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J. Phys. Chem. B 2003, 107, 14124-14129
Sonoluminescence Emission from Aqueous Solutions of Organic Monomers Gareth J. Price,*,† Muthupandian Ashokkumar,‡ and Franz Grieser‡ Department of Chemistry, UniVersity of Bath, BATH, BA2 7AY, U.K., and Particulate Fluids Processing Centre, School of Chemistry, UniVersity of Melbourne, Melbourne, Victoria 3010, Australia ReceiVed: February 13, 2003; In Final Form: August 18, 2003
The quenching of multibubble sonoluminescence, MBSL, by seven organic monomers in aqueous solution using ultrasound at two frequencies is reported. The results at 515 kHz closely parallel those reported for other simple organic compounds in solution. However, at 20 kHz, significant differences from those at the higher frequency were observed with lower degrees of quenching. In some cases, enhancement of the MBSL was observed. These effects are accounted for in terms of a model where monomer adsorbs to the bubble/ solution interface and different types of cavitation can be observed at the two frequencies. The implication of the results for polymerization of these monomers is discussed.
Introduction Over the past two decades, a number of uses have been developed for high-intensity ultrasound in synthetic and materials chemistry.1-3 Many chemical reactions are enhanced under sonication and benefits have accrued in the preparation of pharmaceutical intermediates, metal and other inorganic particles, colloidal systems and polymers. The sonochemistry of heterogeneous systems is particularly rich and diverse. Although there has been less study of homogeneous, single-phase systems,4 these are important in a number of areas. For example, radical production arising from sonication has been used as an initiation method in vinyl monomers. There is also considerable interest in the sonochemistry of aqueous solutions from the point of view of environmental remediation.5,6 However, despite these applications, there remain doubts as to the precise mechanisms of many of these reactions. The effect of experimental factors such as the ultrasound frequency is also incompletely understood. Most sonochemical processes result from cavitation7sthe growth and violent collapse of microscopic bubbles as the alternate compressions and rarefactions of the sound wave propagate through the liquid. The high temperatures that result in the core of the collapsing bubble cause dissociation of molecules and the creation of radicals and other excited-state reactive intermediates. The main species formed during sonication of water are H• and OH• radicals.8 These may recombine to form water, H2 gas, or H2O2. Alternatively, OH• radicals may react with volatile solutes that evaporate into the bubble during its growth or they may diffuse away and react with dissolved species in solution or at the bubble-solution interface. A parallel reaction pathway exists for volatile solutes that can enter the bubble and may be pyrolyzed by the high temperatures. One consequence of cavitation is the emission of a brief flash of light, known as sonoluminescence, SL. This arises from vibronically excited states of molecules produced as a result of the high temperatures and pressures that are generated within * Author for correspondence. Tel: +44 1225 386504. Fax: +44 1225 386231. E-mail:
[email protected]. On sabbatical leave at the University of Melbourne. † University of Bath. ‡ University of Melbourne.
the core of bubble.9,10 It is well documented that the intensity of SL and the emission spectra depend on the nature of the liquid and any dissolved gases or solutes.9,11,12 It has recently been shown that measurement of SL can give valuable information regarding sonochemical processes. For example, millimolar concentrations of aliphatic alcohols, amines, and carboxylic acids in aqueous solution cause efficient quenching of the SL from 515 kHz ultrasound by evaporation into the bubbles and reaction with excited-state intermediates and/or by lowering the effective temperature at the bubble core.13,14 Conversely, SL emission can be enhanced by the addition of anionic and cationic surfactants11,15 or salts such as NaCl or MgSO4. This was attributed to electrostatic effects modifying the interactions between bubbles, hence changing the nature of the bubble field. Ashokkumar et al. recently showed that quenching of 515 kHz SL by carboxylic acids depended on the pH and occurred only under conditions where un-ionized acid was present in solution.13,14 Didenko et al.16 showed that the SL behavior at 20 kHz depended on the nature of the solute studied. For example, addition of 0.1-0.4% benzene to water under argon gas (which will produce higher bubble core temperatures than saturation with air) quenched the SL intensity and the emission spectra contained bands corresponding to excited C2* formed during decomposition of benzene in the bubble. Solutions containing similar concentrations of carbon tetrachloride also showed C2* emission but no quenching of the overall SL was observed whereas carbon disulfide increased the SL but did not show C2* bands. The enhancement of SL was attributed to collisioninduced fluorescence of CS2. The effect of the ultrasound frequency on sonochemical processes is generally not well understood. One suggestion is that “stable” or “ repetitive transient” bubbles give rise to SL at high frequencies. These are bubbles that exist for many (thousands of) acoustic cycles. At lower frequencies such as 20 kHz, transient cavitation, where bubbles exist for only a few cycles, is the primary source of SL although Crum and Reynolds17 observed SL from both stable and transient bubbles at a frequency of 20 kHz. Recently, it has been demonstrated that SL quenching shows different behavior with the sound frequency.18 For example, addition of C1 to C4 alcohols showed
