Sonoluminescence intensity and ultrasonic cavitation temperature in organic solvents: Effects of generated radicals.

Ultrasonic cavitation in organic solvents remains poorly understood in contrast with aqueous systems, largely because of complexities related to solvent decomposition. In this study, we sonicated different types of organic solvents (i.e. linear alkanes, aliphatic alcohols, aromatic alcohols, and acetate esters) under argon saturation. The average temperature of the cavitation bubbles was estimated using the methyl radical recombination method. We also discuss the effects of the physical properties of the solvents, such as vapor pressure and viscosity, on the cavitation temperature. The average cavitation bubble temperature and sonoluminescence intensity were higher in organic solvents with lower vapor pressure; for aromatic alcohols, these values were particularly high. It was found that the specific high sonoluminescence intensities and average cavitation temperatures exhibited in aromatic alcohols are caused by the highly resonance-stable generated radicals. The results obtained in this study are very useful for acceleration of sonochemical reaction in organic solvents, which are indispensable for organic synthesis and material synthesis.

Periodic Expansion and Contraction Phenomena in a Pendant Droplet Associated with Marangoni Effect.

A spontaneous oscillation between the expansion and contraction of a nitrobenzene pendant droplet containing di-(2-ethylhexyl)phosphoric acid (DEHPA) was observed in an aqueous phase under alkaline conditions. We described this phenomenon as the spontaneous oscillation of the oil-water interfacial tension. The oscillation characteristics such as the induction period and the interfacial-tension oscillation frequency were investigated under different temperatures and aqueous phase polarities. The effects of the interfacial tension of the biphasic pendant-droplet, the surface excess of the surfactant molecules, and the amount of nitrobenzene elution from the droplet to the aqueous phase on the oscillation characteristics were investigated. Consequently, the periodic expansion-contraction oscillation mechanism was explained through the adsorption-desorption cycle of DEHPA with respect to the aggregate formation of the inverted micelle of DEHPA. This study was based on a simple vibration phenomenon of interfacial tension, and is extremely important for clarifying the predominant factors that cause fluctuations in the free interface energy, which has been ambiguous.

Sonochemical degradation of surfactants with different charge types: Effect of the critical micelle concentration in the interfacial region of the cavity.

Ionic surfactants tend to accumulate in the interfacial region of ultrasonic cavitation bubbles (cavities) because of their surface active properties and because they are difficult to evaporate in cavitation bubbles owing to their extremely low volatilities. Hence, sonolysis of ionic surfactants is expected to occur in the interfacial region of the cavity. In this study, we performed sonochemical degradation of surfactants with different charge types: anionic, cationic, zwitterionic, and nonionic. We then estimated the degradation rates of the surfactants to clarify the surfactant behavior in the interfacial region of cavitation bubbles. For all of the surfactants investigated, the degradation rate increased with increasing initial bulk concentration and reached a maximum value. The initial bulk concentration to obtain the maximum degradation rate had a positive correlation with the critical micelle concentration (cmc). The initial bulk concentrations of the anionic surfactants were lower than their cmcs, while those of the cationic surfactants were higher than their cmcs. These results can be explained by the negatively charged cavity surface and the effect of the coexisting counterions of the surfactants.

Mechanism for sonochemical reduction of Au(III) in aqueous butanol solution under Ar based on the analysis of gaseous and water-soluble products.

When an aqueous Au(III) solution containing 1-butanol was sonicated under Ar, Au(III) was reduced to Au(0) to form Au particles. This is because various reducing species are formed during sonication, but the reactivity of these species has not yet been evaluated in detail. Therefore, in this study, we analyzed the effects of Au(III) on the rates of the formation of gaseous and water-soluble compounds (CH4, C2H6, C2H4, C2H2, CO, CO2, H2, H2O2, and aldehydes), and the rate of Au(III) reduction as a function of 1-butanol concentration. The following facts were recognized: 1) for Au(III) reduction, the contribution of the radicals formed by the pyrolysis of 1-butanol was higher than that of the secondary radicals formed by the abstraction reactions of 1-butanol with ·OH, 2) ·CH3 and CO acted as reductants, 3) the contribution of ·H to Au(III) reduction was small in the presence of 1-butanol, 4) aldehydes and H2 did not act as reductants, and 5) the types of species that reduced Au(III) changed with 1-butanol concentration.

