J. Phys. Chem. B 2005, 109, 5095-5099
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The Effect of Surface-Active Solutes on Bubble Coalescence in the Presence of Ultrasound Judy Lee,† Sandra E. Kentish,† and Muthupandian Ashokkumar*,‡ Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, and School of Chemistry, UniVersity of Melbourne, VIC 3010, Australia ReceiVed: June 1, 2004; In Final Form: NoVember 30, 2004
The sonication of an aqueous solution generates cavitation bubbles, which may coalesce and produce larger bubbles. This paper examines the effect of surface-active solutes on such bubble coalescence in an ultrasonic field. A novel capillary system has been designed to measure the change in the total volume resulting from the sonication of aqueous solutions with 515 kHz ultrasound pulses. This volume change reflects the total volume of larger gas bubbles generated by the coalescence of cavitation bubbles during the sonication process. The total volume of bubbles generated is reduced when surface-active solutes are present. We have proposed that this decrease in the total bubble volume results from the inhibition of bubble coalescence brought about by the surface-active solutes. The observed results revealed similarities with bubble coalescence data reported in the literature in the absence of ultrasound. It was found that for uncharged and zwitterionic surface-active solutes, the extent of bubble coalescence is affected by the surface activity of the solutes. The addition of 0.1 M NaCl to such solutes had no effect on the extent of bubble coalescence. Conversely, for charged surface-active solutes, the extent of bubble coalescence appears to be dominated by electrostatic effects. The addition of 0.1 M NaCl to charged surfactant solutions was observed to increase the total bubble volume close to that of the zwitterionic surfactant. This suggests the involvement of electrostatic interactions between cavitation bubbles in the presence of charged surfactants in the solution.
1. Introduction In the absence of ultrasound, bubble coalescence plays an important role in both chemical and biochemical gas/liquid reactors and in absorbers, such as agitated tanks and bubble columns.1-3 For this reason, the effect of surface-active solutes, salts, and viscosity on bubble coalescence in the absence of ultrasound has been widely studied.1-12 However, very few workers have examined the effect of surface-active solutes on bubble coalescence in the presence of ultrasound. It has been known since the early 1960s that sonic vibration can be used to improve adsorption and boiling processes by enhancing mass and heat transfer.13-15 Since then, ultrasound has been adopted in numerous industrial and medical applications, ranging from the deaeration of process fluids,16 emulsification,17 degradation of pollutants,18 formation of nanoparticles,20,21 to lithotripsy22 and drug delivery.23 In most of these applications, the presence of surface-active solutes and salts can alter bubble population and sizes. The bubble population and sizes, which may also be affected by the extent of coalescence, are important because they affect the interfacial area available for mass transfer as well as for sonochemical reactions. Therefore, a fundamental understanding of coalescence and how this phenomenon is affected by surface-active solutes and salts in the presence of ultrasound is warranted. In the absence of ultrasound, the process which leads to bubble coalescence can be described in three steps:1,7 (1) Bubbles must come into contact and form a film of 10-3-10-4 cm in thickness. This is controlled by the hydrodynamics of the bulk liquid. (2) The film thins. The rate of thinning is controlled by the hydrodynamics of the liquid film. (3) Rupturing of the film occurs once the film is sufficiently * Corresponding author. E-mail:
[email protected]. † Department of Chemical and Biomolecular Engineering. ‡ School of Chemistry.
