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Steer et al.
+ rx is the sum of the two radii. Equation 6 has been theoretically shown by Logan24to reduce to the Bronsted-Bjerrum equation for primary kinetic salt effect at low ionic strength. With increasing ionic strength of the solution Logan has suggested that the Debye correction factor F tends to unity, i.e., the ions behave kinetically as if they were neutral. Recently Buxton et a1.z6have also suggested that in 10 M LiCl the Debye correction factor equals unity. Since the Debye correction factor converges to unity at the same ionic strength for all ions independent of their chargez4iridate and ferricyanide should behave similarly. However, comparison of Figures 2 and 3 shows that iridate and ferricyanide behave differently. Further differences appear between the behavior of ferricyanide in LiCl and in CsCl solutions (Figure 3). We assume that both these differences are due to the formation of undissociated salts (complex formation) the formation of which differs for different substrates.
Acknowledgment. One of the authors (E.H.) thanks the Alexander von Humboldt Foundation for a grant. We thank Mrs. M. Pruchova for technical assistance and Dip1.-Phys. F. Schworer and his staff for excellent operation of the pulse radiolysis equipment. References and Notes (1) M. Anbar and E. J. Hart, J. Phys. Chem., 69, 1244 (1965). (2) E. J. Hart and M. Anbar, "The Hydrated Electron", Wiley-Interscience, New York, N.Y., 1970, p 172.
(3) A. K. Pikaev, B. G. Ershov, and I. E. Makarov, J . Phys. Chem., 79, 3026 11975). (4) A. K. kkaev: T. P. Zhestikova, and G. K. Sibirskaya, J. Phys. Chem., 76, 3765 (1972). (5) J. Holzwarth and L. Strohmaier, Ber. Bunsenges. Phys. Chem., 77, 1145 (1973). (6) N. Getoff and F. Schworer, Radiat. Res., 41, 1 (1970). (7) 8. Cercek, Int. J. Radiat. Phys. Chem., 3 , 231 (1971). (8) K. D. Asmus, A. Wigger, and A. Henglein, Ber. Bunsenges. Phys. Chem., 70, 862 (1966). (9) (a) E. J. Hart, S.Gordon, and J. K. Thomas, J . Phys. Chem., 08, 1271 (1964); (b) M. Anbar and E. J. Hart, J. Am. Chem. Soc., 66, 5633 (1964). (10) F. Barat, L. Gilles, B. Hickel, and B. Lesigne, J. Phys. Chem., 77, 1711 (1973). (11) M. S.Matheson, "Physical Chemistry. An Advanced Treatise", Vol. VII, Academic Press, London, 1975, p 570. (12) J. Jortner, Radiat. Res., Suppl., 4, 24 (1964). (13) Landolt-Bornstein, "Zahlenwerte and Funktionen", Vol. 5, Springer-Verlag, Berlin, 1969. (14) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions", London, Butterworths, 1959, p 306. (15) M. Anbar and E. J. Hart, Adv. Chem. Ser., No. 81, 79 (1968). (16) F. S. Dainton and R. Rumfeldt, Proc. R. SOC.London, Ser. A , 296, 239 (1967). (17) P. A. Lyons and J. F. Riley, J . Am. Chem. Soc., 76, 5216 (1954). (18) M. J. Pilling and S. A. Rice, J. Chem. SOC.,Faraday Trans. 2 , 71, 1563 (1975). (19) R. J. Woods, 8. Lesigne, and L. Gilles, J. Phys. Chem., 79, 2700 (1975). (20) R. Mills, J. Phys. Chem., 61, 1631 (1957). (21) K. Schwabe, "Polarographie und chemische Kostitution organischer Verbindungen", Akademie-Verlag, Berlin, 1957. (22) J. W. Fletcher and W. A. Seddon, J. Phys. Chem., 79, 3055 (1975). (23) G. V. Buxton, F. S.Dainton, and D. R. McCracken, Trans. Faraday SOC.,69, 243 (1973). (24) S . R. Logan, Trans. faraday SOC.,62, 3416 (1966). (25) G. V. Buxton, F. C. R. Cattell, and F. S.Dainton, J. Chem. SOC., Faraday Trans. 1 , 71, 115 (1975).
Sonoluminescence of Aqueous Solutions' C. Sehgal, R.
P. Steer," R. G. Sutherland, and R. E. Verrall
Department of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OW0 (Received July 22, 1977) Publication costs assisted by the University of Saskatchewan
The effects of adding efficient H atom and OH radical scavengers on the spectral distribution and intensity of sonoluminescence from argon saturated aqueous solutions have been investigated. The results indicate that the emissive continuum is due to a chemiluminescent process, likely H + OH + M HzO + M + hu.