Quantitative Analytical Method for Single Rain Droplets via Crystal Formation in Photocrosslinking Polymer Gel.

Ion composition contained in individual rain droplets provides important information to investigate the chemistry in rain and clouds, but general rain sampling equipment temporally and spatially averages the information. Determination of the SO42- concentration in an individual rain droplet was achieved by precipitate production in synthesized acrylamide polymer gel. Concentration of the target ion was calculated from the droplet print diameter and precipitation area measured from digital microscope images. We investigated the effects of the interior electrolyte concentration in the polyacrylamide gel and UV irradiation time on the physical properties of the gel and precipitate formation. The precipitated components were identified by scanning electron microscopy with energy dispersive X-ray analysis. We also clarified the effects of coexisting ions on the reaction between the interior and exterior electrolytes. For actual rainwater, the SO42- concentration estimated by this method was in agreement with the results obtained by ion chromatography.

Sonochemistry of aqueous NaAuCl4 solutions with C3-C6 alcohols under a noble gas atmosphere.

The effect of the type of C3-C6 alcohol, solution temperature, and dissolved gas on the rate of Au(III) reduction was investigated in NaAuCl4 aqueous alcohol solution with a 200-kHz ultrasound irradiation system. It was confirmed in the presence of C3-C6 alcohol that more highly hydrophobic alcohols more effectively accumulated at the argon bubble interface region, and the reducing radicals formed here. To avoid changes in the bubble temperature during collapsing bubble, the effects of the solution temperature on the rate of Au(III) reduction and on the rate of formation of the gaseous compounds (CO, CO2, CH4, C2H2, C2H4, C2H6) were investigated in the presence of low concentration (1.0-mM) of 1-hexanol. Both of the rates showed a good relationship with the gas solubility: the amount of dissolved gas at different solution temperatures affected the number of high-temperature bubbles formed. The changes in the concentrations of the gaseous compounds formed from 1-hexanol degradation suggested that CO and the pyrolysis radicals acted as reductants. Finally, the effect of the type of dissolved gas was investigated in the presence of 1.0-mM Au(III) and 1.0-mM 1-hexanol. The rates of 1-hexanol degradation, Au(III) reduction, and gaseous compound formation increased in the order He

Effects of Na2SO4 or NaCl on sonochemical degradation of phenolic compounds in an aqueous solution under Ar: Positive and negative effects induced by the presence of salts.

Sonochemical degradation of 4-chlorophenol, phenol, catechol and resorcinol was studied under Ar at 200 kHz in the absence and presence of Na2SO4 or NaCl. The rates of sonochemical degradation in the absence of salts decreased in the order 4-chlorophenol>phenol>catechol>resorcinol and this order was in good agreement with the order of log P (partition coefficient) value of each phenolic compound. The effects of salts on the rates of sonochemical degradation consisted of no effect or slight negative or positive effects. We discussed these unclear results based on two viewpoints: one was based on the changes in pseudo hydrophobicity and/or diffusion behavior of phenolic compounds and the other was based on the changes in solubility of Ar gas. The measured log P value of each phenolic compound slightly increased with increasing salt concentration. In addition, the dynamic surface tension for 4-chlorophenol aqueous solution in the absence and presence of Na2SO4 or NaCl suggested that phenolic compounds more easily accumulated at the interface region of bubbles at higher salt concentration. These results indicated that the rates of sonochemical degradation should be enhanced by the addition of salts. On the other hand, the calculated Ar gas solubility was confirmed to decrease with increasing salt concentration. The yield of H2O2 formed in the presence of Na2SO4 or NaCl decreased with increasing salt concentration. These results suggested that sonochemical efficiency decreased with decreasing gas amount in aqueous solution: a negative effect of salts was observed. Because negative and positive effects were induced simultaneously, we concluded that the effects of salts on the rates of sonochemical degradation of phenolic compounds became unclear. The products formed from sonochemical degradation of 4-chlorophenol were also characterized by HPLC analysis. The formation of phenol and 4-chloro-1,3-dihydroxy benzene was confirmed and these concentrations were affected by the presence of salts.