thin. This rupturing step is faster than the previous two steps and will not occur if the contact time is shorter than the thinning process, making the rate of thinning of liquid film a ratecontrolling step.24 Some of the factors that can affect the thinning of the liquid film are the presence of surface-active solutes and electrolytes. It is known that surface-active solutes, as the name suggests, adsorb at the air/water interface.25 The extent of adsorption, which subsequently lowers the interfacial tension, will depend on the surface activity of the solute. As two bubbles are brought together in a solution containing surface-active solutes, the thinning process increases the surface area and induces a surface concentration gradient of the adsorbed surface-active solutes. It is believed that this surface concentration gradient is responsible for the restriction of surface mobility7,11 and the retardation of the flow of the draining liquid between the film24 (Marangoni effect26) and consequently for a reduction in the coalescence rate. It has been shown that the frequency of bubble coalescence decreases dramatically across a narrow range of surfactant concentration.2,7,27 For alcohols, the concentration at which this transition occurs was found to decrease with increasing surface activity or with increasing hydrocarbon chain length.7,27 The transition region was found to be broader for less surface-active solutes. Similar inhibition in bubble coalescence behavior was observed in the presence of electrolyte solutions.3,6,28 In the presence of ultrasound, there have been numerous studies on the effect of surface-active solutes on singlebubble29-33 and multi-bubble sonoluminescence,34-37 and on the dynamics of bubbles.38-43 A number of investigations have commented on the role of inhibition of bubble coalescence by charged surface-active solutes, thereby affecting the intensity of multi-bubble sonoluminescence.29,34-37 However, no attempts have been made to quantify the strength of the inhibition of bubble coalescence.
10.1021/jp0476444 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/01/2005
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Figure 1. The capillary cell showing the change in liquid level (∆VT) that occurs during sonication.
Bubble coalescence in the presence of ultrasound is more complicated than that in the absence of ultrasound. Existing microcavities in the liquid will be forced to expand when sufficient acoustic pressure is applied, giving rise to the generation of bubbles. These bubbles can grow via rectified diffusion, coalesce, dissolve, collapse, or be translated by acoustic forces.44,45 The primary Bjerknes force arises from the acoustic pressure waveform that moves bubbles to the nodes and antinodes of the acoustic wave pattern.46 The secondary Bjerknes force arises from neighboring oscillating bubbles. Two bubbles of the same size will oscillate in phase and attract, whereas bubbles of different sizes will oscillate out of phase and repel.44,47 These effects make direct investigations into bubble coalescence very difficult. The presence of surface-active solutes will further complicate the system. This paper presents a novel technique for measuring the change in total bubble volume in an acoustic field and links the changes caused by surface-active solutes with the inhibition of bubble coalescence. The results show that there is a sharp decrease in the change in total bubble volume when small amounts of surface-active solutes are present in the solution. 2. Experimental Details 2.1. Materials. Methanol (99.8%) and ethanol (99.5%) were purchased from Ajax Chemicals. Propan-1-ol (99.5%), sodium chloride (NaCl) (99.9%), and sodium dodecyl sulfate (SDS) (99.0%) were purchased from BDH. Sodium dodecyl benzene sulfonate (SDBS) and alkanol XC or sodium alkyl-naphthalene sulfonate (AXC) were purchased from Sigma Aldrich. Dodecyl trimethylammonium chloride (DTAC) was purchased from Kodak Chemicals, and n-decyl-N,N-dimenthyl-3-ammonio-1propanesulfonate (ZW3-10) (99%) was purchased from Calbiochem. ZW3-10 is a zwitterionic surfactant with a quaternary ammonium alkyl sulfonic acid inner salt which retains its dual charge over a very wide pH range in aqueous solutions.48 The solutions were made using Milli-Q water with a conductivity of less than 10-6 S cm-1 at 20 °C. 2.2. Methods. 2.2.1. Equipment. The equipment used in this experiment consists of a Pyrex capillary cell with a base volume of approximately 40 mL and with a long capillary neck of 12.5 µL cm-1 (see Figure 1). The capillary cell was fitted over a 35 mm diameter, 515 kHz flat plate transducer, which was connected to a 515 kHz Undatim Ultrasonics D-reactor (generator) operating at 20 W. The cell temperature was not controlled, but monitored within the range 20-21 °C. A 50 mL syringe with a 19-gauge needle was used to transfer the solution into the capillary cell. 2.2.2. Varying DissolVed Gas Concentrations. Milli-Q water was degassed using a Javac vacuum pump and stored in an airtight container overnight. The dissolved air concentration was varied by exposing the solution to the atmosphere, and the dissolved oxygen concentration was monitored until the desired gas concentration was reached. An YSI 5739 Field Probe, with an accuracy of 0.2%, was used to measure the dissolved oxygen content. 2.2.3. Surface-ActiVe Solutes. Experiments involving solutions containing the desired solutes were fully saturated with air. In
Figure 2. Average total bubble volume ∆VT as a function of the percentage of dissolved air in water. 1.7 µL of the total bubble volume is caused by thermal expansion.
