18461
2007, 111, 18461-18463 Published on Web 11/29/2007
Estimation of Cavitation Bubble Temperatures in an Ionic Liquid Parag M. Kanthale, Muthupandian Ashokkumar,* and Franz Grieser* Particulate Fluids Processing Centre, School of Chemistry, UniVersity of Melbourne, Melbourne, Victoria 3010 Australia ReceiVed: October 19, 2007; In Final Form: NoVember 14, 2007
The cavitation bubble temperatures in a room-temperature ionic iquid, 1-Ethyl-3-methylimidazolium-ethyl sulfate (EMIS), are presented that have been estimated using the methyl radical recombination (MRR) method. The temperatures measured in pure EMIS are of the same order as those observed in other solvents, that is, in the range of 3000-5000 K.
Room-temperature ionic liquids (RTILs) possess a number of remarkable intrinsic properties, such as, negligible vapor pressure, high thermal stability, and in many cases can act as “green solvents” with low chemical reactivity, in which to conduct organic and inorganic chemical reactions. These physicochemical features have led to the use of RTILs as replacements for common organic solvents in chemical syntheses, biotechnology applications, and in electrochemical and separations processes.1-4 There have been only a few reports on the use of ultrasound in RTILs for the synthesis of organic compounds.5,6 Suslick’s group has investigated some sonochemical reactions in a few RTILs.7,8 These studies have reported a significant increase in the rates of sonochemical reactions relative to other solvent systems as well as some selectivity in the reaction products. Sonochemistry (SC) originates from the phenomenon of acoustic cavitation in a fluid, that is, ultrasound-induced formation, growth, and violent collapse of microbubbles in a liquid medium.9 The implosion of cavitation bubbles is sufficiently violent to generate localized temperatures and pressures on the order of 5000 K and hundreds of atmospheres, respectively. These localized conditions lead to the formation of very reactive radicals and excited-state species within the bubbles.9 Because the efficiency of sonochemical reactions is directly related to the temperatures generated within collapsing bubbles, it is of fundamental interest to know the temperatures developed within cavitation bubbles. To the best of our knowledge, there has been no experimental investigation of the cavitation bubble temperatures generated in RTILs. In this report, the cavitation bubble temperatures in RTILs are presented that have been estimated using the methyl radical recombination (MRR) method.10 The MRR method relies upon the relative amounts of ethane, ethylene, and acetylene from methyl radicals generated within the high-temperature conditions of cavitation bubbles. The full details of the method can be found in our previous work;11-13 it is suffice to say that the cavitation bubble temperatures were estimated from the concentration ratio {([C2H4] + [C2H2])/[C2H6]}.10-13 The acoustic cavitation was generated in the present study using a plate-type transducer (ELAC Nautik) operated at an ultrasonic frequency of 213 kHz and at an acoustic power of * Corresponding authors. E-mail:
[email protected]; franz@ unimelb.edu.au.