10.1021/jp034375t CCC: $25.00 © 2003 American Chemical Society Published on Web 11/22/2003
Sonoluminescence Emission of Organic Monomers no quenching of SL at 20 kHz whereas efficient quenching was observed at 515 kHz. The enhancement of SL due to addition of a surfactant or salt was also markedly reduced at the lower frequency. In addition to the applications of aqueous sonochemistry outlined above, a number of workers have applied ultrasound to the radical polymerization of vinyl monomers, both as solutions in water and in emulsion reactions.19-23 Though there has been some speculation on the initiating species in these reactions, no conclusive evidence has been presented. Thus, to provide information on the mechanism of the polymerization and the behavior of these compounds under sonication, it was of interest to determine whether unsaturated vinyl compounds behave in the same manner as the previously studied saturated compounds. This communication presents SL quenching measurements as well as reaction product analyses for seven vinyl monomers in aqueous systems using ultrasound frequencies of 515 kHz (for comparison with previously reported results) and (because most polymerization work has been conducted at lower frequencies) 20 kHz. Experimental Section SL measurements were performed in air using a cylindrical Pyrex glass cell placed over the transducer-horn assembly. The photoemission intensities were measured on a Hamamatsu endon photomultiplier that was responsive between 300 and 650 nm. Signals were displayed and averaged on a digital oscilloscope and stored on a personal computer for further data processing. Each reported value was averaged over at least 500 pulses. The variation in SL emission intensity15 over the 500 pulses was 10 MΩ. For experiments involving acrylic or methacrylic acids, the pH was controlled by adding small volumes of NaOH or HClO4. Over the short period of sonication, no significant change of pH was noticed. To prevent any effects due to varying ionic strength, a constant background concentration of 0.1 M NaClO4 was maintained for these measurements. No background electrolyte was added for experiments with the neutral monomers. In separate experiments, portions of the monomer solutions were placed in septum-sealed vials and sonicated on the apparatus described above. Periodically, the headspace above the solutions was sampled and 100 µL was injected onto a Shimadzu GC17A gas chromatograph with a J&W GSCarbonPLOT column at 80 °C. Reaction products were identified by comparison with known standards.
J. Phys. Chem. B, Vol. 107, No. 50, 2003 14125
Figure 1. Sonoluminescence emission (relative to pure water) for airsaturated aqueous solutions of methacrylic acid sonicated at 515 kHz at various pH values: (×) 9.7; ([) 8.8; (9) 7.8; (O) 6.9; (*) 6.0; (b) 5.6; (4) 5.1; (0) 4.4; (2) 3.2; (]) 2.2.
Figure 2. Sonoluminescence emission (relative to pure water) for airsaturated aqueous solutions of methacrylic acid sonicated at 20 kHz at the indicated pH values.
The surface tensions of aqueous solutions of the compounds were measured at room temperature using a McVan Analite Surface Tension Meter with a glass Wilhemy plate. Results The SL emission (relative to that of pure water) for aqueous solutions of methacrylic acid sonicated at 515 kHz is shown in Figure 1. It is clear that the SL is highly pH dependent. Quenching occurs at low pH; for example, over 90% of the emission is quenched at concentrations around 50 mM for pH < 4. Conversely, at high pH, the SL is within (10% of that in water across the whole concentration range studied and there is no significant degree of quenching. In contrast, the equivalent measurements carried out using an ultrasound frequency of 20 kHz, shown in Figure 2, show no discernible pattern. There is considerably less quenching than at the higher frequency; the minimum observed emission is around 80-85% of the water value. Similar results were obtained for acrylic acid so that there is a clear difference in the SL quenching at the two frequencies. The five neutral monomers each caused effective quenching of SL arising from sonication at 515 kHz as shown in Figure 3. Even for the least efficient quencher, methyl acrylate, a concentration of 10 mM was sufficient to quench 90% of the luminescence observed from pure water. Again, there is a very significant frequency effect, the results recorded using 20 kHz being shown in Figure 4. The efficiency of quenching follows an order different from that at the higher frequency and the
14126 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Figure 3. Sonoluminescence emission (relative to pure water) for airsaturated aqueous solutions of vinyl monomers sonicated at 515 kHz.
