J . Phys. Chem. 1990, 94, 3625-3628
3625
Chemical Action of Pulsed Ultrasound: Observation of an Unprecedented Intensity Effect Maritza Cutierrez and Arnim Henglein* Hahn- Meitner- Institut Berlin GmbH, Bereich Strahlenchemie, 1000 Berlin 39, FRG (Received: August 14, 1989)
The oxidation of iodide in aerated aqueous solution by pulses of 1-MHz ultrasound in the intensity range from 0.3 to 3 W/cm2 was investigated. At low intensities, pulses of ultrasound are as efficient as continuous irradiation. At higher intensities, the chemical yield decreases with decreasing pulse length and decreasing on/off ratio. There exists a very narrow intensity range in which the yield drops from a maximum value to almost zero. This range is shifted to lower intensities with decreasing on/off ratio. A mechanism is proposed in which the coalescence of chemically active cavitation bubbles is regarded as a high-order process with respect to the sound intensity. The rapid coalescence at high intensities decreases the concentration of chemically active bubbles. In addition, the effects of stable nuclei for cavitation are considered. Such nuclei are very effective with respect to the initiation of chemically active bubbles but can also be destroyed by ultrasound. The re-formation of stable nuclei is a rather slow process, which requires minutes.
Introduction The chemical effects of pulsed ultrasound are often different from those initiated by continuous sonication. This has recently been shown in the cases of nitrate formation in aerated solutionla and the oxidation of iodide and luminol.Ib Pulse trains of ultrasound were applied, and the chemical yield y was compared with the yield yo of continuous irradiation. In order to expose the chemical system to the same amount of acoustic energy, a longer irradiation time has to be applied under pulsed conditions. If to is the time of continuous irradiation, time t for irradiation with pulses is t = to(1 1/R) (1)
+
where R = T:Tois the on/off ratio of the pulse train ( T , length of a pulse; To,length of the interval between the pulses). It was found that the relative yield y / y o depends on both the pulse length and the on/off ratio. Generally, the reduced yield increases with increasing T a n d increasing ratio T T , . Studies of this kind are also of interest with respect to the application of ultrasound in medicine, Le., diagnosis, therapy, and kidney stone distintegration, in which intense acoustic pulses are used. In the present paper an unexpected intensity effect in the use of pulsed ultrasound is reported, the intensity varying between 0.3 and 3 W/cm2. The effect is understood in terms of cavitation bubble dynamics, i.e., the nucleation, formation, disappearance, and coalescence of gas bubbles in the ultrasonic field. Chemical effects in aqueous solutions occur in the presence of a mono- or diatomic gas, such as air. The ultrasound produces tiny gas bubbles which oscillate in the periodically changing pressure field; temperatures of several thousand kelvin arise during the adiabatic compression or collapse of the bubbles? and at these temperatures molecules of the solvent and of dissolved substances are decomposed.3 The investigation of the intensity effect described here required good reproducibility of the sonication experiments. It is shown that under certain conditions a change a few percent in the intensity of the sound can lead to a change in the chemical yield ( I ) (a) Henglein, A.; Gutierrez, M. Int. J . Radial. Bioi. 1986,50, 527. (b) Henglein, A.; Ulrich. R.; Lilie, J. J . A m . Chem. Soc. 1989, 111, 1974. (2) (a) Noltingk, B. E.; Neppiras, E. A. Proc. Phys. Soc. 1950,863. 674. (b) Neppiras, E. A. Phys. Rep. 1980,61, 159. (c) Flynn, H. G. In Physical Acoustics: Principles and Methods; Mason, B. W . P., Ed.; Academic Press: New York, 1964; Vol. I . (3) See, for example: (a) Henglein, A. Ultrasonics 1987, 25, 6. (b) Carmichael, A. J.; Mossoba, M. M.; Riesz, P.; Christman, C. L. IEEE Tram. Ultrason. Ferroelectr. Freq. Control 1986, 33, 148. (c) Suslick, K. S.,Ed. Ultrasound, Its Chemical, Physical and Biological Effects; VCH Publishers: Weinheim, 1988. (d) Mason, T. J.; Lorimer, J. P. Sonochemistry, Theory, Applications and Uses of Ultrasound in Chemistry; Ellis Horwood Ltd. Verrall, Publishers: Chichester, 1988. (e) Mead, E. L.; Sutherland, R. G.; R. E. Can. J . Chem. 1976. 54, 1114.