10.1021/jp710148k CCC: $37.00
40 W that was measured calorimetrically. 1-Ethyl-3-methylimidazolium-ethyl sulfate (EMIS) was selected for this study because of its low toxicity and the ability to cause acoustic cavitation within the fluid. EMIS was supplied by Sigma Aldrich (>95% purity). In a typical experiment, 8 mL of EMIS, in pure form or with different concentrations of added t-butyl alcohol, was placed into a 15 mL glass reaction vial and then purged with argon for 10 min prior to sonication and sealed from the atmosphere. Milli-Q (Millipore) water (250 mL) was placed in a glass cell that was fitted on top of the transducer plate. The vial was immersed in the Milli-Q water in such a way that the bottom was always 5.5 cm from the transducer plate. The temperature of the cell contents was maintained at about 55 °C by circulating thermostated water through the jacketed glass cell. After sonication, the headspace of the vial was analyzed for hydrocarbon gases using gas chromatography (GC) (Shimadzu GC-17A). GC analysis revealed that the hydrocarbon (HC) gases generated during sonication were methane, ethane, ethylene, acetylene, and some other unidentified gases. Figure 1 shows the amount of selected hydrocarbon products produced in pure EMIS and at different concentrations of t-butanol in EMIS. It can be observed in Figure 1 that low, but measurable, quantities of hydrocarbon products were produced in pure EMIS. This indicates that some EMIS is decomposed under the sonication conditions used. Previous studies have also found that RTIL may decompose upon sonication.7,8 Because EMIS is a nonvolatile liquid, the pyrolytic products may come from either the decomposition of some entrained nanodroplets of EMIS within a collapsing bubble or from the decomposition of material at the bubble interface; the bubble interface can reach temperatures of around 2000 K.14 In Figure 1, it can also be seen that the quantities of the HC gases produced strongly depend upon the concentration of t-butanol. The amount of HC gases produced initially increases with an increase in the concentration of t-butanol. However, above 10 mM t-butanol, the quantity of gases produced decreases. These results are similar to those reported for the sonication of t-butanol and other aliphatic alcohols in water.10,13 The generation of HC gases in larger quantities with added t-butanol, compared to that in pure EMIS, can be explained by the fact that t-butanol is volatile and can therefore evaporate © 2007 American Chemical Society
18462 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Figure 1. Effect of t-butanol concentration on hydrocarbon gases produced following 30 min of sonication. The yields of ethane, ethylene, and acetylene were estimated using a standard calibration procedure. (The concentrations were also corrected for the solubility of these gases in EMIS.)
Figure 2. Bubble temperatures in pure EMIS and in EMIS containing 50 mM t-butanol.
into the bubble core. The evaporated alcohol molecules may then undergo thermolytic reactions within the bubbles because of the high local temperature conditions leading to the formation of HC products. As the concentration of t-butanol increases in EMIS, it can be expected that more alcohol would evaporate into the bubbles during the expansion phase of a bubble oscillation. This would have two consequences: an increase the amount of material that can be decomposed, hence more (HC) gases, and a counter-effect of a decrease in the core temperature of a bubble due to the energy from the collapse being used in endothermic bond rupture reactions as well as alterations in the thermal characteristic of the bubble interior.10-14 The two opposing trends, in terms of HC product formation, will have the effect of producing a product yield maximum, as can be seen in Figure 1 for ethane and ethylene. Figure 2 shows the temperature of cavitation bubbles in pure EMIS and in the presence of 50 mM t-butanol in EMIS following different periods of sonication. On first inspection, it is perhaps surprising to have obtained such a relatively low temperatures in EMIS. Acoustic cavitation in a nonvolatile liquid can be expected to generate a significantly higher temperature compared to that generated in volatile liquids, such as water or other organic solvents. However, the temperatures measured in pure EMIS are of the same order as those observed in other solvents, that is, in the range of 3000-5000 K.10-15 The injection of liquid droplets into a cavitation bubble as well as the decomposition of solvent molecules at the bubblesolution interface may be responsible for the observed lower temperatures in RTIL. It should be noted that lower than expected cavitation bubble temperatures have been observed in other systems, for example in SBSL studies, due to the interference of endothermic chemical reactions.16-18 An interesting feature to note from Figure 2 is that the temperature of cavitation bubbles after 10 minutes sonication
Letters
Figure 3. Effect of t-butanol concentration on bubble temperature for 30 min of sonication.
