relative ∆VT decreases with an increase in SDS concentration up to ∼1 mM and then increases with a further increase in SDS concentration; (ii) the relative Ncrit increases with an increase in SDS concentration up to ∼1 mM and then decreases with a further increase in SDS concentration; (iii) the relative MBSL intensity increases with an increase in SDS concentration up to ∼1 mM and then decreases with a further increase in SDS concentration. Surface active solutes are known to hinder bubble coalescence in static and dynamic conditions.5,7,21 In an acoustic field, where the bubble/solution interface is under constant oscillation, the dynamics of surfactant adsorption play a major role in controlling acoustic cavitation events. At low SDS concentrations, the adsorption of negatively charged SDS molecules at the bubble/ solution interface causes three major effects. First, the electrostatic repulsion between bubbles leads to an open cluster arrangement where the physical contact between the bubbles is restricted.22 (Note that the open cluster arrangement is valid at high-frequency conditions only since low-frequency sonication did not show such effects.5 Lee et al.5 have shown that the clustering/declustering mechanism does not operate at low frequencies due to the absence of standing waves. At 20 kHz,
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the type of cavitation is transient cavitation compared to high frequencies where stable cavitation dominates.) Second, the adsorption of the surface active solute causes steric hindrance to bubble coalescence. Finally, the surfactant adsorption also leads to a decrease in the surface tension. All of these effects ultimately reduce the extent of bubble coalescence. At low SDS concentrations, the hindrance to bubble coalescence is mainly due to the electrostatic repulsion between bubbles. At high SDS concentrations, where the excess SDS acts as a salt, the electrostatic effect is negated and the hindrance to bubble coalescence is mainly due to the steric and surface tension effects that can be observed with any neutral surface active solute. Thus, the decrease in ∆VT observed at low SDS concentrations, Figure 1a, is due to the hindrance to bubble coalescence caused by the electrostatic repulsion between bubbles due to surfactant adsorption leading to a smaller number of larger inactive bubbles and hence a relatively low ∆VT. The hindrance to bubble coalescence by the electrostatic, steric, and surface tension factors affects the MBSL in two ways at low SDS concentrations. We have shown in our previous study9 that a certain number of acoustic pulses are required to grow the bubble nuclei to the resonance size range and to reach a steady state active bubble population. The bubble nuclei can reach this size range in two pathways, viz., coalescence and rectified diffusion.9,23 In the absence of any solutes, the coalescence between small bubble nuclei will allow the bubbles to reach the resonance size range relatively quickly and so require a smaller number of acoustic pulses (a lower Ncrit). In the presence of surface active solutes such as SDS, Ncrit increases (Figure 1b) since the bubble coalescence pathway is restricted (by electrostatic, steric, and surface tension factors) and the bubble growth can only occur via a rectified diffusion pathway, which is a relatively slow process.9 An accompanying effect of coalescence is a reduction in the total number of bubble nuclei, which otherwise could become active bubbles. In other words, the coalescence between bubble nuclei during the initial growth phase leads to a decreased number of steady-state active bubbles. This ultimately reduces the overall MBSL intensity compared to that observed when coalescence is inhibited. Thus, the steady state MBSL intensity observed for pure water (Figure 1c at SDS ) 0 mM) is relatively low due to the loss of bubble nuclei due to coalescence. The increase in this intensity at low SDS concentrations is due to an increase in the number of active bubbles, which may be related both to the more open bubble cluster caused by electrostatics and the steric and surface tension effects caused by the adsorption of the surfactant. However, as will be discussed later, the electrostatic effect is dominant. It can be seen from the ∆VT data (Figure 1a) that the extent of coalescence increases above 1 mM and plateaus at values between 4 and 10 mM SDS. This reversal in the data probably results from the increasing ionic strength of excess SDS reversing the electrostatic effects discussed above.7 The Ncrit and MBSL data show a similar decline in values at similar concentrations (parts b and c of Figure 1) which again can be related predominantly to a reclustering mechanism. However, a major difference in the trends observed is that while the steady state MBSL intensity returns to that observed in pure water, the Ncrit falls to a value even lower than that in pure water (Ncrit ∼ 0). Conversely, the ∆VT never returns to the pure water value. These are important observations that provide crucial mechanistic detail on the individual contributions of the electrostatic repulsion between bubbles and the physical adsorption of the surfactant molecules on the surface of the bubbles.