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Introduction The propagation of acoustic waves through a liquid medium is known to result in the phenomenon of acoustic cavitation. Cavitation is a three stage process consisting of the formation, growth, and collapse of gas or vapor filled bubbles suspended in the liquid phase. The presence of dissolved gas or microparticles reduces the liquid strength and hence favors the initiation of cavity formation. In aqueous solutions the decomposition of water into H atoms and OH radicals is thought to result from either "adiabatic" heating or electrical discharge during the irreversible collapse of transient bubbles. Reactions may occur among these primary species and other reactive molecules present in the cavity, and the contents of collapsed bubbles may subsequently diffuse into solution where secondary reactions may occur. Reactions may also occur at the gas-liquid bubble interface. The mechanism of production of luminescence from aqueous solutions subjected to ultrasonically induced The Journal of Physlcal Chemlstry, Vol. 81, No. 26, 1977
cavitation has been the subject of considerable speculation since the phenomenon was first reported by Marinesco and Trillat in 1933.2 Low resolution spectra3 show that there are likely two contributions to the emission from water saturated with the noble gases; a broad continuum extending from about 250 nm into the near infrared with a broad maximum near 400 nm, and a second, banded component in the near UV. The latter has been assigned unequivocally to the well-known A(?Z+) X(%) transition of the hydroxyl radical, while the former, which contributes the majority of the emission at high ultrasonic frequencies, has been attributed variously to black body i n c a n d e s c e n ~ ebrem~strahlung,~ ,~~ and chemiluminescent H + OHQ or ion-electron recombination7s9at the moment of cavity collapse. No definitive mechanism has been established for the production of either emissive component, however. We have carried out studies of the effects of adding various radical scavengers on the spectra and intensity of
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2619
Sonoluminescence of Aqueous Solutions
minescence intensity to near zero is strong evidence that the emissive continuum is the result of a chemiluminescent process. If the continuum were due to “black body” radiation from incandescent collapsing bubbles, then a diminution in radiant intensity with increasing added concentration (due, for example, to a lowering of the temperature of the cavity contents) should be accompanied by a shift in the spectrum to longer wavelengths. Such is not the case and therefore cavity incandescence cannot contribute substantially to sonoluminescence. Spectroscopic inve~tigationsl~ of Hz-02 flames have been shown that an emissive continuum, likely resulting from the process15 H + O H + M - H,O+ M + hv
9
7
-. 5 3
I 0
0.5
1 .o
3
1.5
I 2.0
C O N C E N T R A T I O N X 10 M
Flgure 1. Plot of ratio of total sonoluminescence intensity in the absence of scavenger, IO, to total intensity, I , vs. concentration of scavenger in bulk solution. The insonation frequency is 459 kHz.
emission from ultrasonically cavitated solutions with the objective of obtaining chemical evidence for the mechcanism of sonoluminescence production.
Experimental Section Air-free argon-saturated aqueous solutions in a closed, temperature regulated (T = 285 f 2 K) system were insonated a t 333 or 459 kHz using transducers and a generator previously described.1° Sonoluminescencefrom the cylindrical cell was observed axially by means of a GCA/McPherson Model EU-700 grating monochromator coupled to a cooled RCA C31034A photomultiplier tube and single photon counting detection system.ll Measured volumes of reagent grade liquid scavengers were added to the system by microsyringe injection through a specially designed inlet and were allowed to mix thoroughly with the liquid. Spectral measurements were made using first-order diffraction from the grating and intensity measurements were made with the monochromator in place using the spectrally undispersed zero order diffraction to obtain a higher signal-to-noise ratio. Results and Discussion Sonoluminescence spectra of argon-saturated water obtained with 333-kHz insonation showed a strong band a t 307 nm with weaker bands at 281 and 343 nm, attributable to the 0-0,1-0, and 0-1 transitions of OH A(?P) X(211),superimposed upon a strong continuum extending from ca. 250 to 700 nm, in agreement with previous observations.2 The OH emission bands