Price et al.
Figure 5. pH variation of the relative SL emission at two frequencies from 25 mM solutions of methacrylic acid, MAA. The dashed line shows the concentration of free acid calculated from the Literature pKa value.
TABLE 1: Physical Properties of Monomers25
styrene butyl acrylate methyl methacrylate vinyl acetate methyl acrylate acrylic acid methacrylic acid a
Figure 4. Sonoluminescence emission (relative to pure water) for airsaturated aqueous solutions of vinyl monomers sonicated at 20 kHz. (Inset shows magnification at low concentrations.)
minimum emission was ∼40% of that from water. The major difference is that at very low concentrations, the emission was enhanced by up to 50%. This is seen clearly in the inset to Figure 4. This enhancement was not seen at 515 kHz and has not previously been reported. In contrast to the results from the higher frequency, concentrations of 10 mM caused reductions of only around half of the SL even with the most efficient quencher. Further information on the sonochemistry occurring is given by analysis of the reaction products. Gas chromatograms recorded after 15 min sonication of 5 mM aqueous solutions of butyl acrylate at 515 kHz and 6 h sonication at 20 kHz indicated significant differences. At the higher frequency, significant amounts of methane and C2 hydrocarbons (corresponding to the four peaks detected) were present. This is indicative of pyrolysis of the monomers occurring in the bubble. Previous work24 has shown that these compounds are major products of sonicating organic solutes. Because these hydrocarbon products are poorly soluble in water, they can diffuse into and accumulate in the cavitation bubbles. For solutions of styrene, the major product was acetylene, as expected given the lack of alkyl groups. For the other monomers, pyrolysis produced methyl and other hydrocarbon radicals, reactions of which give rise to methane and the C2 hydrocarbons. In solutions of acrylic and methacrylic acids, relatively large amounts of methane, acetylene, and ethene were detected after 15-30 min of 515 kHz sonication, at pH 1.9. However, sonication of up to 120 min under the same conditions at pH 9.0 produced amounts of these products of less than 1% of those at low pH, indicating that little material was subjected to pyrolysis under these conditions.
vapor pressure at 25 °C/Torr
log10(P)a
water solubility at 20 °C/mM
6.4 5.5 38.5 90.1b 86.6 3.97 0.99
2.95 2.36 1.38 0.73 0.80 0.35 0.93
3.0 15.6 150.0 232.6 574.4 13900 1035
P ) octanol - water partition coefficient. b Value at 20 °C.
Differences in behavior between the two frequencies used in this work are again manifest in the GC results. At 20 kHz, sonication of either of the acids at low or high pH or of the other monomer solutions for 6 h produced no detectable methane, acetylene, or ethene. It should be noted that due to the difference in ultrasound frequency, the time periods used involve approximately equivalent numbers of acoustic cycles. The same general phenomena were observed for all the monomers investigated. Discussion The pH dependence of the SL quenching for solutions containing methacrylic acid is clearly demonstrated in Figure 5, which shows the relative emission at both frequencies from concentrations of 25 mM. The pKa of MAA is 4.6525 and the percentage of free, un-ionized acid remaining in solution calculated from this value is shown in Figure 5 as the dashed line. It is apparent that the SL is quenched when free, un-ionized acid exists in solution but not when it is all converted to its ionic form. In the molecular form, methacrylic acid has appreciable volatility, as shown by its vapor pressure listed in Table 1, and can therefore evaporate into the bubble where it can undergo pyrolysis. This may quench the SL by reaction with emitting intermediates and/or lowering the core temperature of the bubble during collapse. However, the ionic form of the acid is nonvolatile and thus cannot enter the bubble to quench the SL. These results are similar to those obtained in previously reported studies on aqueous solutions of saturated aliphatic carboxylic acids13,14 sonicated at 515 kHz. Also shown in Figure 5 are the results obtained for solutions with the same concentration using an ultrasound frequency of 20 kHz. There is rather higher uncertainty in these results due to the less well-defined nature of the sound field. However, it is clear that no significant quenching occurred across the whole pH range. As shown in Figure 6, similar effects were noted for
Sonoluminescence Emission of Organic Monomers
J. Phys. Chem. B, Vol. 107, No. 50, 2003 14127
Figure 6. pH variation of the relative SL emission at two frequencies from 25 mM solutions of acrylic acid, AA. The dashed line shows the concentration of free acid calculated from the Literature pKa value.