these cases, the solution was prepared to the appropriate concentration and then left overnight to allow full air saturation and equilibrium to be reached. The concentrations of the surfaceactive solute and NaCl used were very low and did not affect the equilibrium dissolved air concentration.49,50 Prior to each experiment with a surface-active solute or salt-containing solution, a water run was conducted as a control and was found to be very reproducible. 2.2.4. Change in Total Solution Volume. At the start of all experiments, the ultrasound generator was allowed to stabilize for 15 min. For every measurement, approximately 40 mL of solution was then transferred into the capillary cell, with care being taken to avoid the creation of any visible gas pockets adhering to the walls of the cell. The solution was then sonicated for 5 s. During this time, the liquid interface was observed to rise up the capillary (see Figure 1). In all cases, as soon as the ultrasound was switched off, the interface dropped by a small, but noticeable, amount. This initial drop can be attributed to the dissolution of smaller uncoalesced gas bubbles that are below or at the resonance size for this frequency, that is, around 6 µm. Equation 1, derived by Epstein and Plesset,51 predicts that such cavitation bubbles should dissolve in the absence of ultrasound in approximately 2 s.
( ) 2DCs FgRo2
t)
( )
2 2Mσ (1 + δ); δ ) 3δ BTFgR0
(1)
D is the diffusion coefficient, Cs is the saturated dissolved gas concentration, Fg is the gas density in the bubble, Ro is the initial bubble radius, t is the time, M is the molecular weight of the gas, σ is the surface tension of the liquid, B is the universal gas constant, and T is the temperature of the liquid. However, after this initial drop, the position of the interface stabilized and remained at the same height for at least 1 min after the ultrasound was turned off. It is this volume change (∆VT) after the initial drop that is the main focus of this paper. 3. Results/Discussion 3.1. Effect of Air Saturation. Figure 2 shows the magnitude of the steady-state volume increase (∆VT) as a function of the air saturation. This volume is constant at approximately 4 µL for air saturation levels of less than 40% and then increases rapidly to 23 µL at approximately 83% air saturation where it plateaus at air saturation levels exceeding 83%. This increase in the total solution volume (∆VT) may be attributed to two distinct effects, thermal expansion of the solution and cavitation bubbles. An increase in temperature will result in the thermal expansion of the liquid. The change in the solution temperature for a given period of sonication is shown in Figure 3. For a 5 s
Effect of Solutes on Bubble Coalescence
J. Phys. Chem. B, Vol. 109, No. 11, 2005 5097
Figure 3. Change in solution temperature as a function period of sonication time for water at 100% saturation.