Preliminary study of quantitative analysis of ammonium ions in a raindrop following Liesegang ring formation.

NH(4)(+) concentration in an individual droplet was determined by forming a Liesegang ring on a gelatin film containing NaB(C(6)H(5))(4). The NH(4)(+) concentration (mol L(-1) abbreviated to M) was calculated from the NH(4)(+) amount (mol) ascertained in a droplet using pixel count measurements. The droplet volume (L) was calculated by measurement of the diameter of a droplet print on the gelatin film. For rainwater, the NH(4)(+) concentration estimated using this method corresponded with results obtained using ion chromatography.

Threshold for spontaneous oscillation in a three-phase liquid membrane system involving nonionic surfactant.

This study of self-oscillation was conducted using a new three-phase liquid membrane system of ethanol aqueous solution, benzyl alcohol solution with nonionic surfactant, and pure water. Relations of the initial ethanol concentration to the oscillation amplitude and frequency, and to the induction period before oscillations were investigated. The oscillation amplitude is independent of the initial ethanol concentration, but the frequency and the induction period are related to it. The oscillation frequency increased concomitantly with the increased ethanol initial concentration, but the induction period before the electrical oscillations decreased with increasing concentration. To estimate the influence of ethanol diffusion on the electrical oscillations, the ethanol concentration in each phase was measured using separate experiments after different durations of oscillation. The diffusion coefficient was calculated using Fick's second law. Results show successful estimation of the threshold for oscillations. The threshold is defined in terms of the ethanol concentration at the interface between the benzyl alcohol phase and the pure water phase.

Influence of adding salt on ultrasonic atomization in an ethanol-water solution.

Ethanol was enriched by ultrasonic atomization. Enrichment ratios were increased by adding salt to the ethanol solution. Different enrichment ratios were observed for different types of salts in a range of low ethanol concentrations. The enrichment ratio was significantly improved by adding K(2)CO(3) or (NH(4))(2)SO(4). It is concluded that this is due to enhanced interfacial adsorption of the ethanol. Addition of Na(2)CO(3) to the ethanol solution also enhanced the interfacial adsorption of the ethanol, but the effect was relatively small. Addition of NaCl to the ethanol solution did not enhance the interfacial adsorption of the ethanol.

Effect of reaction vessel diameter on sonochemical efficiency and cavitation dynamics.

The influence of reaction vessel diameter on the sonochemical yield was investigated by using reaction vessels with five different diameters. It was revealed that the formation of H(2)O(2) and chloride ion, from the sonolysis of pure water and 1,2,4-trichlorobenzene aqueous solution, was affected by the reaction vessel diameter. That is, these yields increased as the reaction vessel diameter increased up to ø 90 mm and then decreased over ø 90 mm. From the analyses of the measurement of sonochemiluminescence and the calorimetry, it was suggested that active cavitation bubbles were formed at certain zones. In the case of a larger diameter reaction vessel, it was suggested that bubble nuclei that have not grown up to the resonance size, escaped from the sonication zone to the non-sonication zone and dissolved away. As a result, the number of active cavitation bubbles and the yields of H(2)O(2) and chloride ion would decrease in the case of a larger diameter reaction vessel.

Sonochemical decomposition of organic acids in aqueous solution: understanding of molecular behavior during cavitation by the analysis of a heterogeneous reaction kinetics model.

The sonochemical decomposition of a low concentration of butyric acid was performed in an aqueous solution by use of 200 kHz ultrasound to discuss the reaction kinetics and molecular behavior during cavitation. Taking into account a Langmuir-type adsorption model, we propose a heterogeneous reaction kinetics model, which is based on the local reaction zone at the interface region of the cavitation bubbles, where the adsorption and desorption of butyric acid molecules from the bulk solution occur during bubble oscillation and then the existing molecules inside the local reaction zone are finally decomposed. To confirm our proposed kinetics model, the rates of decomposition were investigated as a function of the initial concentration of butyric acids in the different pH solutions. It was confirmed that our model could be reasonably applied to explain the obtained results and the pseudo rate constant (k) and the equilibrium constant (K) were able to be calculated: k is 8.0 microM min(-1) (pH 2) and 3.5 microM min(-1) (pH 10), and K is 5.7 x 10(-3) microM(-1) (pH 2) and 8.0 x 10(-3) microM(-1) (pH 10), respectively. By the analysis of the obtained K values, it was clear that the ionized organic acid molecules are relatively difficult to accumulate at the reaction zone, because of their lower hydrophobicity compared with that of the neutral ones. The results obtained in the sonochemical decomposition of benzoic acid were also able to be analyzed with the proposed kinetics model. In addition, we proposed an opinion toward the interpretation of a Langmuir-type adsorption model which has often been applied to explain heterogeneous reaction systems.

