Effect of surfactants on inertial cavitation activity in a pulsed acoustic field

It has previously been reported that the addition of low concentrations of ionic surfactants enhances the steady-state sonoluminescence (SL) intensity relative to water (Ashokkumar; et al. J. Phys. Chem. B 1997, 101, 10845). In the current study, both sonoluminescence and passive cavitation detection (PCD) were used to examine the acoustic cavitation field generated at different acoustic pulse lengths in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). A decrease in the SL intensity was observed in the presence of low concentrations of SDS and short acoustic pulse lengths. Under these conditions, the inhibition of bubble coalescence by SDS leads to a population of smaller bubbles, which dissolve during the pulse "off time". As the concentration of surfactant was increased at this pulse length, an increase in the acoustic cavitation activity was observed. This increase is partly attributed to enhanced growth rate of the bubbles by rectified diffusion. Conversely, at long pulse lengths acoustic cavitation activity was enhanced at low SDS concentrations as a larger number of the smaller bubbles could survive the pulse "off time". The effect of reduced acoustic shielding and an increase in the "active" bubble population due to electrostatic repulsion between bubbles are also significant in this case. Finally, as the surfactant concentration was increased further, the effect of electrostatic induced impedance shielding or reclustering dominates, resulting in a decrease in the SL intensity.     

Effect of alcohols on the initial growth of multibubble sonoluminescence

The effect of alcohols on the initial growth of the multibubble sonoluminescence (MBSL) intensity in aqueous solutions has been investigated. With increasing concentrations of the alcohols, the number of pulses required to grow the MBSL intensity to a steady state (N(crit)) increases (relative to that of water) initially to a maximum for all the alcohols used in this study, followed by a decrease for methanol and ethanol. The cause of the initial increase in N(crit) is attributed to the inhibition of bubble coalescence in the system. This inhibition in bubble coalescence results in a population of bubbles with a smaller size range and thus a larger number of pulses is required to grow the bubbles to their sonoluminescing size range. It is suggested that the decrease in the N(crit) at higher alcohol concentrations may be caused by an increase in the bubble growth by rectified diffusion.     

Sonochemistry and sonoluminescence in ionic liquids, molten salts, and concentrated electrolyte solutions

Ionic liquids have favorable intrinsic properties that make them of interest as solvents for various chemical reactions. The same properties that make the liquids effective solvents also make them interesting liquids for studies involving sonochemistry, acoustic cavitation, and sonoluminescence. Recent interest in using ultrasound to accelerate chemical reactions conducted in ionic liquids necessitates an understanding of the effects of acoustic cavitation on these solvents. Here, we review our previous results on the effects of cavitation on some room-temperature ionic liquids, including the sonoluminescence spectra of molten salt eutectics and concentrated aqueous electrolyte solutions. In all cases, regardless of the essentially nonexistent vapor pressure of the solution atomic and small molecule emitters are observed in the spectra which arise from sonolysis of the ionic liquids.     

Cavitation, shock waves and the invasive nature of sonoelectrochemistry

The invasive nature of electrodes placed into sound fields is examined. In particular, perturbations of the sound field due to the presence of the electrode support are explored. The effect of an electrode on the drive sound field (at approximately 23 kHz) is shown to be negligible under the conditions investigated in this paper. However, scattering of shock waves produced by cavity collapse is shown to exhibit a significant effect. To demonstrate this, multibubble sonoluminescence (MBSL) and electrochemical erosion measurements are employed. These measurements show an enhancement, due to the reflection by the solid/liquid boundary at the electrode support, of pressure pulses emitted when cavitation bubbles collapse. To first order, this effect can be accounted for by a correction factor. However, this factor requires accurate knowledge of the acoustic impedance of the interface and the electrolyte media. These are measured for two commonly employed substrates (soda glass and epoxy resin, specifically Epofix). A scattering model is developed which is able to predict the acoustic pressure as a function of position over a disk-like electrode substrate. The effects of shock wave reflection and materials employed in the electrode construction are used to clarify the interpretation of the results obtained from different sonoelectrochemical experiments. Given the widespread experimentation involving the insertion of electrodes (or other sensors) into ultrasonic fields, this work represents a significant development to aid the interpretation of the results obtained.     

