Relationship between the bubble temperature and main oxidant created inside an air bubble under ultrasound

Abstract

Numerical simulations of nonequilibrium chemical reactions in a pulsating air bubble have been performed for various ultrasonic frequencies (20 kHz, 100 kHz, 300 kHz, and 1 MHz) and pressure amplitudes (up to 10 bars). The results of the numerical simulations have indicated that the main oxidant is OH radical inside a nearly vaporous or vaporous bubble which is defined as a bubble with higher molar fraction of water vapor than 0.5 at the end of the bubble collapse. Inside a gaseous bubble which is defined as a bubble with much lower vapor fraction than 0.5, the main oxidant is H2O2 when the bubble temperature at the end of the bubble collapse is in the range of 4000-6500 K and O atom when it is above 6500 K. From the interior of a gaseous bubble, an appreciable amount of OH radical also dissolves into the liquid. When the bubble temperature at the end of the bubble collapse is higher than 7000 K, oxidants are strongly consumed inside a bubble by oxidizing nitrogen and the main chemical products inside a bubble are HNO2, NO, and HNO3.

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…For a bubble with high water vapour content the main oxidant is •OH, however in bubbles with less water content, temperature dependence becomes apparent. When the bubble temperature is in the range of 4000 -6500 K, H 2 O 2 is the dominant product, whilst above ~6500K it is the O atom [101]. There are three reaction sites in sonochemical experiments; the gas phase region inside the bubble (pyrolysis reactions), the bubble surface (reactions occurring in pressure temperature gradients) and the bulk solution [100,[102][103][104].…”

Critical factors in sonochemical degradation of fumaric acid

“…Since the ratios of specific heats of organic vapours are lower than that of air, volatile organics in or around cavitation bubbles may decrease the collapse temperature of cavitation bubbles, resulting in slower volatile organics decomposition [39,40]. As seen in Table 1, the rate constant of fumaric acid degradation at 0.1 mM is over 3.6-times higher at 1.0 mM, indicating that the rate of sonochemical degradation of fumaric acid is directly linked to the availability of the active species such as OH Å , O 2 ÅÀ as well as O atoms that are generated in solution [18,19,23,41], i.e. the concentration of active species formed by sonication is, relatively speaking, insufficient for the rapid oxidation of higher initial concentrations of fumaric acid.…”

“…[18,19]. Inside a gaseous bubble the main oxidant is H 2 O 2 when the bubble temperature at the end of the bubble collapse is in the range of 4000-6500 K and O atom when it is above 6500 K [19]. Such active species react with volatile solutes in the gas phase or react with dissolved solutes in an interfacial area or even in the bulk solution [20].…”

“…[18,19]. Inside a gaseous bubble the main oxidant is H 2 O 2 when the bubble temperature at the end of the bubble collapse is in the range of 4000-6500 K and O atom when it is above 6500 K [19].…”

The decomposition of protoporphyrin IX by ultrasound is dependent on the generation of hydroxyl radicals

“…Because the solutions were air saturated, the hydrogen atoms formed from inertial cavitation were immediately scavenged by oxygen [5]. Oxygen atoms produced inside the acoustic bubbles also could result in PpIX decomposition, however, the large inhibitory effects of the hydroxyl radical scavengers exclude oxygen as a major role [35]. Hydroxyl radicals were mainly responsible for the sonochemical activation of PpIX.…”