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of several hundred percent. The reproducibility of the data was within 5%.
Experimental Section The I-MHz ultrasound was emitted from a quartz plate (5.5-cm-diameter metal plated; 7.0-cm total diameter). As shown by Figure 1 , the acoustic energy is emitted through a X metal plate P into the glass vessel V which had a plane X/2 glass bottom B. The coupling fluid between P and B was water. To avoid loss of acoustic energy, the oscillating system, i.e., quartz, Q, and metal plate P, connected to flange F via the thin-walled metal part T (0.5" thickness). In fact, P, T, and F were made from one piece of brass. As shown in the figure, a stream of cooling water removed the heat that was generated in the vessel and metal flange. The quartz was insulated by immersing the lower part of the apparatus into an oil bath. The rate of temperature increase of the irradiated liquid is plotted in Figure 2 as a function of the high-frequency (hf) power supplied to the quartz. It increases linearly at lower intensities. From this initial increase the intensity of the sound was calculated from the formula I [W/cm2] = 4.18rw/A
(2)
where r is the rate of temperature increase, w the weight of irradiated liquid (25 g), A the square area of the quartz emitting into the vessel (24 cm2), and 4.18 J/(g grad) the specific heat of water. The intensity is shown on the upper abscissa scale. It was assumed here that the intensity increases proportionally to the hf power also at higher powers. In the following figures, the intensity is given in terms of the hf power delivered to the quartz. The oxidation of iodide in aerated 0.2 M KI solution was the only reaction studied. The concentration of I2 formed was determined spectrophotometrically (e = 2.4 X IO4 M-' cm-' at 350 nm). The irradiation time was 20 s to 1 min depending on intensity. All solutions were made with deionized water and stored for several days before use. They were irradiated under open air without bubbling of gas.
Results The absorbed sound energy is proportional to the hf power picked up by the quartz only at powers below about 50 W (or exposure intensities below about 1.3 W/cmZ) as can be seen from Figure 2. At higher intensities, the curve passes through a maximum. The decrease above about 80 W in Figure 2 is explained by the existence of a great number of gas bubbles in the solution which scatter the sound waves to the walls of the vessel or back to the transducer. Less energy remains in the liquid in this way, although the vessel is exposed to higher and higher intensities. 0 I990 American Chemical Society
3626 The Journal of Physical Chemistry, Vol. 94, No. 9, 1990
Gutiirrez and Henglein
Figure 1. Schematic drawing of the apparatus: Q, quartz plate; P, X/2 plate: T, thin-walled metal; F, flange (P,T, and F made from one piece of brass); W,, W,,cooling water in and out; B, X/2 bottom of glass vessel V containing solution S ; W, water; 0, oil. "0
0.4
-
0.3
u
-
60
80
100
120
Figure 4. Application of 100-ms pulse trains. Yield is a function of hf power for various on/off ratios.
e 0.2 r[l
40
power [WI
-
c
20
I
9'
-
0.1 -
76 E
o
I
' I
I
N M
x
3
E c
-
I
-N
x
3'
0
/i
-
O
i
Figure 5. Application of 10-ms pulse trains. Yield is a function of hf power for various on/off ratios.
d P "0
20
10 Dower [ W I
9,
40
60 power
80
100
120
[WI
Figure 3. Application of I-s pulse train at various on/off ratos and continuous irradiation. to in eq I was 20 s. Iodine yield is a function of the hf power supplied.