Figure 7. Relative SL emission at 515 kHz as a function of surface excess concentration of the monomers.
solutions of acrylic acid (pKa ) 4.25). With this monomer, there was a small amount (∼20%) of residual quenching even at high pH. NMR analysis showed the monomer sample to contain ∼12% of an impurity that was not removed by column purification. If this were volatile and/or surface active, it could preferentially enter the bubble and cause quenching even when the acid was in the ionic form. However, again there was no significant difference in quenching between pH 2 and pH 10 at 20 kHz even though quenching of 60-80% occurred at this concentration at low pH under sonication at 515 kHz. The order of quenching efficiency revealed in Figure 3 reflects that of the hydrophobicity of the compound as measured by the octanol-water partition coefficients shown in Table 1. Previously published results11,13 concerning aliphatic carboxylic acids, bases, and alcohols showed that the SL quenching correlated not with the bulk concentration of solute but with the concentration at the bubble-solution interface. The interfacial concentration can be estimated26 from the variation of the surface tension, γs, with concentration, c, in terms of the interfacial excess, Γ, and the Gibbs adsorption isotherm.
Γs ) -
( )
( )
c dγs 1 dγs )RT dc RT d ln c
(1)
Figure 7 shows the SL results as a function of the surface excess concentration of methacrylic acid in both ionic and molecular forms together with those for the neutral monomers. This figure has been constructed in the same way as reported in previous studies.11,13 The measurements reported here for MAA correlate well with the previously published data on other aliphatic acids and
Figure 8. Relative SL emission at 20 kHz as a function of surface excess concentration of the monomers.
alcohols. The four aliphatic monomers, methyl methacrylate, methyl acrylate, vinyl acetate, and butyl acrylate show results very similar to each other although they are rather different from those for the acids and alcohols, being more efficient quenchers at lower initial concentrations. These results reinforce previous conclusions on the importance of the bubble-solution interface in determining sonochemical properties when high ultrasound frequencies are used. In particular, the monomers are more hydrophobic than the acids and alcohols and so will adsorb to the interface more effectively. Unlike the results obtained with aliphatic alcohols, organic acids, and bases, the SL quenching by the monomers is not uniformly linked to their surface excess value. The variation shown in Figure 7 suggests that there are other as yet unidentified factors involved, which affect the SL quenching efficiency. The treatment in this manner of the results obtained at 20 kHz is presented Figure 8. As noted above, the systems show very significant differences from those at the higher frequency. Whereas styrene was one of the most efficient quencher at 515 kHz, it shows only enhancement of SL when irradiated at 20 kHz. Note that the maximum solubility of styrene in water at 25 °C is ∼3 mM so that it is not possible to determine whether quenching would occur at higher concentration values. Butyl acrylate enhances the SL at very low concentrations but quenches as the amount in solution increases. The other three monomers increase in quenching efficiency with rising vapor pressure or decreasing hydrophobicity although the effect does not follow a simple quantitative relationship. The major conclusion to be drawn here is that the behavior at 20 kHz seems to be influenced by a wider range of factors than simply the interfacial concentration, which was the major controlling factor at 515 kHz. The lack of enhancement or quenching at 20 kHz noted here with the two acidic monomers parallels that recently reported with short chain alcohol solutes.18 Thus, there are two competing processes in operation, one which enhances the SL and one which quenches. The observed behavior is the resultant of these effects. Clearly then there are very significant differences in the SL emission and quenching processes at the two frequencies. One explanation for these observations is the potentially different nature of inertial cavitation at the two frequencies. At 515 kHz, the predominant effect will be “stable” cavitation where a bubble undergoes many oscillations during its lifetime.7 This leads to significant amounts of dissolved material adsorbing to the bubble-solution interface and evaporating into the bubble. The lifetime of each individual collapse and associated hot spot is 0.1 M) amounts of relatively hydrophilic solutes such as methanol (log P ) -0.77) and ethanol were required to prevent air bubbles from coalescencing together but that smaller amounts of more hydrophobic compounds (e.g., 0.2 mM for pentanol; log P ) 1.51) are needed to give the same effect. Thus, we speculate that the most