TABLE 1: The Change in Volume for Water, and 0.1 mM and 1 mM SDS at 30% and 100% Air Saturation
water 0.1 mM SDS 1 mM SDS
30% air saturation (µL)
100% air saturation (µL)
5.5 4.4 3.2
25 10 5
sonication, our measurements recorded a small temperature increase of 0.2 °C (which gives a thermal expansion of 1.7 µL for water). Therefore, it seems reasonable to conclude that the plateau at approximately 4 µL below 40% air saturation predominantly results from this temperature-induced liquid density change. It has been suggested52-54 that the presence of stabilized gas nuclei or microparticles of dust is responsible for the cavitation activity. Increasing the air saturation level will increase the number of gas nuclei available for such activity. The rate of bubble growth via the process of rectified diffusion will also increase, resulting in an increase in the size of bubbles. As both the number and the size of cavitation bubbles are thus increased, the number and size of the coalesced larger bubbles that contribute to ∆VT will also increase as a direct function of the air saturation level. As these larger bubbles are above the resonance size, they are relatively stable and are not further influenced by the ultrasound itself. Further, they will persist for much longer times once the ultrasound is switched off. In many cases, these bubbles are visible to the naked eye. We conclude that these larger bubbles are the cause of the increase in ∆VT above 4 µL. Above 80% air saturation, the ∆VT plateaus. This indicates that the number of gas nuclei and the bubble size has reached a steady state and further addition of air in the liquid will not have a significant effect on ∆VT. It is clear that measurements should be performed at air saturation either below 50% or above 80% where the ∆VT is not a strong function of dissolved gas content. Table 1 demonstrates the effect of SDS on the ∆VT at 30% and 100% air saturation. The table shows that the presence of SDS lowers the ∆VT (this is discussed in the next section) and that the relative lowering of the ∆VT relative to that of water is very small at 30% as compared to that at 100%. At low air saturation, there are a few bubbles undergoing coalescence activity, and, as discussed above, the ∆VT is predominantly caused by the thermal expansion effect. It would be a difficult task to measure ∆VT for a range of solutes and of solute concentrations without compromising the accuracy of the measurements. At 100% saturation, the bubble coalescence is the dominant effect and the large change in ∆VT provides practical convenience for data collection; therefore, fully saturated water was used for all remaining experiments. 3.2. Effect of Surface-Active Solutes in the Absence and Presence of a Salt. The effect of surface-active solutes on ∆VT
Figure 4. Average relative change in total bubble volume as a function of alcohol concentration. The effect of 0.1 M NaCl addition to methanol solutions is also included. The changes in total bubble volume have been normalized relative to the average change in total volume for water.
Figure 5. Average relative change in total bubble volume as a function of surfactant concentration. The effect of 0.1 M NaCl addition to AXC, DTAC, and ZW3-10 solutions is also included. The changes in total bubble volume have been normalized relative to the average change in total volume for water.
is shown in Figures 4 and 5 for alcohols and surfactants, respectively. In these figures, ∆VT is normalized with respect to the ∆VT for water. Note that the concentrations are plotted on a log scale. It is apparent from these figures that with the addition of surface-active solutes there is a significant decrease in ∆VT. It is highly unlikely that this decrease in ∆VT in the presence of surface-active solutes is caused by any change in the percentage of air saturation of the solutions. This is supported by the reproducibility of the water runs (with an error of ∼5%), indicating that there was no significant variability in the initial gas nuclei present in the water used from day to day. It is further unlikely that this decrease in ∆VT in the presence of surfaceactive solutes is brought about by the decrease in the initial population of gas nuclei present. In fact, some literature suggests that surface-active solutes can stabilize gas pockets.53,54 This would result in an increase in the population of gas nuclei and would increase ∆VT rather than decrease ∆VT as observed. In fact, surfactants are known to both reduce the acoustic pressure threshold for rectified diffusion and increase the growth rate via this phenomenon.39 These effects would again work in opposition to our observed results, tending to increase rather than decrease ∆VT. The only plausible cause of the decrease in ∆VT is thus that the surface-active solutes inhibit bubble coalescence. This will result in a population of bubbles with a smaller radius. To reinforce this argument, the rapid reduction in ∆VT as a function of concentration is very similar to the transition zone reported in the literature for the inhibition of bubble coalescence by a range of surface-active solutes in the absence of ultrasound.7,27 Adopting the same approach as other workers,7,27 the concentration at which the relative ∆VT decreases to 50% has
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TABLE 2: Experimental Results from This Current Study and Literature Values on the Effect of Surface-Active Solutes on ∆VT
solute methanol ethanol propan-1-ol sodium dodecyl benzene sulfonate (SDBS) sodium dodecyl sulfate (SDS) sodium alkylnaphthalene sulfonate (AXC) dodecyl trimethylammonium chloride (DTAC) n-decyl-N,N-dimenthyl3-ammonio-1propanesulfonate (ZW3-10) a
CT [M] Oolman and Blancha
CMC [mM]
CT [M]
CH3OH C2H5OH C3H7OH C12H25(C6H4)SO3-Na+
1.8 0.14
4b
0.40 0.05 0.02 0.085 × 10-3
C12H25SO4-Na+
8b
0.085 × 10-3
0.0062 × 10-3
(C3H7)3(C10H4)SO3-Na+
10b
0.085 × 10-3
C12H25N+(CH3)3Cl-
20c
0.085 × 10-3
C10H21N+(CH3)2(CH2)3SO4-
25-40c
0.650 × 10-3
chemical formula
Reference 7. b Experimental value obtained using a pendant drop tensiometer. c Reference 61.