The rate of iodine formation is plotted in Figures 3-6 as a function of the hf power supplied. In each figure one finds the curve for continuous irradiation and various curves for the irradiation with pulse trains of different on/off ratio R . The pulse length was 1 s in Figure 3, 100 ms in Figure 4, 10 ms in Figure 5, and 2 ms in Figure 6. The yield from continuous irradiation increases above a threshold of 3 W in a linear manner until a broad maximum is reached at higher hf powers. Curves of this kind have already been reported by previous author^.'^,^ Note the very steep decrease in yield above about 100 W which was observed in the present investigation. In previous studies of this kind, including our own,lb the decrease at higher intensities occurred much more slowly. It seems that the favorable geometry in our present irradiation equipment, which avoids significant loss of acoustic energy, is the main reason for the steep dependence observed. As was pointed out previously,Ib the character of cavitation changes in passing through the maximum of the curve. At this maximum (4) (a) Lindstrom. 0. J . Acousr. SOC.Am. 1955, 27, 654. (b) Degrois, M.; Badilian, B. C. R . Acad. Sci. 1962, 254, 837. (c) Haissinsky, M.; Klein, R.: Rivayrand, P. J . Chim. Phys. 1962, 59, 61 1. (d) Sehgal, C. M.; Wang, S. Y . J. Am. Chem. Sor. 1981, 103, 6606.
-
6
-
y1
E
-
o
-
I
N
M
3-
0
0
20
40
60
EO
100
120
p o w e r lW1
Figure 6. Application of 2-ms pulse trains. Yield is a function of the hf power for various on/off ratios.
there was a strong fountain at the surface of the liquid, and a lot of aerosol was formed from the solution. The low yields above 100 W cannot be simply explained by the lower acoustic energy deposition in the liquid (Figure 2 ) as this effect amounts to less than a factor of 2. The novel effect to be described here are the yield vs hf power curves observed for pulsed irradiation. At low sound intensities, the yields are the same as for continuous irradiation independtntly of the on/off ratio. For example, in Figure 3, where I-s pulse trains were used, the yields coincide with the yield for continuous irradiation up to about 40 W. At higher hf-powers, the yields
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3627
Chemical Action of Pulsed Ultrasound
1
9 ,
jllontinuaui
I
T-1s
T, Is1
Figure 7. Yield as a function of the interval time, To,for various values of the hf power (I-s pulse trains, data from Figure 3).
for pulsed irradiation are always smaller than for continuous irradiation, the effect being most pronounced for R = 1 : l O (or an interval length of IO s between the I-s pulses). Similar relations are seen in Figures 4-6 where shorter pulses were applied. In the case of the IO- and 2-ms pulses (Figures 5 and 6), the curves for pulse trains deviate from that of continuous irradiation already at rather low intensities. The yield in the pulsed irradiation decreases at higher intensities, this decrease occurring earlier than in the case of continuous irradiation. For the rather long pulses in Figure 3, the steepness of the decrease is not so strong and the yields for various on/off ratios do not differ very much. In Figure 7 one finds the results of Figure 3 presented in a different way; the yield is plotted here as a function of the time interval Tobetween the pulses for various hf powers. Again, one can see that the yield is independent of the sound intensity at low hf powers (10 and 20 W). At 40 W a slight decrease with increasing interval time becomes noticeable. A stronger effect occurs at 60 W, where the curves goes through a minimum at To = IO s. A more drastic effect occurs at 80 W, where the yield rapidly drops at short values of To,goes through a minimum, and recovers only a little up to the longest interval of 50 s. In the case of shorter pulses, such as the 10- and 2-ms pulses in Figures 5 and 6, respectively, the decrease in yield at higher intensities is extremely steep and occurs at lower hf powers with decreasing on/off ratio R . Furthermore, the maximum yield obtainable for a given on/off ratio becomes smaller. The figures demonstrate that low-intensity ultrasonic pulses produce significant chemical effects only in a rather narrow intensity range. The 100-ms pulses in Figure 4 show a behavior intermediate between those of the long pulses in Figure 3 and the short ones in Figures 5 and 6. The yield vs hf power curve is practically the same for all lower on/off ratios but tends toward zero at a lower intensity than for continuous irradiation.
