J. Phys. Chem. 1980, 84, 529-531
529
Selective Quenching of Species That Produce Sonoluminescence C. Sehgal, R. G. Sutherland, and R. E. Verrall" Depatfment of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0 WO (Received July 23, 1979) Publication costs assisted by the University of Saskatchewan
The effect of added nitrates on the spectral distribution and intensity of sonoluminescence from argon-saturated solutions has been investigated. The results indicate that NO; quenches OH emission preferentially and so uncovers the emission peak due to HzO* at -275 nm. Perhydroxyl radical (HO,) formed by the reaction between NO3- and OH undergoes a radiative association with H to give a continuum, the sharp short wavelength cutoff of which gives an upper limit of 87.8 kcal/mol for H02-H bond dissociation energy, in good agreement with the thermochemical value.
Introduction According to the theory of acoustic cavitation, minute bubbles formed at a pressure amplitude above a threshold value either oscillate nonlinearly or collapse depending upon their initial radius, insonation frequency, external pressure, surface tension, etc. During the bubble's compression phase the contents are heated adiabatically to produce free radicals from the decomposition of molecules within the cavity. These radicals in turn produce luminescence either by radiative relaxation to the ground state or by radliative recombinations. Sonoluminescence emission from argon-saturated water extends from 240 nm to the near infrared and mainly consists of OH and H 2 0 band systems due to 2E+ zrI and 3111 Z+ (3B17lAl) transitions, respectively, overlaying a broad continuum resulting from the radiative recombination of H and OH.1,2 Since all the emitting species are extremely reactive and very similar in their chemical behavior, it is difficult to exclusively scavenge only one of them and so there is no direct chemical evidence in support of the formation of H20* t3B1)during sonolysis of water. The purpose of this study was to scavenge OH preferentially to provide chemical evidence for HzO* and also to show that NO, can react with free radicals to produce changes in the sonoluminescence spectra of argon-saturated HzO.
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Experimental Section Air-free argon-saturated aqueous solutions were insonated a t 459 kHz by using the equipment previously de~ c r i b e d .Sonoluminescence ~ from the cylindrical cell4was observed axially by single-photon counting technique^.^ Measured volumes of' argon-saturated solutions of H N 0 3 (reagent grade) were added to the system by syringe injection through a specially designed inlet port and were allowed to mix thoroughly with the liquid before rapid scans of sonoluminescence spectra were recorded. Total sonoluminescence intensity and the emission at 310 nm were measured after each injection of H N 0 3 solution. Sonoluminescence spectra from saturated alkali nitrate solutions were also observed and recorded. Results and IDiscussion Sonoluminescence spectra of argon-saturated water consists of an OH band with a maximum at 310 nm, superimposed on a H 2 0 band with a maximum at 275 nm and a continuum due to the radiative association of radH 2 0 + hv.2 When nitric acid icals viz. H + OH + M is added to the solution the sonoluminescence intensity at 310 nm decreases rapidly (curve I1 of Figure 1) and
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0022-3654/80/2084-0529$0 1.OO/O
strongly suggests that OH radicals are scavenged. The total sonoluminescence flux (curve I of Figure 1) also diminishes with increasing concentration of nitrate ions and then increases. Similar curves were obtained for Nz-saturated solutions of nitric acid (Figure 2). The initial decrease in emission is due to the scavenging of various emitting species (HzO*, OH*, OH, etc.). The subsequent increase in flux leads to the conclusion that chemiluminescent species are formed by the secondary reactions between OH and NO3- in an ultrasonic field e.g., NO2, H02, etc. (see below). Since appreciable interaction between H20* and NO3- is not expected until the scavenger's concentration exceeds unit molarity: NO3- must scavenge the hydroxyl radicals preferentially, thereby uncovering