Acoustic emission from cavitating solutions: implications for the mechanisms of sonochemical reactions

The acoustic emission from collapsing cavitation bubbles generated using ultrasound of 20 kHz and 515 kHz frequencies in water has been measured and correlated with sonoluminescence and hydroxyl radical production to yield further information on the frequency dependence of sonochemical reactions. A reasonable correlation was found, and the results suggest differences in the predominant types of cavitation observed under laboratory conditions.     

Sonochemistry and sonoluminescence of room-temperature ionic liquids

The sonication of ionic organic liquids leads to decomposition of the liquids. Multibubble sonoluminescence spectra and headgas analysis reveal a variety of decomposition products from the sonolysis of N,N'-dialkylimidazolium ionic liquids. The decomposition is a result of acoustic cavitation, which generates localized hot spots from the implosive collapse of bubbles in the ionic liquids. Despite the negligible vapor pressure of the ionic liquids, reaction still occurs in a heated shell of the bubbles or from microdroplets thrown into the collapsing bubbles.     

A comparison between multibubble sonoluminescence intensity and the temperature within cavitation bubbles

Cavitation bubble temperatures have been measured using a methyl radical recombination method and compared with the changes in the sonoluminescence intensity in aqueous ethanol solutions over a range of concentrations. Whereas the sonoluminescence intensity was decreased by more than 90% at low ethanol concentrations (<0.1 M), the measured bubble temperatures seem to be unaffected at this level of additive. The cavitation bubble temperatures were noticeably decreased at substantially higher ethanol concentrations (0.5 M). It has been concluded that the methyl radical recombination method does not report on the true sonoluminescence temperatures. However, it does report on the average bubble temperatures at which sonochemical reactions occur.     

On the mechanism of multi-bubble sonoluminescence

The thermal chemiluminescent model of multibubble sonoluminescence (MBSL) is considered, and the contradictions that follow from its basic propositions are analyzed. It is shown that, if the thermal mechanism of sonoluminescence (SL) is operative, the continuous spectrum should be emitted before the band spectrum. It is established that, if the thermal recombination mechanism of a solid is operative, the duration of SL should be at least two orders of magnitude longer than that observed experimentally (<5 ns); according to the chemiluminescent model of SL, the SL burst time must be several orders of magnitude longer. Consideration is given to the previously proposed mechanism of emission of spectral lines of metals from solutions of alkali and alkaline-earth metal salts, which correlates with results of sonolysis of salt solutions under the action of ultrasound (US) pulses. A mechanism based on the theory of local electrification of cavitation bubbles in the US field is put forward to treat MBSL. It is shown that this mechanism agrees with experimental results: the SL burst time corresponds to the characteristic time of fluorescence.     

The dynamics of a bubble ensemble in a cavitation field

A system of equations was obtained to describe the dynamics of bubbles in a cavitation cloud taking into account the interaction of pulsating bubbles involved in translational motion. The kinetics of cavitation bubble concentration changes, changes in the compressibility of the liquid, and phase transitions within a cavitation bubble and in the neighboring volume of the liquid were taken into account. The role played by bubble deformation in a cavitation cloud was considered. The Bernoulli pressure effect was shown to be negligible. The interaction of cavitation bubbles was a substantial factor that strongly influenced the dynamics of bubbles. It was suggested that there was at least one more mechanism that reduced sonoluminescence intensity from the multiple-bubble cavitation field, namely, a fairly high efficiency of sonoluminescence quenching could additionally be related to the arrival of a cumulative liquid stream at the central cavitation bubble region, where the concentration of active species was high. The dynamics of bubbles in the cavitation field is not only related to the expansion and compression of cavitation bubbles in the acoustic field, but also governed to a great extent by their interaction, translational motion, deformation, and the influence of cumulative streams penetrating the bubbles.     

