158
J. Phys. Chem. 1993,97, 158-162
Sonochemistry and Sonoluminescence: Effects of External Pressure Arnim Henglein' and Maritza Gutierrez Hahn-Meitner- Institut Berlin GmbH, Bereich S, 1000 Berlin 39, FRG Received: July 20, 1992; In Final Form: October 13, 1992
The oxidation of iodide by 1-MHz continuous ultrasound and the sonoluminescence in water created by pulsed ultrasound were investigated in the pressure range from 0.7 to 3 bar. A t low intensities, the chemical yield and the luminescence intensity decrease with increasing pressure. At higher intensities the yields increase moderately with increasing pressure. There is a critical intensity above which a steep decrease in the chemical and luminescence yields occurs. This critical intensity is strongly shifted to higher values with increasing pressure. Thus, by applying higher pressures a t intensities above the critical intensity, the chemical yields and the luminescence intensity are increased by a factor of more than 10. The effects are not dependent on the nature of the gas under which the irradiation occurs. The effects are discussed in terms of the current theories of cavitation and gas bubble dynamics. The effects can be rationalized by assuming that there is a decreased rate of bubble formation with increasing pressure but that the probability that a particular bubble initiates chemical reactions increases with increasing pressure.
Introduction Redox processes, such as the oxidation of iodide in aqueous solution, are initiated by ultrasound, if a monoatomic or diatomic gas is present in the solution. Tiny gas bubbles are formed which grow to a certain resonance size, a t which they violently oscillate and collapse. Temperatures of several thousand Kelvin are produced during the adiabatic compression phase of the oscillating and collapsing bubbles, and these temperatures are high enough to cause the dissociation of molecules. For example, water vapor in the bubbles is decomposed to yield H atoms and OH radicals.' Only a few observations have been reported in the literature on the effect of the external atmospheric pressure under which a solution is irradiated. Polotskii2 found that the yields of H202, NO2-, an NO3-, which are formed during the irradiation of water under air, were greatly reduced if the pressure was only 0.1 bar. When the pressure was increased, the yields increased until a maximum at about 1.5 bar was reached, but at about 4 bar the yields were reduced to zero. A significant increase with pressure was also observed by Hart and Henglein3 in the decomposition of N 2 0 in argon bubbles. A systematic investigation of the pressure effects has not yet been made. In the first part of the present paper, the oxidation of iodide is studied at various ultrasonic intensities and external pressures between 0.7 and 3.0 bar. It is shown that the effects of pressure are complex, with both promoting and retarding effects being possible depending on the irradiation conditions. In the second part of this paper, the effects of the external pressure on the sonoluminescence of water are investigated. As in the previous luminescence studies? pulsed ultrasound was used. This allows one to obtain information not only about the luminescence intensity but also about the temporal buildup of luminescence. The frequency was 1 MHz. It has recently been shown5 that an 'unprecedented" intensity effect can be observed using ultrasound of such high frequency: The yield increases above a small threshold value with increasing intensity but decreases dramatically to a few percent within a very narrow intensity range above a critical intensity, and remains almost constant at this low value at still higher intensities. This 'fall-off", which has also been reported in a less dramatic fashion by other authors6 (to observe the effect, certain requirements on the irradiation geometry have to be fulfilled, see Figure l), is shown in the present paper to depend in an extremely sensitive manner on the external pressure. 0022-3654/58/2097-0158%04.00/0
v2
Figure 1. Schematic description of the geometry of the irradiation setup: Q,quartz plate (1 MHz); M, metal flange (1X); B. bottom (1 X) of glass vessel V I ; L, liquid; V2, upper part of vessel with stopcocks CI and C2 (inlet and outlet for gas bubbling).
Experimental Section Figure 1 shows schematically the irradiation setup. The ultrasound is emitted from the quartz plate Q and passes through the metal flange M (1X thick) and the bottom B (1X thick) of the glass vessel V Iinto the liquid L. A film of water between B and M ensures acoustic transparency. It is very important for efficient acoustic conduction that the thickness of the polished glass bottom B does not deviate from X by more than 0.05 mm. The steep fall-off can be observed only under these optimized conditions. The upper part V2 of the vessel carries two stopcocks CIand C1,which were closed during irradiation. Parts V Iand Vz are pressed together by a strong metal clamp. Joint J was lubricated by grease (in the case of underpressures) or sealed by a Teflon inset (in the case of overpressures). The desired gas was bubbled through the vessel before irradiation and the pressure was then adjusted to the desired value. The closed vessel was shaken for 1 min before irradiation to make sure that the solution reached equilibrium with the gas. The gases used were either oxygen or a mixture of argon (80%) and oxygen (20%); in this Ar-02 mixture, redox processes occur with especially high yields.' 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97,No. 1, 1993 159
Sonochemistry and Sonoluminescence
4.0
-2.0 N
Y
1.0
50 100 hf-power [ w a t t s ]
0
150
Figure 2. Yield of iodide oxidation as a function of hf power for various volumes of the irradiated solution.