been denoted as the transition concentration (CT). Table 2 shows that as the surface activity of alcohol increases (governed by the length of the hydrocarbon chain), CT decreases. Further, the CT values for alcohols are much higher as compared to those for surfactants, which are more surface active. These results are consistent with the results reported by Oolman and Blanch7 and Drogaris and Weiland27 for two captured bubbles in the absence of ultrasound. However, a relationship between CT and the surface activity of the solute was not observed for ionic surfactants. Table 2 shows that CT appears to be the same for the four ionic surfactants, despite the difference in their surface activity, which is reflected in the magnitude of the critical micelle concentration (CMC). In addition, the CT for the zwitterionic surfactant, ZW3-10, is higher than that observed for the ionic surfactants. The CT for SDS is also an order of magnitude higher in our ultrasonic work than in the literature values. The results obtained by both Oolman and Blanch7 and Drogaris and Weiland27 are based on two stationary bubbles in a stagnant liquid. In such a system, surface-active solutes will have time to reach equilibrium adsorption. It has been demonstrated by Ashokkumar et al.30 that there is a good correlation between the single-bubble sonoluminescence intensity and the equilibrium concentration of the alcohols at the bubble interface. Therefore, it is highly plausible that the alcohols have reached equilibrium adsorption at the interface, which explains the CT values comparable to those of Oolman and Blanch7 and the correlation with the surface activity of the alcohols. However, the surfactants used in this work have a much higher molecular weight as compared to alcohols. It has been shown in the literature55-57 that it takes about 3-10 ms for SDS solutions to reach near equilibrium interfacial adsorption levels. The bubbles are oscillating at a frequency of 515 kHz (approximately 2 µs per cycle), and therefore it is unlikely that these surfactants will reach equilibrium adsorption. This may explain why the transition concentration for SDS in the presence of ultrasound is an order of magnitude higher than that of the static, equilibrium system considered by Oolman and Blanch. However, there appears to be some agreement between our result and that obtained by Lobo and Svereika58 in the study on the effect of SDS on coalescence during emulsification of oil in water, a nonequilibrium system. These authors showed that in a rotor-stator mixer, no detectable coalescence was observed
above 0.8 mM of SDS as complete inhibition in bubble coalescence has occurred at this concentration. This agrees with the minimum ∆VT reached for SDS at a surfactant concentration of approximately 1 mM (shown in Figure 5). When charged surfactants adsorb to the bubble interface, the bubble wall becomes charged29 and this change can prevent the bubbles from coalescing. This electric repulsion barrier is commonly observed in emulsion stablisation.59,60 The strength of the electric repulsion depends on the concentrations of adsorbed ionic surfactants.59 The overlapping of the curves shown in Figure 5 for the ionic surfactants suggests that the strength of the electric repulsion caused by the adsorbed ionic surfactants is similar in magnitude. It is unlikely that different surfactants with different surface activity will give rise to equal amounts of surface charge. However, it is plausible that due to the nonequilibrium adsorption described above and the complexity of cavitation activity, the variation in the amount of surface charge is small. The strength of the electrostatic repulsion for the neutral ZW3-10 solute will be much weaker, and hence a relatively higher concentration of this solute is required to cause inhibition of coalescence. For this solute, as with the uncharged alcohols coalescence inhibition appears to be governed predominantly by surface activity rather than electrostatic interaction. To support the