The influence of acoustic power on multibubble sonoluminescence in aqueous solution containing organic solutes

The effect of varying the applied acoustic power on the extent to which the addition of water-soluble solutes affect the intensity of aqueous multibubble sonoluminescence (MBSL) has been investigated. Under most of the experimental conditions used, the addition of aliphatic alcohols to aqueous solutions was found to suppress the MBSL intensity, although an enhancement of the MBSL intensity was also observed under certain conditions. In contrast, the presence of an anionic surfactant sodium dodecyl sulfate (SDS) in aqueous solutions generally enhanced the observed MBSL intensity. For a series of aliphatic alcohols and SDS, a strong dependence of the MBSL intensity on the applied acoustic power (in the range of 0.78-1.61 W/cm(2)) at 358 kHz was observed. The relative SL quenching was significantly higher at higher acoustic powers for the alcohol solutions, whereas the relative SL enhancement was lower at higher acoustic powers in SDS solutions. These observations have been interpreted in terms of a combination of material evaporation into the bubble, rectified diffusion, bubble clustering and bubble-bubble coalescence.     

Line emission of sodium and hydroxyl radicals in single-bubble sonoluminescence

Spectroscopic studies of single-bubble sonoluminescence (SBSL) in water and aqueous sodium chloride solutions with a defined concentration of argon were performed as a function of the driving acoustic pressure. The broad-band continuum ranging from 200 to 700 nm is characterized by fits using Planck's law of blackbody radiation. The obtained blackbody temperatures are in the range of 10(4) K and are revealed to be independent of the presence of a salt and the acoustic pressure, whereas the SL intensity increases by a factor of more than 10 within the studied acoustic pressure range. The different trends followed by SL intensity and blackbody temperatures question the blackbody model. In solutions with 70 mbar of argon, line emissions of OH(?) radicals and Na* are observed. The shape of the OH(?) radical emission spectrum is very similar to that in MBSL spectra, indicating the strong similarity of intrabubble conditions. An increase of the acoustic pressure causes the continuum to overlap the lines until they become indistinguishable. The emission line of Na* in NaCl is observed only at high NaCl concentrations. When sodium dodecylsulfate is used a pronounced Na* line is already observed in a 1 mM solution thanks to enrichment of sodium ions at the interface. The results presented in this work reveal the strong similarity of SBSL and MBSL under certain experimental conditions.     

A sonoluminescent method for on-line monitoring of the natural water quality

Ultrasonic impact on natural waters was applied to identify hydrochemical anomalies from the changes in the characteristics of sonoluminescence generated by ultrasonic cavitation in a flowing water sample. Sonoluminescence excitation spectra characteristic for various natural water pollutants were obtained for the first time. The suitability of changes in the sonoluminescence intensity and in the shape of the sonoluminescence excitation spectra as indicators of hydrochemical anomalies was elucidated.     

Multibubble sonoluminescence of Tb3+ ion in aqueous solutions of dimethyl sulfoxide

The intensity and spectra of multibubble sonoluminescence of TbCl₃ solutions in water-DMSO mixtures saturated with air and argon are studied. The spectra represent the superposition of the characteristic glow of Tb³⁺ ions and the continuum of emission of electronically excited products of solvent sonolysis (with H₂O*, OH*, and SO₂* as main emitters). Abnormal action of DMSO and sulfur dioxide on the characteristic luminescence of Tb³⁺ ions during sonolysis of aqueous solutions is revealed. These additives enhance the sonoluminescence of water to different extent, quench the sonoluminescence of Tb³⁺, and differently influence the photoluminescence quantum yield of this ion (DMSO acts as activator, whereas SO₂ acts as quencher). Sulfur dioxide quenches the sonoluminescence of Tb3+ much more efficiently than the photoluminescence of Tb³⁺. The abnormal effect of DMSO on sonoluminescence is most probably due to the quenching action of sulfur dioxide formed during sonolysis of DMSO on Tb³⁺* ions in cavitation bubbles.     

On the emitters of sulfuric acid sonoluminescence

A comparative study of the sonoluminescence spectra of water and argon-saturated aqueous H₂SO₄ solutions was carried out. At an H₂SO₄ concentration of 18 mol L⁻¹, the sulfuric acid sonoluminescence is fifty times more intense than water sonoluminescence. The sulfuric acid luminescence spectrum differs from the water sonoluminescence spectrum caused by the emission of excited water molecules and OH radicals from the gas phase of cavitation bubbles. The sulfuric acid sonoluminescence spectrum exhibits maxima at 330, 420, 500, and 630 nm. Emitters of sonoluminescence of sulfuric acid are the singlet (330–340 nm) and triplet (∼420 nm) excited SO₂ molecules formed by sonolysis of H₂SO₄ molecules. Another product of sonolysis of H₂SO₄, atomic oxygen, is assumed to be responsible for the luminescence at λ = 630 nm. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1742–1745, August, 2005.     

