Chemistry Reference
In-Depth Information
Chapter 16: Sonochemistry
TIMOTHY J. MASON AND P. CINTAS
1 Introduction
acoustic cavitation [9]. There is an argument that the
term sonochemistry should refer only to those reac-
tions that are influenced chemically by cavitation
but it is often difficult to separate the chemical from
the physical consequences of cavitation and so both
aspects will be considered in this contribution.
Sonochemistry is the term used to describe the
effect of ultrasonic sound waves on chemical reac-
tions. This terminology is in keeping with that of the
longer established techniques, which use light (pho-
tochemistry) and electricity (electrochemistry) to
achieve chemical activation. Unlike these, however,
sonochemistry does not require some special at-
tribute of the system being activated, e.g. the pres-
ence of a chromophore or a conducting medium,
respectively. For chemical applications, ultrasound
requires only the presence of a liquid in which to
generate cavitation in order to transmit its power.
Increasingly, sonochemistry is being seen as one of
the first options when looking at new technology in
a number of fields as a processing aid in terms of
energy conservation and waste minimisation (Table
16.1). Not only has the subject broadened in scope
and gained acceptance in chemical engineering for
scale-up, but it has also been found particularly ben-
eficial when applied jointly with other techniques.
Such joint applications have resulted in major
advances in these specialisms, some examples being
ultrasound with electrochemistry, biotechnology and
extraction processes.
The interest in sonochemistry has been accompa-
nied by a surge in the development of new equip-
ment for the generation of ultrasound. In recent
years a number of studies have been taking place
using frequencies outside of those that are normally
associated with sonochemistry (the most common
frequencies used are 20 kHz for probe systems and
40 kHz for ultrasonic baths). In essence, any sound
frequency that can generate cavitation in a liquid can
be used in sonochemistry. This would encompass all
sound frequencies, from infrasound (below 16 Hz)
through the audible sound range and all the way up
1.1 Sonochemistry
The first commercial application of ultrasound dates
back to 1917 with the echo sounding technique
developed by Langevin for estimation of the depth
of water. From this has developed a whole range of
sophisticated techniques for non-destructive testing
and medical imaging, all essentially based on the
pulse-echo technique. Such diagnostic uses of ultra-
sound use low powers and very high frequencies (in
the MHz range) and do not affect the physical or
chemical character of the medium that is probed. If,
on the other hand, a lower frequency (generally in
the 20-40 kHz range) and a higher power are applied
to a fluid, it is possible to produce significant physi-
cal and chemical changes in the medium through
the generation and subsequent collapse of cavitation
bubbles. It is acoustic cavitation produced by power
ultrasound that is the basis of sonochemistry and a
number of processing techniques.
The history of this use of power ultrasound is
shorter than that for diagnostic ultrasound and
began in the years preceding World War II, when
it was being developed for a range of processing
including emulsification and surface cleaning. By the
1960s the industrial uses of power ultrasound were
well accepted [1,2] and have since continued [3].
Paralleling these developments in processing there
were a growing number of chemists interested in the
chemical effects of power ultrasound, which became
known as sonochemistry. It was not until 1986,
however, that the first ever international symposium
on a subject identified as sonochemistry was held
at Warwick University, UK, as part of the Autumn
Meeting of the Royal Society of Chemistry and sig-
nified the beginning of serious interest in the uses of
cavitation in chemistry as a study in itself [4]. Since
then, the subject has developed to generate an ever-
expanding number of applications [5-9] and a
growing interest in the underlying driving force—
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