the peak due to excited water at -275 nm which is imbedded under the hydroxyl band system in the presence of OH emission (Figure 3). Furthermore, a continuum extending from 320 nm to the near infrared is observed, which at higher HN03 concentrations splits into two broad bands (I1 and TI1 of Figure 3). The assignment of the origin of bands I1 and 111 was obtained by a further study of the sonoluminescence spectra of LiN03, NaN03, and KNOB (Figure 4). It can be seen that in all three cases there is emission from alkali metal atom from the first excited state,4 and there is also a broad continuum with a sharp short wavelength cutoff a t 324 f 8 nm. The continuum is attributed to the radiative recombination of perhydroxyl radicals (formed by the reaction between NO; and OH) and hydrogen in the presence of a third body, i.e. NO3- + OH HO2 + H
-
NOz-
+ HOz
Hz02
+ M + hv
-
+M NO3- + OH + H
NOz-
+ H2Oz
The short wavelength cutoff gives the dissociation energy of HzOz a t 0 K (Le., H202 HOz + H) as 88.3 f 2 kcal/mol. Since the kinetic energy of the products formed at room temperature is -0.5 kcal/mol, the heat of reaction at room temperature is 88.3 - 0.5 = 87.7 kcal/mol, in good agreement with the thermal value of 90 f 4 kcal/mol.6 The reaction between NO3- and OH in the ultrasonic field to give nitrite and perhydroxyl radicals likely involves several steps but it is not yet possible to say anything definite about the intermediates formed during this reaction. However, due to the close similarity between radiolysis and sonolysis it may occur either via NO: or NO?formation.8 However, the second possibility is unlikely 0 1980 American Chemical Society
530
The Journal of Physical Chemistry, Vol. 84, No. 5, 1980
Sehgal, Sutherland, and Verrall 6
LiN03(IN)
(672nm)
I
I
4-
e
ZP
+
700
2%
800
c
W
1 k
a
I
10
20
CONCENTRATION
30
IO'M
X
-I W CL
Figure 1. Plots of (I) relative intensity of total emission vs. concentration of nitric acid in argon-saturated water, and (11) relative intensity of emission at 310 nm vs. concentration of nitric acid in argon-saturated water insonated at 459 kHz.
I
I
600
500
400
700
I 800
Figure 4. Low resolution sonoluminescence spectra of argon-saturated alkali nitrate solutions insonated at 459 kHz.
0
0
10
30
20
CONCENTRATION
X
102M
been questioned by Anbar and Pechtg and Haissinsky and Klein.lo NO?- has been reported from ESR studies in frozen solutions of sodium nitratel' but this does not necessarily mean that NOS2-would exist at the extreme conditions of temperatures and pressure associated with cavitation. Moreover, in an acidic medium the hydrated electrons would preferentially react with H+. The formation of products during insonation also can be explained readily by alternate mechanisms involving formation of NO3 i.e. 2N03- + 20H 2N03 20H-
Flgure 2. Plots of (I) relative intensity of total emission vs. concentration of nitric acid in N,-saturated water, and (11) relative intensity of emission at 310 nm vs. concentrationof nitric acid in N,-saturated water insonated at 459 kHz.
2N03
+ 20H
+
20H-
+ OH
NOS + H
A(nm)
+
-
net
2H20
-
+ HzO
NO3 + OH-
+ HOz NO + HzO H+ + NOZ- + l/zH2 HOz + H HzOz H' + OH- G HzO NO3- + H + OH NOz- + H202 +
Figure 3. Low resolution sonoluminescence spectra of argon-saturated nitric acid solutions insonated at 459 kHz: I,emission from HzO*; 11, emission from H HOz; and 111, emission from NOz*.
F!
2H302
2H2O2+ N02-
2H
or
NO3-
2N02
2H02
2Ht net NO3- + 40H
+ 2H02 NOz- + NO; + 2HS
+ + + --
2N02 + HzO 2H
since it involves a reaction between nitrate ion and hydrated electron, i.e. NO3- + eaq- NOS2As yet no direct experimental evidence has been obtained for the formation of eaq- during the sonolysis of aqueous solutions and the hypothesis of its formation has
+
+
+
NO
--
While further work is needed before any definite conclusions can be drawn about the mechanism of the above reaction, the net equations explain the results obtained by Margulis who observed the formation of NOz- and HzOz
The Journal of Pbysical Chemistry, Vol. 84, No. 5, 1980 531
Selective Quenchiing of Sonoluminescence
1
I
240
300
. I 400 500
600
I
1
700
800
A(nml
Flgure 5. Low rescllution sonoluminescence spectrum of argon-saturated KNOBsolution after 5 h of insonation at 459 kHz.
