5.1 Historical introduction
There were two main interests in the use of ultrasound in chemistry from the beginning of research in this field: one involved the use of low-power ultrasound for analysis and the other with chemical changes that could be affected by high-power ultrasound. This was identified by Weissler in his seminal paper “Ultrasonics in chemistry” which was published in 1948 [1]. In the introduction, he stated that:
There are two main fields in which ultrasonics contributes valuable information to chemistry. One of these is the investigation of molecular properties of fluids by measurement of the velocity of weak ultrasonic waves; the other is the study of chemical reactions which arecaused or accelerated by intense ultrasonic irradiation.
Ultrasound used for chemical analysis is not one of the topics of this book but is a research field that has attracted a lot of interest. This was presented in the first sonochemistry symposium in 1986 [2] but since it involves low power, usually high-frequency measurement of velocity, attenuation and scattering of ultrasound it fits more squarely with non-destructive evaluation of materials and acoustics. Nevertheless, at much higher ultrasound powers, there must be a connection between the way in which sound waves interact with a medium and the creation of acoustic cavitation. This was what first brought the attention of chemists to a new branch of chemistry –sonochemistry – a term that was first used by Weyl [3] and Weissler [4] in the 1950s (see Volume 1, Chapter 1).
In 1986, Tim Mason published a short review on the uses of ultrasound in chemical synthesis [5]. In this paper, he championed the use of the term “sonochemistry”: “A new word has recently appeared in the chemical literature to cover this rapidly expanding field, the use of ultrasound in chemistry which is now generally referred to as sonochemistry.” He also made the prediction that:
Sonochemistry may be as important a topic within chemistry as photochemistry, thermochemistry or high-pressure chemistry. It might even be argued that it could become more important because of its greater general applicability.
Together with Jim Lindley, a colleague from Coventry University, over 100 references on the synthetic aspects of sonochemistry were gathered together and reviewed in the following year, 1987 [6].
It is our opinion that 1986 should be considered to be the year which saw the renaissance (rebirth) of sonochemistry. During that year, the first-ever international symposium on a subject identified as sonochemistry was organized at Warwick University, United Kingdom, as part of the Autumn Meeting of the Royal Society of Chemistry [7]. This meeting signified the beginning of serious interest in the uses of ultrasound in chemistry, which now spreads across almost all possible areas of chemical sciences and beyond.
5.1.1 Mechanistic aspects
Many researchers who became involved in sonochemistry began asking questions about how sound energy could cause changes in chemical reactions. It had been recognized from the very beginning that there could not be a direct interaction between ultrasound and the bonds holding together atoms in molecules but, despite this, ultrasound could influence chemical reactions. In 1927, Richard and Loomis had considered the direct effect of acoustic vibrations observing that the frequencies of ultrasonic waves are much lower than the vibrations of molecular bonds [8]. The words that they used in their paper were:
A third possible effect should be mentioned, although it cannot be treated in detail in this communication, namely, the effect of the vibration frequency of the sound wave itself on an unstable molecule, apart from its local kinetic effect upon molecule
