Chapter 5 Sonochemical synthesis

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 are caused 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 molecules collectively. Although the frequencies used in the work described below (289,000 per second unless otherwise stated) were of a magnitude far below that of molecular vibration, certain effects, to be discussed later, seems to substantiate such an hypothesis.

Scientists began asking deeper questions about the reasons for the interaction between sound and chemistry. This included delving into the energies evolved during cavitation bubble collapse particularly in terms of sonoluminescence [9]. The most accepted explanation emerged from the idea that acoustic bubbles generated by the passage of ultrasound through a solution of chemicals would be subject to collapse through normal cavitation processes. Such cavitation bubble collapse can produce high local temperatures and pressures around each bubble, and this was identified by Fitzgerald et al., in their paper in which "hot spot" chemistry was introduced to scientists for the first time [10]. The authors raised the question about why do any chemical reactions occur when a system is irradiated with high-intensity ultrasound? They explored the influence of different gases upon the cavitation threshold of liquids and the outcomes below and above that threshold. The conclusion was that:

Since the threshold of cavitation is strongly affected by so many factors, we would like to emphasize that studies of chemical effects of ultrasonics must always include a measurement of the threshold of cavitation for the particular experiments being undertaken.

It is our opinion that cavitation threshold is indeed an important factor in sonochemistry. There are, however, two problems associated with this measurement. Firstly, the values are normally obtained for very pure solvents (and chemistry seldom uses materials of such purity), and secondly, chemical reactions almost always involve mixtures and these also change the cavitation threshold of a liquid. It is an area of research which traditionally belongs to the physicist or physical chemist but maybe some new research should be opened to investigate "A list of liquids (common laboratory solvents) and their cavitation threshold as a function of ultrasonic frequency and power".

When sonochemistry became the subject for conferences, there were some questions about whether sonochemical effects were mainly mechanical rather than chemical. This was a reasonable point given that cavitation collapse could produce effects similar to high shear mixing:

• extremely good mixing,

• emulsification,

• powder deaggregation and dispersal,

• particle size reduction,

• surface cleaning,

• mass transfer to surfaces.

Some scientists were not happy that sonochemistry might be considered simply to be the result of some form of super mixing. In the 1990s, there were attempts made to predict the effect of power ultrasound on reactions themselves and to try and formulate rules governing such predictions. It was Jean-Louis Luche who made the most concerted effort to introduce some order in this part of chemistry [11]. He suggested that sonication promotes reactions proceeding through radical pathways [12, 13] and began to examine the chemical effects of ultrasound and defined any accompanying mechanical effects as "False sonochemistry". He went on to suggest that "True sonochemistry" could occur either in homogeneous or heterogeneous systems through processes in which the reactive intermediate was a radical or a radical ion since the production of such species could be stimulated by cavitation. He developed three rules covering sonochemical reactions which were written in the following terms in a book published in 1996 entitled Chemistry Under Extreme or Non-Classical Conditions [14].

• Rule 1 applies to homogeneous processes and states that those reactions which are sensitive to the sonochemical effect are those which proceed via radical or radical-ion intermediates. This statement means that sonication is able to affect reactions proceeding through radicals and that ionic reactions are not likely to be modified by such irradiation.

• Rule 2 applies to heterogeneous systems where a more complex situation occurs, and here reactions proceeding via ionic intermediates can be stimulated by the mechanical effects of cavitational agitation. This has been termed "false sonochemistry" although many would argue that the term "false" may not be correct, because if the ultrasonic irradiation assists a reaction, it should still be considered to be aided by sonication and thus "sonochemical". In fact, the right test for "false sonochemistry" is that similar results should, in principle, be obtained using an efficient mixing system in place of sonication. Such a comparison is not always possible.

• Rule 3 applies to heterogeneous reactions with mixed mechanisms,...