X(nm)
Figure 6. Low resolution sonoluminescence spectrum of argon-saturated solution of hydrogen peroxide (10 % by volume) insonated at 459 kHz.
as the final products during the sonolysis of dilute solutions of nitrates in the preseme of argon gases His results also show that prolonged sonolysis diminishes the nitrite concentration due to its oxidation by OH to give NOz, i.e. INOz- + OH NO2 + OHIf NOz is formed one should observe emission from excited NOz.l2 In verification of this proposal, a sonoluminescence spectrum was recorded after -5 h of insonation of KNOB solution (Figure 5), and, as expected, emission from NOz* was observed above 400 nm. T o provide further chemical evidence in support of H + HOz emission an experiment was devised to produce perhydroxyl raldicals from different sources and compare the sonoluminescence spectra with that from nitrate solutions. Radiation studies13 show that OH formed during irradiation combines with hydrogen peroxide to produce per-
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hydroxyl radicals, i.e., OH t HzOz HOz t HzO. A sonoluminescence spectrum was obtained from a 10% solution of hydrogen peroxide in water (Figure 6). The continuum is very similar to that obtained from solutions containing nitrate and the cutoff at 320 f 20 nm (89.7 f 6 kcal/mol) supports the previous argument that it is due to radiative recombination of H and HOz. Because of the high volatility of Hz02,large fluctuations were observed in the emission intensity and this necessitated the use of larger time constants (i.e., 30 s) where the galvanometer was sluggish and unable to respond to quick changes in intensity with change in wavelength. As a consequence the cutoff is not sharp but extends over a wider range, thereby giving a larger limit to the error (Le., 6 kcal/mol). Therefore, it would appear that in the sonoluminescence spectra of nitric acid solutions the continuum cutoff at 320 nm (Figure 3) is due to the radiative association of H and H02. At higher concentrations of NO3-, or prolonged insonations, the amount of NO2- formed is large enough to be able to react with OH to form NOz. This is substantiated by the fact that the continuum at X >400 (Le., continuum I11 of Figure 3) is overlaid by a weak NOz band emission.
Acknowledgment. Financial support from the Natural Sciences and Engineering Research Council Canada is gratefully acknowledged. References and Notes (1) C. Sehgal, R. P. Steer, R. G. Sutherland, and R. E. Verrall, J. Pbys. Cbem., 81, 2618 (1977). (2) C. Sehgal, R. G.Sutherland, and R. E. Verrall, J . Pbys. Chem. 84,
388 (1980). (3) E. L. Mead, R. 0. Sutherland, and R. E. Verrall, Can. J. Cbem., 54, 1 1 14 11974). (4) C.Sehgal, d. P. Steer, R. G. Sutherland, and R. E. Verrall, J . Chem. fbys., 70, 2242 (1979). (5) G. Stein, "The Chemistry of Ionization and Excitation", G. R. A. Johnson and G. Scholes; Ed., Taylor and Francis, London, 1967. (6) P. Gray, Trans. Faraday Soc., 55, 408 (1959). (7) E. K. Milner and R. K. Broszkiewcz, Radiat. Pbys. Cbem., 11, 253 (1978). (8) M. A. Margulis, Russ. J . Phys. Chem., 48 (ll),1653 (1974). (9) M. Anbar and I. Pecht, J . Pbys. Chem., 68, 1460 (1964). (10) M. Haissinsky and R. Klein, J . Chem. Pbys., 65, 326 (1968). (1 1) B. G. Ershov, A. K. Pikaev, et al., Dokl. Akad. Nauk SSSR, 159, 1357 (1964). (12) C. Sehgal, R. G. Sutherland, and R. E. Verrall, J . Pbys. Cbem., 84, 396 (1980). (13) J. W. T. Spinks and R. J. Woods, "An Introduction to Radiation Chemistry", Wiley, New York, 1976.