3.0
OO
50 100 hf-power (watts1
150
Figure 4. Iodine yield as a function of hf power for various oxygen pressures.
150 w a t t s ; 0,
4.0 0 10
20
3.0
’
4
30
40
y!
50
volume [mLI
F i p e 3 . Yieldofiodideoxidationasafunctionofthevolumeofirradiated liquid at high hf power.
The concentration of iodine was determined spectrophotometrically (e = 2.6 X lo4 M-I cm-l a t 350 nm). The solution contained 0.2 M potassium iodide; it was buffered with phosphate (pH = 5.0) to make sure that the iodine formed was not decomposed.8 Absolute intensities of the ultrasound were not determined. The hf power picked up by the quartz is given in the figures as a relative measure of the intensity. The luminescence signals produced by the pulsed ultrasound were recorded by the photomultiplier P positioned near thevessel. Details of the method used for data acquisition and processing were described previously.4a
Results ‘Fall-W Effect. Generally 25 mL of solution was irradiated. Figure 2 shows the dependence of the iodine yield on the hf power picked up by the quartz plate. The solution was irradiated under normal atmospheric pressure, i.e. 1 bar. The curve for 25 mL shows that the yield increases up to 130 W and then falls off rapidly. The figure also contains the curves for the irradiation of other liquid volumes in the vessel. With 40 mL, the drop in the yield at high powers was not observed (within the power range available). In the case of the smaller volumes, the drop occurs at substantially lower intensities. With increasing volumes of solution the intensity of the ultrasound in the liquid becomes smaller. In addition, the distribution of the intensity within the liquid is less homogeneous (the parallel propagation of the ultrasound becoming worse because of scattering by the gas bubbles). We explain the early drop in the yield for the smaller volumes by a more homogeneous and especially strong ultrasonic field in the liquid. It is also instructive to describe the effect by plotting the yield versus the volume of the liquid at high hf power (1 50 W). Figure 3 shows that almost nochemistry is brought about insmallvolumes under these conditions. For 27 mL of irradiated solution, the yield starts to rise and it reaches a maximum at 30 mL of solution. At still larger volumes, the yield then slowly decreases again. Pressure Dependence of Iodide Oxidation. Figure 4 shows the yield as a function of the hf power for various oxygen pressures. The curve for 1 bar, Le. irradiation under normal atmospheric
-$- 2.0 N
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1.0
0
1.0
1.5
2.0 p Ibarl
2.5
3.0
Figure 5. Iodine yield as a function of pressure for various hf powers.
pressure, shows the fall-off effect above a critical hf power of 120 W. For the higher pressures, there is a continuous increase in yield with increasing hf power in the investigated power range, and a limiting value is eventually reached at higher hf powers. At the lower hf powers, the yields are smaller than in the irradiation under normal atmospheric pressure. However, they are substantially greater at high hf power, Le. beyond the critical hf power a t 1 bar. In Figure 5 , the yield is plottedversus the pressure for various hf powers. One can see that the yield decreases continuously for small hf powers (30and 50 W). At 80 W, the yield passes through a weak maximum between 1.5 and 2.0 bar and then decreases. At the highest hf powers (130and 150 W), the yields jump up very steeply by a factor of about 10 a t 1.1 bar and remain at a high level even up to 3 bar. In order to find out whether these effects depend on the nature of the gas, experiments with an argon-oxygen (80:20 vol 5%) mixture were carried out. Figure 6 shows the iodine yield as a function of the pressure for two intensities. The yields are much greater than in theirradiation under oxygen (Figure 5 ) , the reason being that argon is a monoatomic gas in which higher temperatures are generated during the adiabatic compression phase of the cavitation bubbles.’ However, the shapes of the curves in Figure 6 are not different from those in Figure 5: for 30 W, one observes a steady decrease in yield with increasing pressure, and for 150 W, a steep increase in the 1-1.1-bar range is followed by large yields up to 3 bar. It is concluded that the nature of the gas is not important for the occurrence of the pressure effects. In the experiments of Figure 7,pressures lower than the normal atmospheric pressure were applied. It can be seen that the falloff effect occurs at lower hf powers with decreasing pressure. The maximum yield reached before the fall-off occurs is greater with increasing pressure. When the hf power was very close to the critical intensity, a finely dispersed fog was emitted from the surface of the irradiated liquid.