anew. However, if To < r2,the subsequent pulse can still act on bubbles formed by the preceding pulse and in this way be more chemically efficient. If the pulse length T i s shorter than T ! , no chemical effects occur. The case of low ultrasonic intensities is discussed first. Under these conditions, the yield of iodide oxidation is independent of pulse length Tand the on/off ratio TIT,. Furthermore, the yield is equal to that of continuous irradiation (Figures 3-7). This means in the framework of the simple theory that successive pulses always find the liquid in an ”activated state” due to the previous pulses regardless of the time that elapses between the pulses. A kinetic explanation is the following. Tiny gas bubbles formed by ultrasound disappear during the interval between pulses either by dissolution (because of the high internal pressure due to the great surface energy) or by combination with each other to form larger bubbles which are not chemically active. The latter process may be formulated as nb
-
b,
(3)
where b means a small bubble and b, a large one. The dissolution
of a bubble is a first-order process, the rate of which is independent
Discussion All previous experiments with ultrasonic pulses have been performed at intensities that correspond approximately to 25-W hf power in the present paper.’-s Under these circumstances it was observed that changing the pulse length T a t constant on/off ratio can lead to almost zero yield below a critical value of T. With decreasing on/off ratio, this critical pulse length became longer. According to a simple theory described previously,l a pulse does not produce chemical reaction immediately but needs some time, the “activation time” T ~to, form gas bubbles of suitable size. After the pulse, the bubbles disappear with a characteristic time, the “deactivation time” T ~ .I f the interval To between the pulses is longer than 72,the following pulse has to activate the solution
of the number of bubbles present. It cannot explain the observed phenomena. The coalescence of bubbles, however, is a process of higher order, its characteristic time becoming shorter with increasing concentration of bubbles. For low intensities, 72 is greater than To and hence the system is chemically reactivated easily to provide for good yields of 12. Several pulses will very soon “top-up” to the level attained for continuous irradiation. At higher sound intensities, the coalescence is so fast that chemical activity is lost very quickly and even with several pulses the system cannot be topped-up to the level obtained on continuous irradiation. Obviously, the longer the Toat these intensities, the more difficult to top-up and the lower the effective yield. In order to explain the minimum in yield in Figure 7, we have to take into consideration a second type of cavitation bubble. Recent luminescence investigations on pulsed ultrasound have shown that there are two types of nuclei which determine the “activity” of a solution to undergo chemical reaction upon sonication.lb The first type of nuclei are stable ones which exist normally in a liquid. The ultrasound acts on them very efficiently to produce chemical reaction. However, at the same time these nuclei are also destroyed by the sound. The second type of bubbles are the ones which the ultrasound produces itself. They are not stable and more or less disappear during the interval between pulses. This picture of two kinds of gas bubble nuclei allows one to rationalize the existence of the minimum in the yield vs To curves observed for rather long pulses of 1-s duration (and rather long interval times at the lower T:Toratios) in Figure 7. During a pulse stable nuclei are destroyed. However, during a long interval part of the stable nuclei may be re-formed. Similarly, for the 100-ms pulses (Figure 4) this effect counterbalances to a certain extent the loss of unstable bubbles during the interval, the result being a dependence of the yield on hf power which is almost the same for all on/off ratios. The nature of the small nuclei has often been discussed in the literature. The conical crevice mode is the most accepted, according to which small dust particles always being present in liquids entrap gas in a crack or crevice! The deactivation of these nuclei by ultrasound is possibly connected with the growth of a bubble as the latter detaches as a whole from the dust particle. Refilling of the crevice by the diffusion of gas, which is dissolved in the liquid, seems to be a relatively slow process requiring of the order of minutes. In conclusion, we would like to emphasize that our mechanism, in which the high-order kinetics of bubble coalescence plays an important part, is pretty much in agreement with the views of Sehgal and Wang expressed several years ago.4d Further studies
( 5 ) (a) Rust, H.H.Angew. Chem. 1953, 65, 249. (b) Henglein, A. 2. Naturforsch. 1954, 96,252.
