Sonochemistry: innovations and challenges

Ultrasound is a sound wave that propagates through the elasticity of the surrounding medium in the form of longitudinal waves alternating between zones of compression and expansion. Like all waves, ultrasound is characterized by a period (T, expressed in seconds) designating the time required for an oscillation, and a frequency (f, expressed in Hertz) which defines the number of periods per unit of time. The frequency range of ultrasound lies between 20 kHz and 200 MHz, above the frequencies of the audible range, but can be divided into two distinct regions: diagnostic ultrasound (between 2 and 200 MHz), used in medical imaging in particular, and power ultrasound (between 20 kHz and 2 MHz) used in chemistry and which is the subject of this article.

The effects of ultrasound are at the origin of the cavitation phenomenon, which is defined by the formation, growth and collapse of gaseous microbubbles in the liquid phase. The intense local effects of the sudden collapse of these cavitation bubbles are at the root of all sonochemical applications. These extreme conditions - physical (shock waves, microjets, microconvection, microemulsion, etc.), thermal (temperatures in excess of a few thousand degrees at the heart of the cavitation bubble) and chemical (production of radical species in solution) - sometimes lead to greater efficiency in the transformation of molecules, materials or polymers, but also to new reactivities or functionalizations, sometimes completely unexpected compared with more conventional conditions. These unique results are a major asset for the development of potential future innovations involving the use of ultrasound.

The chemical industry is faced with new challenges and the need to develop new technological innovations, both to reduce the environmental impact of processes and to increase productivity. As a result, the chemical industry is making increasing use of physical activation methods and disruptive technologies such as ultrasound, microwaves, cold plasma, reactive grinding and supercritical fluids. These advanced technologies can be used to activate chemical synthesis or materials preparation, as well as new applications in eco-extraction, biomass and waste recovery, and liquid effluent decontamination. The use of ultrasonic waves can therefore play a key role in terms of innovation in various applications in a context of green chemistry and decarbonization of industry.

This article outlines the principle of sonochemistry, detailing the various parameters influencing the associated phenomena and discussing the contribution of this technology to greener chemistry. The research carried out in recent years in this field is then reviewed to show the current challenges of sonochemistry in different fields of application....