argument that in charged surfactant solutions the electrostatic effects are responsible for inhibiting the coalescence, ∆VT measurements were carried out in the presence of 0.1 M NaCl. ∆VT measurements carried out in pure NaCl aqueous solutions in the absence of surfactants show that concentrations below 1 M have little effect on ∆VT (Figure 6). The addition of 0.1 M of NaCl to the uncharged/neutral solutes, methanol, ethanol, propan-1-ol, and ZW3-10, also has no significant effect on ∆VT (for clarity, only the results for methanol and ZW3-10 are shown in Figures 4 and 5, respectively). In contrast, for ionic surfactants, 0.1 M of NaCl shifts the curves significantly. While for clarity only the results for AXC and DTAC are shown in Figure 5, in all cases the addition of salt increases the curves to a level consistent with the zwitterionic surfactant ZW3-10. Previous reports on the effect of surfactants on MBSL have suggested that the addition of 0.1 M NaCl eliminates the electrostatic shielding effects observed.34,37 We believe that the present results reflect a similar phenomenon. The salt addition
Effect of Solutes on Bubble Coalescence
Figure 6. Average relative change in total bubble volume as a function of NaCl concentration. The changes in total bubble volume have been normalized relative to the average change in total volume for water.
has reduced the strength of the electrostatic repulsion between the charged surfactant headgroups on bubbles and hence reduced the strength of coalescence inhibition. 4. Conclusions Bubble dynamics in the presence of ultrasound is a complicated system. The novel technique developed by us to measure the ∆VT proves to be a simple method for examining the effect of surface-active solutes on bubble coalescence in the presence of ultrasound. The results show similarities with results obtained for bubble coalescence in the absence of ultrasound. The sharp decline in total bubble volume observed as surface activity increases is indicative of coalescence inhibition by surface-active solutes. This study found that for nonionic surface-active solutes, the coalescence inhibition is governed purely by the surface activity of the solutes and, more specifically for alcohols, by the length of the hydrocarbon chain. However, for charged surfactants, electrostatic effects also play a significant role in the extent of coalescence inhibition. As mentioned in the Introduction, the findings reported in this paper are relevant to a range of both industrial and medical applications of ultrasound, where bubble population and sizes are important. Acknowledgment. We would like to acknowledge Prof. Franz Grieser for his valuable comments and suggestions, and Oliver Winchester and Timothy Lauricella for their help in repeating some experimental work to confirm the reproducibility of the data. The financial support from Kodak and from the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council, is also acknowledged. J.L. receives a postgraduate stipend from the Department of Chemical and Biomolecular Engineering within the University of Melbourne, and this support is also gratefully acknowledged. References and Notes (1) Cain, F. W.; Lee, J. C. J. Colloid Interface Sci. 1985, 106, 70. (2) Machon, V.; Pacek, A. W.; Nienow, A. W. Trans Inst. Chem. Eng. 1997, 75, 339. (3) Deschenes, L. A.; Barrett, J.; Muller, L. J.; Fourkas, J. T.; Mohanty, U. J. Phys. Chem. B 1998, 102, 5115. (4) Christenson, H. K.; Yaminsky, V. V. J. Phys. Chem. 1995, 99, 10420. (5) Crabtree, J. R.; Bridgwater, J. Chem. Eng. Sci. 1971, 26, 839. (6) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. J. Phys. Chem. 1993, 97, 10192. (7) Oolman, T. O.; Blanch, H. W. Chem. Eng. Commun. 1986, 43, 237. (8) Valkovska, D. S.; Danov, K. D.; Ivanov, I. B. Colloids Surf., A 1999, 156, 547. (9) Valkovska, D. S.; Danov, K. D.; Ivanov, I. B. Colloids Surf., A 2000, 175, 179. (10) Weissenborn, P. K.; Pugh, R. J. J. Colloid Interface Sci. 1996, 184, 550.
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