( 6 ) (a) Apfel, R. E. J . Acousr. SOC.Am. 1970, 40, 1179. (b) Apfel, R. E. Ulfrasonics 1984, 2, 167.
J . Phys. Chem. 1990, 94, 3628-3636
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on other chemical reactions and especially on the luminescence of single pulses are necessary to elucidate the details of the chemical processes occurring during bubble nucleation and disappearance in ultrasonic fields. The present studies were limited to intensities below 3 W/cm2. Chemical effects at intensities of some IO W/cm2 have been observed by various authors, mainly by using transducers with a horn. It is not yet known quantitatively how the chemical yield responds to pulsing at these high intensities. Only two investigations at higher intensities have yet been carried out which showed that the time interval between pulses also
determines the efficiency of the pulses under these condition^.^^^' .4cknowledgment. The authors thank Dr. Eberhard Janata for construction of the electronic equipment. This work was supported by Deutsche Forschungsgemeinschaft and Fonds der Industrie. Registry KO. I-. 20461-54-5. ( 7 ) Henglein. A,, Gutitrrez. M.: Ulrich, R. In!. J. Radiat. Biol. 1988, 54,
123.
Square-Wave Voltammetry for ECE Mechanisms John J. O'Dea, K. Wikiel,+ and Janet Osteryoung* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214 (Received: January 31, 1989; In Final Form: December 6, 1989)
The theory of square-wave voltammetry is extended to include the ECE mechanism in which an intermediate chemical step occurs between sequential electrode reactions. Exemplary calculations show how the thermodynamic and kinetic parameters of this mechanism affect the shapes of square-wave voltammograms. The theory is used with a nonlinear least-squares technique to obtain first-order rate constants for the chemical step of the reduction of p-nitrosophenol. This procedure discriminates against several experimental artifacts and does not require the usual separate determination of a normalization current. The rate constants obtained for the intermediate chemical step are 0.46 s-l at 25 "C, 2.40 s-' at 50 "C, and 17.3 s-I at 80 "C (12% uncertainty at 95% confidence) in good agreement with previous studies.
The ECE mechanism, that is, electron transfer to generate an intermediate which subsequently reacts to produce a second electroactive species, is common in organic electrochemistry. Techniques for studying this mechanism, in particular by cyclic voltammetry, are well established. The modern pulse technique of square-wave voltammetry offers many advantages over cyclic voltammetry with respect to both the quality of the experimental signal and computation of theoretical curves. These advantages can be exploited readily by employing the COOL algorithm, which provides an unbiased way of comparing theory with experiment.' With the increased availability of automated instrumentation, the analytical and mechanistic capability of square-wave voltammetry continues to be extendede2J In this work we elaborate the theory of square-wave voltammetry to include ECE mechanisms and demonstrate its practical application for determination of the rate constant of the intermediate chemical step for the case of reduction of p-nitrosophenol. The general scheme of the ECE mechanism is A nle s B (1)
+
k
B-C C
+ n2e
(2) G!
D
(3)
in which the electron-transfer steps are characterized by potentials E," (eq 1) and EZ0 (eq 2) and we assume that the homogeneous chemical reaction (eq 2) is totally irreversible. In the present work we restrict ourselves to the case in which both electron-transfer steps are Nernstian. Then the homogeneous cross reaction, reaction 3 minus reaction 1 B + C s A + D is also at equilibrium at the electrode surface.
(4)
Permanent address: Institute of Precision Mechanics, Duchnicka 3,00967 Warsaw. Poland.
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Of particular interest is the case in which E,"