were almost completely buried in the continuum when 459-kHz insonation was used, as predicted,2 and the shape of the continuum was invariant (although attenuated in intensity) with the addition of benzene and several aliphatic alcohol scavengers. Figure 1shows that the total sonoluminescence intensity diminishes with increasing concentration of benzene and several alcohols in a Stern-Volmer-like fashion over the 0 to 2 X M bulk solution concentration range. The observed effects cannot be due to the suppression of cavitation by the organic component because the addition of higher concentrations of poor radical scavengers such as CC14 is known12J3to enhance slightly the intensity of sonoluminescence from aqueous solutions containing dissolved noble gases. The fact that the presence of small concentrations of reactive free radical scavengers can reduce the sonolu-
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may be observed from the near UV through the visible region of the spectrum. The intensity of this continuum is known16 to increase markedly relative to that of the OH A(22+) X(Q) bands with increasing pressure. It is therefore quite conceivable that, at the very high pressures encountered during bubble collapse, radiation from this source could dominate the sonoluminescence spectrum. Chemical scavenging of either H atoms or OH radicals is therefore most likely responsible for the observed attenuation of the sonoluminescence intensity with increasing benzene or alcohol concentration. Benzene and the aliphatic alcohols are known to be efficient OH radical scavengers in aqueous solution” and, although Arrhenius parameters for the corresponding reactions have not been measured, estimation of these quantities by semiempiricalla means indicates that the same should be true for the gas phase a t high temperature. Bimolecular gas phase reactions of these scavengers with H atoms are also relatively rapid, having rate constants at 900 K of 1.3 X lo’, 9.4 X lo7, 1.9 X lo8, 8.8 X lo8, and 1.6 X lo8 M-l s-l for H + MeOH, EtOH, n-PrOH, t-BuOH, and C6H6,respe~tively.’~ Quantitative correlation of the relative efficiencies of sonoluminescence attenuation with scavenging reaction rate constant is unfortunately not yet possible. Temperatures inside the collapsing bubbles are unknown. Intracavity concentrations of the scavengers are not known accurately since equilibrium vapor pressures are not established within the bubbles. Arrhenius parameters for OH alcohol reactions in the gas phase have not been measured. Nevertheless, the efficiencies of sonoluminescence attenuation increase in the order MeOH EtOH < n-PrOH t-BuOH < C6H6,which, qualitatively, correlates reasonably well with the expected relative rates of H and OH scavenging at high temperature. Further work is underway to elucidate the details of the sonoluminescence and scavenging mechanisms.
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References and Notes (1) Financial support from the National Research Council of Canada and the University of Saskatchewan is gratefully acknowledged. (2) M. Marinesco and J. J. Trillat, Compt. Rend., 196, 858 (1933). (3) K. J. Taylor and P. D. Jarman, Aust. J. fhys., 23, 319 (1970). (4) P. Gunther, E. Heim, and G. Eichkorn, 2.Angew. fhys., 11, 274 (1959). (5) D. Srinlvasan and L. V. Holroyd, J . Appl. fhys., 32, 446 (1961). (6) B. E. Noltingk and E. A. Neppiras, f r o c . fhys. SOC.(London), 63, 674 (1950). (7) M. A. Margulis, Sov. fhys.-Acoust., 15, 135 (1969). (8) T. K. Saksena and W. L. Nyborg, J . Chem. fhys., 53, 1722 (1970). (9) M. Degrois and P. Baldo, Ultrasonics, 12, 25 (1974). (10) E. L. Mead, R. G. Sutherland, and R. E. Verrall, Can. J. Chem., 54, 1114 (1976). (11) T. Oka, A. R. Knight, and R. P. Steer, J . Chem. fhys., 6 3 , 2414 (1975). (12) K. Negishi, J. fhys. SOC. Jpn., 16, 1450 (1961). (13) V. Griffing, J . Chem. fhys., 18, 997 (1950). (14) A. G. Gaydon, “The Spectroscopy of Flames”, Chapman and Hall, London, 1957. (15) P. J. Padley, Trans. Faraday Soc., 56, 449 (1960). The Journal of Physical Chemistry, Vol. 8 I , No. 26, 1977
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A.
(16) J. Diederichsen and H. G. Wolfhard, Proc. R . SOC. London, Ser. A , 236, 89 (1956). (17) Farhataziz and A. 8. Ross, Natl. Stand. Ref. Data Ser., Nafl. Bur. Stand., 59 (1977).
Djavanbakht, J. Lang, and R. Zana
(18) S.W. Benson, "Thermochemical Kinetics", 2nd ed, Wiley, New York, N.Y., 1976. (19) J. A. Kerr in "Comprehensive Chemical Kinetics", Vol. 18, C. H. Bamford and C. F. H. Tipper, Ed., Elsevier, Amsterdam, 1976, p 39.