hydrophobic monomers, styrene and butyl acrylate, will adsorb to the surface of the bubble and hence keep the small, SL active bubbles separated, preventing their coalescence, as would happen in water. In this way, the number of bubbles that are able to undergo SL emission increases from that in the absence of solute and the size of the bubble field is also larger, giving the observed enhancement in the emission intensity. We favor this explanation over the possibility of the creation of additional emitting species inside the bubble because the latter would be expected to change the appearance of the SL spectrum. No significant differences in the (albeit relatively low resolution) spectra were observed. That the enhancement in SL does not occur at the higher frequency may be explained in two ways. The better defined sound field and smaller bubble size at the higher frequency will maintain a better separation of emitting bubbles so that there is less difference between the bubble population in water and dilute solution. Acoustic streaming is also less effective at the higher frequency so that there is a smaller chance of small bubbles colliding and coalescing. Additionally, any increase in the “active” bubble population in the presence of the solutes may be masked by the dominating effect of the SL quenching. It would appear that the enhancement effect at 20 kHz is important at low concentrations and overcomes any tendency to quench because there will be only small amounts of material available inside the bubble. With the less hydrophobic monomers, it seems that any interfacial adsorption is not sufficient to prevent coalescence. With all the monomers, material is able to enter the bubble and quench the SL but the quenching efficiency follows the trend opposite to that at high frequency and here the vapor pressure seems to be the major factor. This is explained because the amount of material that is able to evaporate into the bubble in a single or a small number of collapses rather than that at the interface will now be the limiting factor. This explanation is currently somewhat speculative and further work is underway to clarify the reasons for the observed effects. The results reported here have implications for the use of ultrasound in aqueous chemical processes. Significant differences in the types of compound undergoing degradation at different frequencies in studies of sonochemical environmental remediation have been reported.29-31 These results strongly suggest that for those processes requiring “mechanical” effects such as shear or homogenization, low frequencies would be expected to be more efficient but for reactions involving production and subsequent reaction with OH• radicals, better results would be expected at 500 kHz. Our ongoing studies on acoustic emission spectra from aqueous solutions at these differing frequencies provide additional evidence for the existence of transient/stable cavitations at the two frequencies. This will be discussed more fully in a forthcoming manuscript.32 The majority of published work on sonochemical polymerization has involved 20 kHz frequency ultrasound and the most studied monomers are styrene and methyl methacrylate. The latter polymerizes easily23 under sonication even in the absence of an added initiator. Some workers have reported the polymerization of styrene in an emulsion under similar conditions although others have reported that a chemical initiator is necessary, so thermal effects may not have been completely eliminated. The results here show MMA quenches SL to a greater extent than styrene at 20 kHz, implying that more monomer enters the bubble to react and form initiating radical species, which can then enter the solution and start polymeri-
Sonoluminescence Emission of Organic Monomers zation. Conversely, the amount of styrene available for sonochemical reaction in and around the bubble is limited by its solubility. Conclusions The work described here reinforces previous work that suggests that there is a significant frequency effect in the SL emission from dilute aqueous solutions. In contrast to previous work on simple organic alcohols and acids, enhancement of SL has been observed when solutions of vinyl monomers are sonicated at 20 kHz. The results for these compounds at 515 kHz show properties similar to those displayed by other organics where the dominating effect is the interfacial concentration around the bubble. At the lower frequency, other factors such as inter-bubble interactions become important. Thus the work suggests a basis for explanation of observed frequency effects in sonochemical systems although further work is necessary to provide a complete explanation. Acknowledgment. We acknowledge the support of the Australian Research Council and the EU COST D-10 program. G.J.P. also gratefully acknowledges financial support from the University of Melbourne (Visiting Research Scholars Award), The Royal Society (Study Visit Award), and the Royal Society of Chemistry (JWT Jones Travelling Fellowship). We also acknowledge the excellent experimental assistance provided by T. D. Cowan for some of the work presented. References and Notes (1) Suslick, K. S.; Price, G. J. Ann. ReV. Mater. Sci. 1999, 29, 295. (2) Cains, P. W.; Martin, P. D.; Price, C. J. Org. Process Res. DeV. 1998, 2, 34. (3) Cintas, P.; Luche, J. L. Green Chem. 1999, 1, 115. (4) Segebarth, N.; Eulaerts, O.; Kegelaers, Y.; Vandercammen, J.; Reisse, J. Ultrasonics Sonochem. 2002, 9, 113. (5) Weavers, L. K.; Malmstadt, N.; Hoffmann, M. R. EnViron. Sci., Technol. 2000, 34, 1280.
