Influence of US Frequency on Swan Band Sonoluminescence and
Sonochemical Activity in t-BuOH Aqueous Solutions
Pflieger R.*, Ndiaye A. A., Chave T., Nikitenko S. I.
Institut de Chimie Séparative de Marcoule, UMR5257, CEA-CNRS-UM2-ENSCM, Centre
de Marcoule, Bat. 426, BP 17171, 30207 Bagnols-sur-Cèze, France
The multibubble sonoluminescence (MBSL) spectra from organic solvents[1,2] or from
aqueous solutions of organic compounds[3,4] often show emission from C2* Swan band
(d3Πg → a3Πu). This band is traditionally used as a probe of intrabubble conditions. On the
other hand, the C2* excited species are formed as products of a complex set of chemical
reactions occurring inside the cavitation bubbles. Therefore, MBSL of C2* can be useful to
study the mechanism of organic compounds sonolysis in addition to its application for
cavitation thermometry. However, the relationship between sonochemical activity and MBSL
has just begun to emerge. This work describes the MBSL and sonochemical studies of tBuOH sonolysis in aqueous solutions saturated with noble gases (Ar, Xe) as a function of
ultrasonic frequency.
The experiments have been performed at 20, 204, 362 and 613 kHz using the multifrequency
sonoreactor described recently.[4] The temperature in the reactor during sonolysis was
maintained at 10-12°C. Studied solutions were sparged with gas (Ar or Xe) about 30 minutes
before sonication and during the ultrasonic treatment at a controlled rate of 100 mL min -1.
The light emission spectra were collected in the spectral range 400-600 nm using a SP 2356i
Roper Scientific spectrometer coupled to a liquid-nitrogen-cooled CCD camera with UV
coating.[4] Gaseous products in the outlet gas were analyzed using a Thermo Scientific VG
Prolab Benchtop quadrupole mass spectrometer.
The MBSL spectra of t-BuOH aqueous solutions show emissions for the ∆υ = -1 to ∆υ = +2
vibrational sequences of C2* Swan system (Fig. 1). The ∆υ=+2 emission overlaps with
CH(A-X) emission band. In general, MBSL is stronger at high-frequency ultrasound
compared to 20 kHz ultrasound. However, in Xe the sonoluminescence is so bright that it can
be seen by the unaided eye as a blue glow even at low ultrasonic frequency (Fig. 1d). The
intensity of C2* band emission exhibits a maximum vs. t-BuOH concentration: 0.1-0.2 M at
20 kHz and (2-8)·10-3 M at high-frequency ultrasound. The huge difference in saturating
concentrations of t-BuOH for low- and high-frequency ultrasound is attributed to much
smaller bubble size, or in other words, to much higher surface/volume ratio for the cavitation
bubbles produced at higher frequency.
It was found that the relative intensities of ∆υ bands are strongly influenced by the
experimental conditions. In general, the ratio of ∆υ = +1 band composed of (1-0), (2-1), (3-2),
(4-3), (5-4) and (6-5) vibrational transitions and ∆υ = 0 band composed of (0-0), (1-1), (2-2),
(3-3) and (4-4) vibrational transitions is higher for high ultrasonic frequency compared with
that at 20 kHz, indicating higher vibrational excitation of C2* species (Fig. 1a,b). The
vibrational excitation is strongly increased in the presence of Xe (Fig. 1c). Simulation of
emission spectra using SPECAIR software[5] reveals Boltzmann-like behavior of C2* at 20
kHz in Ar (Tv~5800 K, Tr~4500 K), where Tv and Tr are the vibrational and rotational
temperatures respectively. By contrast, in Xe at 20 kHz strong deviation from thermal
equilibrium is observed (Tv~12000 K, Tr~2200 K). The vibrational temperature increases with
US frequency in Ar: at 204kHz Tv~7000 K, Tr~4000 K and at 362 kHz, Tv~12000 K, Tr~6500
K. The peak area ratio R = (∆υ=+1/∆υ=0) sharply drops with an increase in t-BuOH
concentration indicating quenching of higher vibrational states of C2* by organic molecules.
The MBSL of C2* is in line with the spectroscopic studies of OH sonoluminescence in water
revealing nonequilibrium plasma formation during acoustic collapse.[6]
2.0
v=0
v=+2
0.06
v=+1
+ CH (A-X)
0.04
v=-1
0.02
0.00
400
420
440
460
480
500
520
540
SL intensity, A.U.
0.8
v=+2
+ CH (A-X)
1.5
1.0
b)
v=+1
v=0
0.5
0.0
400
560
, nm
a)
v=+2
+ CH (A-X)
SL intensity, A.U.
SL intensity, A.U.
0.08
420
440
460
480
500
520
540
560
, nm
v=+1
0.6
v=0
0.4
v=-1
0.2
0.0
400
c)
d)
420
440
460
480
500
520
540
560
, nm
Figure 1. MBSL from t-BuOH solutions at 20 kHz, 0.10 M t-BuOH, Ar (a), 362 kHz, 8.5·10-4 M t-BuOH,
Ar (b), 20 kHz, 0.12 M t-BuOH, Xe (c), image of C2* MBSL at 20 kHz in Xe.
Analysis of the gaseous products of sonolysis (H2, CH4, and C2H2) demonstrates that at higher
concentration, when the MBSL is almost totally quenched, the sonochemical activity is still
important, whatever the ultrasonic frequency. Even in the absence of sonoluminescence
extreme conditions can still be formed inside the cavitation bubbles.
References
[1] Suslick K. S., Flint E. B. Nature, 330: 553-555. 1987.
[2] Flint E. B., Suslick K. S. Science, 253: 1397-1399. 1991.
[3] Didenko Y. T., McNamara III W. B., Suslick K. S. J. Am. Chem. Soc., 121: 5817-5818, 1999.
[4] Navarro N. M., Pflieger R., Nikitenko S. I. Ultrason. Sonochem., 21 : 1026-1029, 2014.
[5] Laux C. O., Spence T. G., Kruger C. H., Zare R. N. Plasma Sources Sci. Technol., 12 : 125–138, 2003.
[6] (a) Ndiaye A. A., Pflieger R., Siboulet B., Nikitenko S. I. Angew. Chem. Int. Ed., 52 : 2478-2481, 2013;
(b) Ndiaye A. A., Pflieger R., Siboulet B., Molina J., Dufreche J.-F., Nikitenko S. I. J. Phys. Chem. A, 116 :
4860-4867, 2012; (c) Pflieger R., Brau H.-P., Nikitenko S. I., Chem. Eur. J., 16 : 11801-11803, 2010.
* E-mail: rachel.pflieger@cea.fr