Topics in Current Chemistry (2020) 378:2
1 3
1.3 Sonochemistry
1.3.1 A Brief History
Sound waves not detectable by the human ear with frequencies ranging from 20 kHz
to 200 MHz are referred to as ultrasound (US) waves [10]. The effects of sonication
are linked to the cavitation phenomena, and they can be chemical, physical, mechanical, or optical. The first reference to the cavitation phenomena by Thornycroft and
Barnaby dates from 1895 [11]. By the time Neppiras introduced the term “sonochemistry” in 1980 [12] and Makino et al. showed the formation of radical species
during the sonolysis of water in 1982 [13, 14], the research attraction of sonochemistry had increased dramatically. In general, sonochemistry is linked with the understanding and interpretation of the processes and the effects initiated by US irradiation due to the cavitation phenomena. The main derived results are the enhancement
of the reaction rate, radical species formation, as well as mass and heat transfer [1,
2, 4, 15–17].
1.3.2 Cavitation Phenomena—Mechanistic Aspects on “How Does Everything
Work?”
The formation of cavitation bubbles is due to pressure changes upon the travel of US
waves in a liquid. The initially formed microbubbles, consisting of vaporized solvent
or/and dissolved gases, grow continuously in size by absorbing energy during the
irradiation [18]. After growing to a certain size, they violently collapse, creating a
localized “hot spot.” The local pressure and temperature can be above 1000 bars and
5000 K, respectively [18–20]. The hot-spot concept and the consequential effects
can be described by distinguishing three zones [21, 22]. Zone 1 is inside the bubble (primary sonochemistry), zone 2 is at the gas–liquid interface (secondary sonochemistry), and zone 3 is the bulk liquid phase surrounding zone 2. At the interior
of the cavity, cleavage of bonds and formation of radicals occurs due to the harsh
energetical environment and to the fact that the gaseous concentration is extended
[22–24]. These radical species can also be transported to zone 2, where reaction of
free radicals and pyrolysis can take place, or even to the bulk liquid zone 3. In an
aqueous environment, the sonolysis of water can lead to the formation of hydroxyl
(HO
·
) or hydroperoxyl (HO 2
·
) radicals, and hydrogen peroxide (H 2 O 2 ). These species
can initiate secondary reactions that can play a key role in material synthesis or for
catalytic reactions [15].
Even if the “hot-spot” theory is the most accepted to explain these phenomenas,
several studies lead to the proposition of plasma [25, 26], electrical [27], and supercritical water [28] theories, demonstrating that all the mechanisms involving US are
not completely known. In addition to the chemical effect of US, various other mechanisms exist, such as physical, mechanical, or light emission (sonoluminescence).
The latter one is also valuable in order to determine the active regions and intensity
of the US waves using a hydroxyl radical trapping agent, luminol (through chemiluminescence [29–31]. The physical/mechanical effects can be the formation of microjets, turbulence, microstreaming, shockwaves, and agitations [23]. These effects can
32
Reprinted from the journal
1 3
1.3 Sonochemistry
1.3.1 A Brief History
Sound waves not detectable by the human ear with frequencies ranging from 20 kHz
to 200 MHz are referred to as ultrasound (US) waves [10]. The effects of sonication
are linked to the cavitation phenomena, and they can be chemical, physical, mechanical, or optical. The first reference to the cavitation phenomena by Thornycroft and
Barnaby dates from 1895 [11]. By the time Neppiras introduced the term “sonochemistry” in 1980 [12] and Makino et al. showed the formation of radical species
during the sonolysis of water in 1982 [13, 14], the research attraction of sonochemistry had increased dramatically. In general, sonochemistry is linked with the understanding and interpretation of the processes and the effects initiated by US irradiation due to the cavitation phenomena. The main derived results are the enhancement
of the reaction rate, radical species formation, as well as mass and heat transfer [1,
2, 4, 15–17].
1.3.2 Cavitation Phenomena—Mechanistic Aspects on “How Does Everything
Work?”
The formation of cavitation bubbles is due to pressure changes upon the travel of US
waves in a liquid. The initially formed microbubbles, consisting of vaporized solvent
or/and dissolved gases, grow continuously in size by absorbing energy during the
irradiation [18]. After growing to a certain size, they violently collapse, creating a
localized “hot spot.” The local pressure and temperature can be above 1000 bars and
5000 K, respectively [18–20]. The hot-spot concept and the consequential effects
can be described by distinguishing three zones [21, 22]. Zone 1 is inside the bubble (primary sonochemistry), zone 2 is at the gas–liquid interface (secondary sonochemistry), and zone 3 is the bulk liquid phase surrounding zone 2. At the interior
of the cavity, cleavage of bonds and formation of radicals occurs due to the harsh
energetical environment and to the fact that the gaseous concentration is extended
[22–24]. These radical species can also be transported to zone 2, where reaction of
free radicals and pyrolysis can take place, or even to the bulk liquid zone 3. In an
aqueous environment, the sonolysis of water can lead to the formation of hydroxyl
(HO
·
) or hydroperoxyl (HO 2
·
) radicals, and hydrogen peroxide (H 2 O 2 ). These species
can initiate secondary reactions that can play a key role in material synthesis or for
catalytic reactions [15].
Even if the “hot-spot” theory is the most accepted to explain these phenomenas,
several studies lead to the proposition of plasma [25, 26], electrical [27], and supercritical water [28] theories, demonstrating that all the mechanisms involving US are
not completely known. In addition to the chemical effect of US, various other mechanisms exist, such as physical, mechanical, or light emission (sonoluminescence).
The latter one is also valuable in order to determine the active regions and intensity
of the US waves using a hydroxyl radical trapping agent, luminol (through chemiluminescence [29–31]. The physical/mechanical effects can be the formation of microjets, turbulence, microstreaming, shockwaves, and agitations [23]. These effects can
32
Reprinted from the journal