Wu et al.
Figure 2. ∆VT in SDS solutions at 575, 856, and 1136 kHz.
The electrostatic repulsion is suggested to be the main factor in increasing the number of active bubbles,2 and thus once these effects are reversed the MBSL intensity returns to that of water. If the steric and surface tension effects were dominant, then the number of active bubbles would not return to this value at higher SDS concentrations, since the steric hindrance increases with an increase in the surfactant concentration. In static systems, this surface concentration would stabilize at the critical micelle concentration (∼8 mM). However, because of the dynamic nature of the cavitation bubble interface, the surface concentration never reaches this equilibrium level.5,14 These electrostatic repulsion effects are also the major influence on the number of pulses required to reach steady state. We speculate that the reason for the instant appearance of a steady-state population (Ncrit ∼ 0) at higher surfactant concentrations may be due to an additional increase in the growth rate of the bubbles due to rectified diffusion. Surfactant adsorption is known to increase this growth rate.23 Conversely, the ∆VT data is more strongly influenced by the steric/surface tension effects. As the steric hindrance remains high and the surface tension remains low at higher surfactant concentrations, the extent of coalescence, and thus the ∆VT value, never returns to the level of water. Effect of Frequency. The effects of frequency on ∆VT, Ncrit, and steady-state MBSL intensity in the presence of SDS are complex in nature. It is known that the bubble oscillation time per acoustic cycle decreases with an increase in acoustic frequency. In addition, the lifetime of the acoustic cavitation bubbles also decrease with an increase in acoustic frequency.5 Sunartio et al.5 have shown that the lifetime decreases from ∼0.35 ms at 213 kHz to 0.1 ms for 1062 kHz. A decrease in both of these factors will lead to less surfactant adsorption (for a given bulk concentration) at higher frequencies compared to that at lower frequencies. On the basis of this argument, one can expect a frequency dependence of the ∆VT as a function of SDS concentration. Figure 2 shows that there is little difference in the magnitude of the ∆VT decrease as a function of frequency at lower SDS concentrations. This inflection point occurs at a relatively low SDS concentration for 575 kHz (∼3 mM) and increases to higher concentrations (4-8 mM) with increasing frequency. As mentioned earlier, at low SDS concentrations, the electrostatic factor dominates the observed effects. The data observed in Figure 2 indicates that the electrostatic repulsion between bubbles is strong enough even at very low concentrations and hence a frequency effect could not be observed for the lowering of the ∆VT. Conversely, there appears to be a frequency trend in the inflection point of the ∆VT data, i.e., the concentration at which the extent of coalescence increases again after reaching a minimum. This inflection point occurs at a relatively low SDS
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Figure 3. Ncrit at 575, 856, and 1136 kHz in (a) SDS and (b) zwitterionic solutions (4 ms on and 12 ms off).
