88
6 Ultrasound as Mechanical Force
used, although it has resulted in two distinctive domains being defined: covalent and
non-covalent mechanochemistry, the latter of which is also called soft mechanochemistry (Ribas-Arino and Marx 2012; Zhang et al. 2015; Lavalle et al. 2016; Stauch
and Dreuw 2016; Garcia-Manyes and Beedle 2017). In covalent mechanochemistry,
a mechanical action causes ruptures at the molecular level (although as we shall see
later bond formation can occur as well), leading to a new substance or a material
with different properties. These processes are usually irreversible, meaning that the
force causes a permanent change in the molecules which do not revert to their initial
state when the force is removed. On the other hand, non-covalent mechanochemistry
does not affect chemical bonds and its processes are generally reversible, relying on
conformational processes that affect small fragments and supramolecular entities.
Both fields are important, complementary and are involved in the development of
chemo-mechanoresponsive systems.
Mechanochemistry offers inherent advantages and opportunities for cleaner syntheses and environmentally compatible chemistry. Reactions in the field generally
occur between solids, often in fast and quantitative protocols, require either no or
reduced amounts of volatile solvents, while purification and recycling are greatly
facilitated. Accordingly, most mechanochemical applications involve solvent-free
transformations or slurries containing low solvent quantities, which sets them apart
from ultrasonic reactions that require a liquid phase for the propagation of sound
waves and the induction of cavitation.
6.2 Sonochemistry: Tribochemical Interpretations
Sonochemists are of course aware of the intertwined chemical and mechanical effects
caused by cavitational collapse, with the latter being largely due to the shear forces
and shock waves experienced at the bubble–bulk liquid interface that are capable
of dramatically influencing heterogeneous reactions (Mason and Lorimer 2002).
Useful analogies between sonochemistry and mechanochemistry can be drawn from
tribochemistry, the subdiscipline that deals with the stimulation of chemical transformations through friction at boundary surfaces, leading to mechanical-energy-caused
physico-chemical changes in solids (Boldyrev 1995). For example, atomic dislocations and structural defects in solids often serve as the preferred reaction sites under
ultrasonic action. As expected, mechanical effects, such as chemical production, are
strongly dependent on the parameters that influence cavitational events. In particular, there is an inverse dependence between mechanical and chemical effects and the
frequency of ultrasound (Mason et al. 2011). Moreover, the estimated thresholds for
cavitational, chemical and mechanical effects to occur are also dependent on acoustic
frequency (Nguyen et al. 2017). At frequencies lower than 100 kHz, the mechanical
effect threshold, determined using the erosion of aluminium foil, nearly equals the
threshold required for cavitation. However, the threshold for mechanical effects is
much higher than the cavitation threshold at high frequency.
6 Ultrasound as Mechanical Force
used, although it has resulted in two distinctive domains being defined: covalent and
non-covalent mechanochemistry, the latter of which is also called soft mechanochemistry (Ribas-Arino and Marx 2012; Zhang et al. 2015; Lavalle et al. 2016; Stauch
and Dreuw 2016; Garcia-Manyes and Beedle 2017). In covalent mechanochemistry,
a mechanical action causes ruptures at the molecular level (although as we shall see
later bond formation can occur as well), leading to a new substance or a material
with different properties. These processes are usually irreversible, meaning that the
force causes a permanent change in the molecules which do not revert to their initial
state when the force is removed. On the other hand, non-covalent mechanochemistry
does not affect chemical bonds and its processes are generally reversible, relying on
conformational processes that affect small fragments and supramolecular entities.
Both fields are important, complementary and are involved in the development of
chemo-mechanoresponsive systems.
Mechanochemistry offers inherent advantages and opportunities for cleaner syntheses and environmentally compatible chemistry. Reactions in the field generally
occur between solids, often in fast and quantitative protocols, require either no or
reduced amounts of volatile solvents, while purification and recycling are greatly
facilitated. Accordingly, most mechanochemical applications involve solvent-free
transformations or slurries containing low solvent quantities, which sets them apart
from ultrasonic reactions that require a liquid phase for the propagation of sound
waves and the induction of cavitation.
6.2 Sonochemistry: Tribochemical Interpretations
Sonochemists are of course aware of the intertwined chemical and mechanical effects
caused by cavitational collapse, with the latter being largely due to the shear forces
and shock waves experienced at the bubble–bulk liquid interface that are capable
of dramatically influencing heterogeneous reactions (Mason and Lorimer 2002).
Useful analogies between sonochemistry and mechanochemistry can be drawn from
tribochemistry, the subdiscipline that deals with the stimulation of chemical transformations through friction at boundary surfaces, leading to mechanical-energy-caused
physico-chemical changes in solids (Boldyrev 1995). For example, atomic dislocations and structural defects in solids often serve as the preferred reaction sites under
ultrasonic action. As expected, mechanical effects, such as chemical production, are
strongly dependent on the parameters that influence cavitational events. In particular, there is an inverse dependence between mechanical and chemical effects and the
frequency of ultrasound (Mason et al. 2011). Moreover, the estimated thresholds for
cavitational, chemical and mechanical effects to occur are also dependent on acoustic
frequency (Nguyen et al. 2017). At frequencies lower than 100 kHz, the mechanical
effect threshold, determined using the erosion of aluminium foil, nearly equals the
threshold required for cavitation. However, the threshold for mechanical effects is
much higher than the cavitation threshold at high frequency.