Ultrasonic Absorption in Relation to Hydrogen Bonding in Solutions of Alcohols. 2. Ultrasonic Relaxation Spectra of Solutions of Alcohols in Cyclohexane A. Djavanbakht,+ J. Lang, and R. Zana" CNRS, Centre de Recherches sur ies Macromol6cuies 6, rue Boussingault, 67083 Strasbourg, Cedex, France (Received March 23, 1977) Publication costs assisted by CNRS
The ultrasonic absorption of solutions of ethanol, 1-butanol, 1-octanol, 1-dodecanol, 1-hexadecanol, 3-octanol, 2-methyl-3-heptano1, and 2,4-dimethyl-3-hexanol in cyclohexane at 25 "C has been measured in the frequency range 4-250 MHz. The relaxation spectra of all of the investigated solutions could be fitted to a relaxation equation with a single relaxation frequency. The results have been interpreted on the basis of a reaction mechanism where n alcohol molecules associate to give cyclic and noncyclic n-mers. The absorption is attributed to the perturbation by the sound waves of the association equilibrium leading to noncyclic n-mers. For all primary alcohols no fit to the data could be obtained for n = 3 but equally good fits were obtained for n = 4 and n = 5, except with 1-hexadecanol for which n = 5 provides the best fit. The results indicate that (1) the rate of association of alcohol molecules through H bonds is close to its diffusion-controlled limit, even for the most hindered alcohol investigated; (2) the dissociation rate constant of one alcohol molecule from a noncyclic n-mer is only very slightly dependent on the alcohol chain length for primary alcohols. This rate constant increases with the degree of steric hindrance, reflecting the decreased stability of noncyclic aggregates. For 2methyl-3-heptanol the best fit to the data is obtained for n = 3. Likewise, the results for 2,4-dimethyl-3-hexanol appear to indicate that the association of this alcohol is essentially restricted to dimerization. Larger associated species appear to be present only in very small amount. The relation between ultrasonic absorption data and dipole moment data for octanol solutions is examined.
I. Introduction involves cyclic and noncyclic polymers1~2~5~s~10~15~16~zg according to Infrared NMR,7-9 vapor pressure osmometry,8J0J1 vapor density,ll partition coefficient,13J4 monomer (A) --L small noncyclic polymer (A, ) c r y o ~ c o p ydielectric ,~ constant,15-17relaxation,16-20caloJ/' (1) rimetry,6J3*21-23 ultrasonic a b s o r p t i ~ n , and ~ ~ - chemical ~~ cyclic polymer (A,' n4c) kinetics31 have been extensively used for the study of the association of alcohols in solution through H bonding. In Bordewijk15recently pointed out that the discrimination between different association models cannot be achieved spite of this very large number of studies an examination by investigating properties which vary monotonically with of the literature till about 1970 reveals considerable the total alcohol concentration. Indeed the calculated confusion in the understanding of the association behavior variations of such properties are not very sensitive to the of alcohols. In the past few years, however, some concensus model (for instance to different sets of values of n and n' appears to have been reached among workers about several in reaction 1)as those of properties which show a maxiimportant features of the self-association of alcohols in mum and/or a minimum as the alcohol concentration is nonpolar solvents such as saturated hydrocarbons or CC14. increased. This explains the renewed interest in mea(i) Fairly dilute alcohol solutions (at concentrations below surements of apparent dipole moment pa of alcohols in 0.5 M) appear to contain, in addition to the monomeric solution. y, goes through a maximum a t a concentration alcohol, a t least two associated species.1-6,8-16,29,32 This which depends on the solvent15-17(0.02, 0.04, and 0.4 M conclusion results from the fact that the best fit to the for 1-octanol in cyclohexane, CC14, and benzene, data, whichever the investigated property, is obtained by respectively16). This maximum has been attributed to the using a t least two equilibrium association constants. (ii) relative variations of concentration of high dipole moment At concentration below 0.5 M the association does not linear (but small) polymers and of low dipole moment proceed to large aggregate^.^^^^^^ Trimers8 and tetramers1 It must be noted that studies of static cyclic p01ymers.l~~~~ are most often invoked for the interpretation of the results. constant of alcohol solutions have yielded much Dimers appear to be present only in small a m o u n t s ' ~ ~ ~ ~ Jdielectric ~ more information than dielectric relaxation studies. Inbut the authors disagree on whether this amount can be deed, two out of the three relaxation processes found for neglected in the mass conservation equation.'Jl (iii) The alcohol solutions are of intramolecular origin and make presently accepted model, which agrees with the results extremely complicated the study of the third process which of the most recent studies by means of various methods, results from the intermolecular association of alcohol molecules, in the case of dielectric relaxation.20 Present address: 163 Manoutcheri Ave., Meched, Iran, f
The Journal of Physlcal Chemistry, Voi. 81, No. 26, 1977