Single bubble sonoluminescence--a chemist's overview.

The phenomenon of sonoluminescence has been known for over 60 years but it is only over the last few years that a better understanding of its origins has emerged. In part the discovery of single bubble sonoluminescence, just over 10 years ago, has been a major contributor to the theoretical advances that have been made to account for the event. This Minireview is from the perspective of a physical chemist and considers the progress that has been made in understanding the role of solutes in affecting the sonoluminescence from a solution exposed to ultrasound. The physicochemical properties of solutes that are important in controlling both single bubble and multibubble sonoluminescence are discussed.     

Sonochemiluminescence of aromatic hydrocarbons

Multibubble sonoluminescence of water and a series of aromatic hydrocarbons, viz., benzene, toluene, ethylbenzene, and m-xylene (at 25 °C), and a naphthalene melt (at 110–120 °C) was studied. An analysis of the influence of oxygen and argon on the sonoluminescence intensity and the luminescence spectra of these liquid compounds, as well as the effect of additives of ionol, ethanol, and 9,10-dibromoanthracene on m-xylene sonoluminescence, showed that a considerable contribution to the sonoluminescence of aromatic hydrocarbons is made by chemiluminescence reactions associated with their oxidation. This sonochemiluminescence is observed in both the gas phase of cavitation bubbles and the bulk solution where luminescence is retained for a long time after ultrasonication switching-off (the initial intensity of the residual chemiluminescence is up to 10% of the luminescence intensity during sonolysis). As for thermoinitiated oxidation, the afterglow of m-xylene contains the radical and molecular components.     

Chemical analysis of acoustically levitated drops by Raman spectroscopy

An experimental apparatus combining Raman spectroscopy with acoustic levitation, Raman acoustic levitation spectroscopy (RALS), is investigated in the field of physical and chemical analytics. Whereas acoustic levitation enables the contactless handling of microsized samples, Raman spectroscopy offers the advantage of a noninvasive method without complex sample preparation. After carrying out some systematic tests to probe the sensitivity of the technique to drop size, shape, and position, RALS has been successfully applied in monitoring sample dilution and preconcentration, evaporation, crystallization, an acid–base reaction, and analytes in a surface-enhanced Raman spectroscopy colloidal suspension. Figure We have systematically investigated the analytical potential of Raman spectroscopy of samples in acoustically levitated drops.     

The analysis of structural defects in piezo and ferroelectric materials by the “heat pulse” method

We consider the “heat pulse” technique for the analysis of structural imperfections. This method is based on the propagation of a pulse or packet of pulses of non-equilibrium acoustic phonons with 10¹¹–10¹² Hz frequencies.     

Mechanism of Sonochemical Reactions and Sonoluminescence

Contemporary concepts of the mechanism of sonochemical reactions and sonoluminescence are reviewed. The most elaborated ideas are the thermal theory and the theory of the local charging of cavitation bubbles, wherein the latter theory provides the best fit to experimental data. The primary physical processes occurring upon initiation of sonochemical reactions and sonoluminescence and their similarity and dissimilarity with the corresponding processes relevant to high energy chemistry are considered.     

Sonochemistry as a New Promising Area of High Energy Chemistry

Sonochemistry is one of the most intensively developing areas of high energy chemistry in both theoretical and applied aspects. There has been considerable progress in sonochemistry: many sonochemical reactions in nonaqueous systems were revealed, power yields for some sonochemical reactions and the initial power yields of ultrasonic degradation of water were determined, single-bubble sonoluminescence was discovered, the formation of radicals in an ultrasound field was detected by spin trapping, a method for the generation of powerful low-frequency (of the order of 10–200 Hz) cavitation fields was developed and chemical and physicochemical effects in these fields were revealed, high-performance hydrodynamic apparatus were designed and sonoluminescence and chemical effects were observed in such systems, ultrasound-induced oscillating reactions were discovered, and the generalized electric theory of cavitation processes was elaborated.