Henglein and GutiCrrez
160 The Journal of Physical Chemistry, Vol. 97, No. 1, 1993
I
3.0
5ms
20ms
2ms
I
1ms
2.5
E 2.0
?
E -1.5 N
- 1.0
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1.5
2.5
2.0
3.0
Figure 9. Shape of luminescence pulses of different lengths, at two hf powers. 0n:off ratio = ]:IO.
p Ibarl Figure 6. Iodine yield as a function of pressure for two hf powers. Irradiation under argon4xygen (8020 vol 6%).
1.5
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0.5
,
,
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p Ibarl Figure 10. Intensity of sonoluminescence of water under Ar-02 (80:20 vol 9%) as a function of pressure for various hf powers.
0
150
50 100 hf-power l w a t t s l
0
Figure 7. Iodine yield as a function of hf power. Irradiation under lower pressures of the argon4xygen mixture.
1
2
3
4
5
6
7
8
9
io
ii
12
13
Figure 8. First 13 luminescence pulses generated by the irradiation of water under a Ar-02 (8020 vol %) atmosphere with a 5-ms ultrasonic pulse train (normal atmospheric pressure). 0n:off ratio = 1:lO. (The long time interval between the pulses is not correctly shown in the figure.)
Ressure Dependence of Sonoluminescence. When water is exposed to a pulse train of ultrasound, a luminescence train is produced. Ofspecial interest is the shapeof the first luminescence pulses of the train, as is illustrated in Figure 8. Water was irradiated under the argon-oxygen mixture with a train of 5-ms pulses, the interval between the pulses being SO ms. It can be seen that the first pulses of ultrasound are not as active as the subsequent pulses, because the luminescence intensity continues to build up until it reaches a steady-state value after the 12th pulse. The critical number of pulses required to reach the final height will be called iVcrit. The following pulses all have about thesameshape. This buildup of theluminescenceintensity,which indicates a cooperative action of the ultrasonic pulses, has previously been studied in the case of chemoluminescenceusing dissolved Luminol and has been explained in terms of the
characteristic times involved for the formation and growth of gas bubbles and for the decay of the bubbles in the interval between pulses.' The shape of the luminescence pulses varies with the length of the pulses and with the intensity of the ultrasound. This is shown in Figure 9 for irradiation under normal atmospheric pressure. Water was irradiated with pulse trains of various lengths, keeping the on:off ratio constant (1 :lo). In other words, when the pulse length was changed, the length of the interval between the pulses was altered accordingly. The profiles in Figure 9 were recorded for pulses in which the full luminescence signal had already developed. It can be seen that the luminescence pulses often do not follow the rectangular form of the exciting ultrasonic pulses. In the case of the short pulses (1 and 2 ms in Figure 9), there is a delay in the buildup of the luminescence intensity. This effect has previously been shown to become more pronounced with increasing interval between the P U I S ~ S . ~ .In ~ the discussion below, this effect is called the 'delay" effect. In the case of the longer pulses (5 and 20 ms in Figure 9). the luminescence intensity is built up almost without any delay. However, at the low hf power of 20 W, the intensity decays within about 5 ms until it reaches a constant value. The effect is practically absent at the high hf power of 50 W in Figure 9. After having described the main luminescence phenomena which are observed in the irradiation under normal preasure (Figures 8 and 9), we can now proceed to find out how variations in the external pressure affect (1) the luminescence intensity, (2) the critical pulse number, iVcrit,and (3) the shape of the luminescence pulses. Figure 10 shows the luminescence intensity of water as a function of the pressure of the Ar-02 gas for various hf powers. The comparison with Figures 5 and 6 shows that the sonoluminescence intensity and chemical yield behave quite similarly. It has previously been shown that the chemiluminescence yield from luminol also shows the same fall-off with increasing ultrasonic inten~ity.~ Figure 10 shows that the same effect exists for the sonoluminescence: at 150 W and normal pressure, the intensity
The Journal of Physical Chemistry, Vol. 97, No. 1 , 1993 161
Sonochemistry and Sonoluminescence
at the start of the pulse, regardless of thedecay effect that follows. In the following discussion, the intensity of a pulse always refers to this initial value.