is similar in pure EMIS and in the presence of 50 mM t-butanol. This indicates the generation of a significant amount of volatile gases from the decomposition of EMIS. It can also be observed that the temperature is almost constant in pure EMIS with increasing sonication time. However, for t-butanol solutions, the cavitation temperature decreases significantly with an increase in sonication time. For t-butanol solutions, because of the alcohol’s relatively high volatility, the solute can be expected to evaporate into a bubble leading to the formation of more gaseous products that may themselves accumulate within an oscillating bubble.11 The cavitation bubble temperatures were measured for different concentrations of t-butanol in EMIS, and the results are shown in Figure 3. The trend observed, of a decrease in the measured temperature with an increase in the alcohol concentration, is quite similar to what has been seen previously for t-butanol and other volatile hydrocarbon solutes in water.10-13 It is accounted for, as mentioned already, by the consumption of the heat of collapse in the core of the bubble by the presence of increasing amounts of t-butanol and decomposition products (increased amount of endothermic reactions). Extrapolating the results of Figure 3 to zero alcohol concentration yields a temperature of 3500 K, within error, the same as that obtained for pure EMIS (Figure 2). This provides some self-consistency for the MRR method for the estimation of the mean temperature of cavitation bubbles for the present system. Finally, it should be noted that the estimated temperatures obtained by the MRR method within cavitation bubbles are not peak collapse temperatures; rather, they represent the mean “chemical” temperature during the collapse.11 Nevertheless, the results presented in this study are significant. The speculation that the cavitation bubbles in RTILs can generate temperatures higher than those in more volatile fluids, because of their intrinsic negligible vapor pressure, is debatable based on the outcome of this study. The only way higher temperatures could be reached would be if decomposition products were minimized. This may be possible if lower ultrasound frequencies, such as 20 kHz, were used in the sonochemical process,19 and is the subject of further study. Acknowledgment. Financial and infrastructure support from the Australian Research Council and the Particulate Fluids Processing Centre are gratefully acknowledged. References and Notes (1) Keskin, S.; Talay, D.; Akman, U.; Hortacsu, O. J. Supercrit. Fluids 2007, 43, 150-180. (2) Krossing, I.; Slattery, J. M.; Daguenet, C.; Dyson, P. J.; Oleinikova, A.; Weingartner, H. J. Am. Chem. Soc. 2006, 128, 13424-13429. (3) Wang, Y.; Voth, G. A. J. Am. Chem. Soc. 2005, 127, 12192-12193. (4) Winterton, N. J. Mater. Chem. 2006, 16, 4281-4293.
Letters (5) Rajagopal, R.; Jarikote, D. V.; Srinivasan, K. V. Chem. Commun. 2002, 616-617. (6) Srinivasan, K. V.; Rajgopal, R. Ultrason. Sonochem. 2003, 10, 41-43. (7) Oxley, J. D.; Prozorov, T.; Suslick, K. S. J. Am. Chem. Soc. 2003, 125, 11138-11139. (8) Flannigan, D. J.; Hopkins, S. D.; Suslick, K. S. J. Organomet. Chem. 2005, 690, 3513-3517. (9) Suslick, K. S. Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc.: New York, 1998; pp 517-541. (10) Tauber, A.; Mark, G.; Schuchmann, H. P.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 1999, 2, 1129-1135. (11) Ashokkumar, M.; Grieser, F. J. Am. Chem. Soc. 2005, 127, 53265327.
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18463 (12) Ciawi, E.; Rae, J.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2006, 110, 9779-9781. (13) Rae, J.; Ashokkumar, M.; Eulaerts, O.; Von Sonntag, C.; Reisse, J.; Grieser, F. Ultrason. Sonochem. 2005, 12, 325-329. (14) Suslick, K. S.; Hammerton, D. A., Jr.; Cline, R. E. J. Am. Chem. Soc. 1986, 108, 5641-5642. (15) Ashokkumar, M.; Grieser, F. ChemPhysChem. 2004, 5, 439. (16) Yasui, K. Ultrasonics 1998, 36, 575-580. (17) Didenko, Y. T.; Suslick, K. S. Nature 2002, 418, 394-397. (18) Toegel, R.; Gompf, B.; Pecha, R.; Lohse, D. Phys. ReV. Lett. 2000, 85, 3165-3168. (19) Tronson, R.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. 2002, 106, 11064.