J. Phys. Chem. B, Vol. 107, No. 50, 2003 14129 (6) Peters, D. Ultrasonics Sonochem. 2001, 8, 221. (7) Leighton, T. The Acoustic Bubble; Academic Press: London, 1994. (8) Riesz, P.; Kondo, T.; Krishna, C. M. Ultrasonics 1990, 28, 295. (9) Sehgal, C.; Sutherland, R.; Verrall, R. J. Phys. Chem. 1980, 84, 388. (10) Didenko, Y.; McNamara, W.; Suslick, K. Nature 2000, 407, 877. (11) Ashokkumar, M.; Hall, R.; Mulvaney, P.; Grieser, F. J. Phys. Chem. B 1997, 101, 10845. (12) Didenko, Y. T.; McNamara, W. B.; Suslick, K. S. Phys. ReV. Lett. 2000, 84, 777. (13) Ashokkumar, M.; Mulvaney, P.; Grieser, F. J. Am. Chem. Soc. 1999, 121, 7355. (14) Ashokkumar, M.; Vinodgopal, K.; Grieser, F. J. Phys. Chem. B 2000, 104, 6447. (15) Ashokkumar, M.; Grieser, F. AdV. Colloid Interface Sci. 2001, 8990, 423. (16) Didenko, Y. T.; McNamara, W. B.; Suslick, K. S. J. Phys. Chem. A 1999, 103, 10783. (17) Crum, L. A.; Reynolds, G. T. J. Acoust. Soc. Am. 1985, 78, 137. (18) Tronson, R.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2002, 106, 11064. (19) Biggs, S.; Grieser, F. Macromolecules 1995, 28, 4877. (20) Chou, H. C. J.; Stoffer, J. O. J. Appl. Polym. Sci. 1999, 72, 797. (21) Vivekanandam, T. S.; Gopalan, A.; Vasudevan, T. J. Polym. Mater. 1998, 15, 261. (22) Price, G. J. In NoVel methods of Polymer Synthesis II; Ebdon, J. R., Eastmond, G. C., Eds.; Blackie: Glasgow, 1995; p 117. (23) Bradley, M.; Grieser, F. J. Colloid Interface Sci. 2002, 251, 78. (24) Tauber, A.; Mark, G.; Schuchmann, H. P.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2 1999, 1129. (25) Physical Properties Database (http://esc.syrres.com/interkow/PhysProp.htm); Syracuse Research Corp.: Syracuse, New York. (26) Adamson, A. W., Physical Chemistry of Surfaces, 5th ed.; WileyInterscience: NY, 1990; pp 75-86 (27) Joos, P.; Serrien, G. J. Colloid Interface Sci. 1989, 127, 97. (28) Oolman, T. O.; Blanch, H. W. Chem. Eng. Commun. 1986, 43, 237. (29) Beckett, M. A.; Hua, I. J. Phys. Chem. A 2001, 105, 3796. (30) Sivakumar, M.; Tatake, P. A.; Pandit, A. B. Chem. Eng. J. 2002, 85, 327. (31) Petrier, C.; Francony, A. Water Sci. Technol. 1997, 35, 175. (32) Price, G. J.; Ashokkumar, M.; Grieser, F. Manuscript in preparation.