Sms pulse H
20 w a t t s 1 bar
1
15
Discussion
17
16
18
20
19
21
22
24
23
25
26
21
I
Figure 11. Shape of the luminescence pulses of a train between pulse numbers 15 and 27 (Le. after the buildup is completed) for the irradiation of water under normal atmospheric pressure at an hf power of 20 W.
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5mx'sc 50 w a t t s
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L
1.5 bar
11
2
3
4 ' 5
6
7
I
8
I
9
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10
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Figure 12. Shape of the first 12 pulses in the irradiation at 50 W and 1.5 bar.
5ms pulse H
50 w a t t s 3 bar
1 1
2
3
4
s
6
7
a
9
io
11
121
Figure 13. Shape of the first 12 pulses in the irradiation at 50 W and 3 bar.
is very small, as one is operating beyond the critical intensity (see Figure 4). However, with increasing pressure at 150 W, the luminescence intensity rapidly increases to reach large values in the 1-2-bar range. The luminescence intensity plotted in Figure 10 is the intensity reached after the initial buildup, which needed Nc,i, pulses. It turned out that Ncri,was between 9 and 12 a t all pressures applied. The intensity of a pulse between pulse numbers 12 and 20 was therefore used in Figure 10. At the higher hf powers in Figure 10, these later pulses of a train had practically the same height (see the last three pulses in Figure 8). However, at the lower hf powers, the height of these pulses changed between an upper and lower value. For this reason, both values are indicated in the figure (the curves were drawn through the upper values). An example is shown in Figure 11, where the shape of the pulses of the train between numbers 15 and 27 are shown. One can see that a periodicity exists with respect to the height of the pulses, two strong pulses being followed by a weaker one. The pulses themselves show the 'decay" effect mentioned above (Figure 9). With increasing pressure, the decay effect becomes more pronounced. This can be seen from (Figures 12 and 13, where the first 12 pulses of the luminescence train are shown for 50 W and at two pressures. The shape of the pulses should be compared to the ones in Figure 8, where the pressure was normal (1 bar). Whereas at 1 bar there is practically no decay, it is already very pronounced at 1.5 bar (Figure 12) and even more so at 3 bar (Figure 13). Therefore in order to make comparisons of the data easier, the intensity plotted in Figure 10 is the initial intensity
Sonochemistry. According to the current theory of cavitation inception and gas bubble dynamics,Ic* an increase in the external pressure should lead to an increase in both the cavitation threshold and the intensity of bubble collapse. We have not tried to determine the cavitation threshold as a function of external pressure. However, the decrease in chemical yield with increasing pressure, which is observed at low ultrasonic intensities (for example, 30 W in Figure 5 and 6), may be explained by a higher cavitation threshold. With increasing pressure, a smaller negative pressure phase of the sound wave acts on the liquid and fewer cavitation bubbles are formed. At high intensitiesjust below thecritical intensity (for example, 100 W at 1 bar in Figure 4), a moderate increase in yield with increasing pressure is observed. It is concluded that the shift in the cavitation threshold is no longer the prevailing factor, as one is operating far above the threshold. The main effect is the more violent collapse of the gas bubbles. Most interesting is the great sensitivity of the critical intensity, beyond which the steep fall-off occurs, on the external pressure. In fact, a change in the normal atmospheric pressure by 50-100 mbar can already shift the critical intensity a noticeable extent. A change in the weather conditions can already lead to pressure changes on this scale! If one irradiates at an intensity above the critical one, an increase in pressure may lead to a more than 10-fold increased chemical yield. The steep fall-off has been related to the rapid association of smaller bubbles b, the sizes of which are below the resonance ~ i z e . The ~ . ~association ~ leads to larger bubbles b, that are not chemically active, which escape from the liquid into the atmosphere:
-.
nb b, (1) The formation of the larger bubbles is possibly responsible for the fog development a t intensities above the critical one. If n is significantly larger than 2, the rate of reaction 1 (which is of nth order) will steeply increase as the rate of formation of bubbles b increases. When the lifetime of the bubbles, b, with respect to association becomes shorter than the time of growth to resonance size (which is a first-order process), there is not much time left for chemical action. The shift of the critical intensity to higher intensities with increasing pressure would be understandable, if one assumes that fewer bubbles are produced a t higher pressures. However, the decrease in yield due to the lower rate of bubble formation is more than counterbalanced by the greater efficiency of the fewer bubbles produced, which explains the high yields above the critical hf power (Figure 5,150 W). Sonoluminescence. Two kinds of deviations from the rectangular shape of the luminescence pulses were observed due to (1) the delayed buildupof the luminescence, and (2) the partial decay of the buildup luminescence intensity over about 5 ms, which was especially pronounced at low ultrasonic intensities (Figure 9). The decay effect is due to the fact that a certain amount of time is required to create and then grow small bubbles until they reach the resonance size at which light emission occurs. This time, which is roughly 1 ms, seems to become shorter with increasing intensity of the pulses. The decay effect is more difficult to explain. The decrease in luminescence intensity during a 5-ms pulse seems to indicate a weakening of the ultrasonic field in which the bubbles oscillate. When a pulse starts, it finds the liquid in an acoustically almost transparent state. As bubbles are formed, the ultrasonic wave is partly reflected in a diffuse manner, and this might be the
162 The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 I
1
SO w a t t s
T
3.0 N
'-
U
n
II 2.0 >
1.0
1
2
3
p Ibarl Figure 14. The ratio u (as defined in the upper left part) as a function o f t h e external pressure for various hf powers.