Sonochemical reduction of permanganate to manganese dioxide: the effects of H2O2 formed in the sonolysis of water on the rates of reduction.

Chemical effects of ultrasound have been actively researched in the field of the synthesis of various metal nanoparticles and nanostructured materials. It is very important to understand the reduction mechanism of metal ions, because the reduction processes can be often applied to the synthesis of various materials. In this study, the sonochemical reduction of MnO4- to MnO2 in water under Ar atmosphere was investigated to discuss the reduction mechanism. It has been reported that H, OH, H2 and H2O2 are formed from the sonolysis of water. To understand the roles of H2O2 on the reduction, the reaction of MnO4- with H2O2 without ultrasonic irradiation was investigated. The obtained results suggested the progress of the following reaction: 2MnO4-+3H2O2-->2MnO2+3O2+2OH-+2H2O. In addition, the rates of the sonochemical reduction of MnO4- were investigated in the presence of 1-propanol, where 1-propanol acted as an OH radical scavenger so that the amounts of the sonochemically formed H2O2 molecules were able to be controlled. The results clearly indicated that the sonochemically formed H2O2 molecules as well as H2 molecules and H atoms play an important role for MnO4- reduction. This mechanism was also supported by the analysis of pH changes during ultrasonic irradiation: the pH value increased as the sonochemical reduction of MnO4- proceeded.

Effect of carbon tetrachloride on sonochemical decomposition of methyl orange in water.

Two types of sonicators were used for the sonochemical decomposition of methyl orange (MO) in the presence and absence of carbon tetrachloride (CCl4): One is a 45kHz ultrasonic cleaning bath (a low intensity sonicator) and the other is a 200kHz ultrasonic reactor (a high intensity sonicator). It was clearly confirmed that the rates of the sonochemical decomposition of MO increased with increasing the concentration of CCl4 in both sonicators. The enhancement effect of CCl4 was much higher in the high intensity sonicator than in the low intensity one: by the addition of 100ppm of CCl4, the decomposition ratio of MO with the high intensity sonicator became 41 times larger, while that with the low intensity sonicator became 4.8 times larger. Based on the obtained results, it was suggested that the formed cavitation phenomenon was different between sonicators. It was also suggested that the sonochemical decomposition of MO in the presence of CCl4 would be useful to evaluate the sonochemical efficiency, because the rate of MO decomposition can be effectively enhanced by the sonolysis of CCl4.

Sonochemical degradation of various monocyclic aromatic compounds: relation between hydrophobicities of organic compounds and the decomposition rates.

Various aromatic compounds, i.e., nitrobenzene, aniline, phenol, benzoic acid, salicylic acid, 2-chlorophenol, 4-chlorophenol, styrene, chlorobenzene, toluene, ethylbenzene and n-propylbenzene were decomposed under identical ultrasonic irradiation conditions. The relationships between the initial rates of degradation of these aromatic compounds and their physicochemical parameters were systematically investigated. It was revealed that some correlations between the degradation rates and parameters of volatility, Henry's law constant and vapor pressure, were observed only in the limited high range of parameters. It was suggested that the Henry's law constant and vapor pressure had influenced on the rate of degradation for some of the tested aromatic compounds. In contrast, better correlations between the initial rates of degradation and hydrophobic parameters, water solubility and LogP (water-octanol partition coefficient), were observed over the wide range of chosen parameters. These results meant that the hydrophobicity of the compounds significantly affected their accumulation at the gas-liquid interface of the bubbles and it was the most important factor for the sonochemical degradation of aromatic compounds. In particular, for the sonolysis of water-insoluble organic compounds, LogP was found to be the representative parameter for understanding the hydrophobic properties of water-insoluble compounds.