1.6 Parameters Influencing the Sonochemical Activity
7
on the medium properties by temperature variations. Each liquid possesses its optimum range of cavitation intensity such as water with a 20–50 °C range or ethanol
around 15–27 °C (Niemczewski 1980). Monitoring/controlling of the working temperature actively prevents ‘false sonochemistry’ examples where beneficial effects
are wrongly attributed to an ultrasonic consequence but due in fact to temperature
rise. Low-frequency ultrasonic irradiation, through induced physical effects, creates
an important macroscopic rise of temperature making necessary thermostatic control
of the medium. At high frequency, the rise of temperature is greatly diminished in
reason of the quasi-absence of mechanical effects resulting in a residual increase
in temperature of a very few degrees easily controllable. Working at temperatures
nearby boiling point may also appear inappropriate unless purposely intended. In
these conditions, vaporization bubbles, displaying much bigger diameters than cavitation ones, coalesce with the latter suppressing greatly cavitation powerness. This
phenomenon is usually referred as ‘vaporous’ cavitation. A liquid/gas emulsion may
also take place acting as an ‘acoustic cushioning’ by reflecting sound waves since a
gas phase has a higher attenuation factor than a liquid one.
1.6.4 Solvent Properties
A certain number of solvent properties can influence acoustic cavitation above all
when considering the variety of potential liquid media (water, organic solvents, liquefied gases, molten organic and inorganic salts, etc.). In any case, the weaker the
intermolecular forces insuring the cohesion of a liquid, the easier will be reached
the cavitation threshold but also the less energetic it will be. Thus, liquids with
high viscosities and surface tensions display higher cavitation threshold but also
harsher collapsing conditions as higher acoustic pressures are necessary to tear apart
molecules of the liquid. In addition, very volatile solvents are usually not recommended under low-frequency ultrasound since they may smooth acoustic activity
by making a too dense bubbles cloud creating detrimental bubbles coalescence and
sound wave reflection/attenuation.
1.6.5 Dissolved Gas
The first action of ultrasound on a liquid is degassing, such as when using an ultrasonic
bath to degas solvents prior to HPLC use. Thus, cavitation may occur greatly at early
stages followed by a less intensive period in reason of the degassing phase. For that
reason, entrain gas is often added in order to sustain or even to promote cavitation
activity. Interestingly, the nature of this gas is of crucial aspect since the ratio of
specific heat (or polytropic ratio) of the ambient gas (C p /C v ) exerts a great influence
on temperature and pressure reached during the collapsing phase (Rooze et al. 2013).
Typically, collapsing temperature and pressure reached in the presence of Argon (with
7
on the medium properties by temperature variations. Each liquid possesses its optimum range of cavitation intensity such as water with a 20–50 °C range or ethanol
around 15–27 °C (Niemczewski 1980). Monitoring/controlling of the working temperature actively prevents ‘false sonochemistry’ examples where beneficial effects
are wrongly attributed to an ultrasonic consequence but due in fact to temperature
rise. Low-frequency ultrasonic irradiation, through induced physical effects, creates
an important macroscopic rise of temperature making necessary thermostatic control
of the medium. At high frequency, the rise of temperature is greatly diminished in
reason of the quasi-absence of mechanical effects resulting in a residual increase
in temperature of a very few degrees easily controllable. Working at temperatures
nearby boiling point may also appear inappropriate unless purposely intended. In
these conditions, vaporization bubbles, displaying much bigger diameters than cavitation ones, coalesce with the latter suppressing greatly cavitation powerness. This
phenomenon is usually referred as ‘vaporous’ cavitation. A liquid/gas emulsion may
also take place acting as an ‘acoustic cushioning’ by reflecting sound waves since a
gas phase has a higher attenuation factor than a liquid one.
1.6.4 Solvent Properties
A certain number of solvent properties can influence acoustic cavitation above all
when considering the variety of potential liquid media (water, organic solvents, liquefied gases, molten organic and inorganic salts, etc.). In any case, the weaker the
intermolecular forces insuring the cohesion of a liquid, the easier will be reached
the cavitation threshold but also the less energetic it will be. Thus, liquids with
high viscosities and surface tensions display higher cavitation threshold but also
harsher collapsing conditions as higher acoustic pressures are necessary to tear apart
molecules of the liquid. In addition, very volatile solvents are usually not recommended under low-frequency ultrasound since they may smooth acoustic activity
by making a too dense bubbles cloud creating detrimental bubbles coalescence and
sound wave reflection/attenuation.
1.6.5 Dissolved Gas
The first action of ultrasound on a liquid is degassing, such as when using an ultrasonic
bath to degas solvents prior to HPLC use. Thus, cavitation may occur greatly at early
stages followed by a less intensive period in reason of the degassing phase. For that
reason, entrain gas is often added in order to sustain or even to promote cavitation
activity. Interestingly, the nature of this gas is of crucial aspect since the ratio of
specific heat (or polytropic ratio) of the ambient gas (C p /C v ) exerts a great influence
on temperature and pressure reached during the collapsing phase (Rooze et al. 2013).
Typically, collapsing temperature and pressure reached in the presence of Argon (with