reason for the weakening of the field. This would also explain why theeffect is strongest for low-intensity pulses (20 Win Figure 9), since in the weakened field one operates more closely to the cavitation threshold. The periodicity observed for low-intensity pulses (Figure 11) also indicates that the pulses themselves cause changes in the quality of the ultrasonic field in which the bubbles oscillate. The decay effect becomes more pronounced with increasing external pressure. For example, the effect is almost absent a t 50 W and 1 bar (Figures 8 and 9) but is strong a t 1.5 and 3 bar (Figures 12 and 13). In Figure 14, the decay effect is quantitatively described by the ratio V = Z,/Zz, where ZIand I2 are the respective luminescence intensities at the beginning and the end of a pulse. One recognizes again that, a t constant hf power, the effect becomes stronger with increasing external pressure. At a given pressure, the effect is more pronounced the lower is the hf power. Thus, an increase in pressure seems to have the same effect as a decrease in the intensity of the ultrasound.
Concluding Remarks The present paper is mainly a phenomenological one, which shows that the external pressure controls various features of
Henglein and Gutitrrez sonochemical reactions and of sonoluminescence. Our understanding of the observed effects is in a preliminary state and the explanations given are speculative at the present time, no physical measurements being available which would corroborate them independently. The effects described were observed using 1-MHz ultrasound. Most laboratories carry out sonochemical experiments with commercially available horns which produce ultrasound in the 2040-kHz range. A comparative study of various chemical reactions initiated by 1-MHz and 20-kHz sound has shown that significant differences exist which are due to different cavitation condition^.^ Therefore the results obtained in the present work cannot be readily transferred to 20-kHz sonochemistry; it is desirable to carry out an additional investigation of the pressure effects under horn irradiation conditions.
References and Notes (1) (a) El'piner, I . E. Ultrusound;ConsultantsBureau: New York, 1964. (b) Henglein, A. Ultrasonics 1987, 25, 6. (c) Mason, T. J.; Lorimer, J. P. Sonochemistry; Ellis Horwoodd Limited and John Wiley and Sons: Chichester and New York, 1988. (d) Suslick, K. E. Ultrasound, VCH Publishers: Weinheim, 1988. (e) Young, F. R. Cuvirurion; McGraw-Hill Book Co.: London, 1989. (0 Riesz, P. Advunces in Sonochemistry; Mason, T. J., Ed.; JAI Press Ltd.: London, 1991; Vol. 2, p 23. (2) Polotskii, I. G. Zh. Obsch. Khim. 1947, 17, 1048, (3) Hart, E. J.; Henglein, A. J . Phys. Chem. 1986, 90, 5992. (4) (a) Henglein, A.; Ulrich, R.; Lilie, J. J. Am. Chem. SOC.1989, 1 1 1 , 1974. (b) Henglein, A.; Herburger, D.; Gutitrrez, M. J . Phys. Chem. 1992, 96, 1126. ( 5 ) Gutitrrez, M.; Henglein, A. J . Phys. Chem. 1990, 94, 3625. (6) (a) Lindstrom, 0. J. Acoust. SOC.Am. 1955,27,654. (b) Haissinsky, M.; Klein, R.; Rivayrand, P. J . Chim. Phys. 1962, 59, 61 1. (c) Sehgal, C. M.; Wang, S . Y. J . Am. Chem. SOC.1981, 103, 6606. (7) Hart, E. J.; Henglein, A. J . Phys. Chem. 1985, 89, 4342. (8) Gutitrrez, M.; Henglein, A.; Ibafiez, F. J . Phys. Chem. 1991, 95, 6044. (9) Henglein, A.; Gutitrrez, M. J . Phys. Chem. 1990, 94, 5169.