concentration for 575 kHz and increases to higher concentrations with increasing frequency. This trend could be explained based on the oscillation time and lifetime of the bubbles. As frequency increases, less SDS might be adsorbed onto the bubble/solution interface, as there is less time for mass transfer to occur between oscillations. As the surfactant concentration in the immediate environment of the bubble would be lower due to these dynamic mass transfer effects, the screening of electrostatic repulsion only occurs at a higher bulk concentration. The maxima in the Ncrit (Figure 3a) values are also different for the three frequencies. Compared to 575 kHz, the maxima for 856 and 1136 kHz occur at higher SDS concentrations: about ∼0.8 mM SDS for 575 kHz, ∼1.2 mM SDS for 856 kHz, and ∼1.8 mM for 1136 kHz. Similarly the decline in Ncrit occurs at higher SDS concentrations for higher frequencies. For 575 kHz, the signal returns to a minimum in the concentration range 1-2 mM, whereas this occurs in the concentration range 1.5-2.5 and 2-4 mM for 856 and 1136 kHz, respectively. These observations can also be interpreted in terms of the changes to the oscillation time and lifetime of the bubbles. A greater level of surfactant adsorption at the lower frequency leads to a relatively stronger hindrance to bubble coalescence and hence the number of acoustic pulses needed to attain a steady state population is less. At higher frequencies, more acoustic pulses are required due to the dynamics involved in the adsorption process as discussed above. As in the ∆VT discussion, the effects of excess SDS acting to increase ionic strength in the fluid immediately surrounding the bubbles can also be expected to be effective at lower surfactant concentrations for the lower frequency. In other words, the SDS concentration required for bubble reclustering to occur increases with an increase in the ultrasound frequency. Further, the SDS concentration required to reach Ncrit ) 0 is also expected to increase at higher frequencies due to relatively lower surfactant adsorption resulting in slower rectified diffusion growth of the bubbles. In order to support the electrostatics based argument, further experiments were carried out where the Ncrit data were acquired in the presence of a zwitterionic surfactant (Figure 3b). This surfactant has both a negative and positive charge and hence the overall charge is neutral. The striking contrast between these two sets of data (parts and b of Figure 3) is very clear. When a zwitterionic surfactant is used, the Ncrit does not increase at low zwitterionic concentrations for all three frequencies. Both SDS and the zwitterionic surfactants have similar surface activities. However, the adsorption of the zwitterionic solute does not generate any net charge on the bubble surface, which supports the argument that the changes in Ncrit caused by SDS are electrostatic in nature. Figure 4 shows the steady state MBSL intensity at 575, 856, and 1136 kHz compared to water. While there is a frequency
Figure 4. Normalized MBSL intensity in SDS solutions at 575, 856, and 1136 kHz (4 ms on and 12 ms off).
dependence observed, the data set here is more complex. Specifically, there are two maxima observed at 1136 kHz. Further, for this frequency the MBSL intensity falls below that of pure water for the SDS concentrations intermediate between these two maxima. These MBSL measurements were carried out in a pulsed mode (4 ms “on” and 12 ms “off”). As we have described in an earlier paper, during the pulse off time, the cavitation bubbles tend to dissolve.24 If the dissolving bubbles reach a size where they cannot be grown to a critical size range required for SL to occur by the subsequent acoustic pulse, then the number of active bubbles can decrease. The loss in MBSL intensity observed at 1136 kHz at SDS concentrations of ∼1 mM can readily be explained by such effects. As coalescence is inhibited with increasing surfactant concentrations, the bubble sizes decline and the short pulse duration is not sufficient to grow these bubbles during the next pulse to a size necessary for SL to occur. As SDS concentrations increase beyond this range, the rate of rectified diffusion increases, resulting in larger bubbles which can survive the pulse “off” time. In order to support this argument, further experiments were carried out at 575 and 1136 kHz under varying pulse “off” time conditions (Figure 5). Almost no MBSL is observed at 575 kHz and 1 mM SDS when the pulse off time is increased to 20 ms. This observation suggests that, at this frequency, the bubbles grown during the 4 ms on time dissolve during the 20 ms off time to a sufficiently small size range that they cannot recover during the next pulse. Similarly, the observation of higher MBSL intensity relative to that of water at 1136 kHz when the pulse off time is reduced to 4 ms shows that the general trend observed is similar to that observed of other frequencies at 12 ms off time. Choi and Funayama25 have reported a drop in the MBSL intensity in solutions containing low amounts of SDS (
