Sonochemistry

In chemistry, the study of sonochemistry is concerned with understanding the effect of sonic waves and wave properties on chemical systems. Since acoustic waves have unique physical properties, the corresponding atomic and molecular chemistry is unique as well. Often these effects are most apparent in ultrasonic systems. This is demonstrated in phenomena such as ultrasound, sonication, sonoluminescence, and sonic cavitation.

The influece of sonic waves traveling through liquids was first reported by Robert Williams Wood (1868-1955) and Alfred Lee Loomis (1887-1975) in 1927, but the article was left mostly unnoticed. Sonochemistry experienced a renaissance in the 1980s with the advent of inexpensive and reliable generators of high-intensity ultrasound.

Upon irradiation with high intensity sound or ultrasound, acoustic cavitation usually occurs. Cavitation – the formation, growth, and implosive collapse of bubbles irradiated with sound— is the impetus for sonochemistry and sonoluminessence. Bubble collapse in liquids produces enormous amounts of energy from the conversion of kinetic energy of the liquid motion into heating the contents of the bubble. The compression of the bubbles during cavitation is more rapid than thermal transport, which generates a short-lived localized hot-spot. Experimental results have shown that these bubbles have temperatures around 5000 K, pressures of roughly 1000 atm, and heating and cooling rates above 1010 K/s. These cavitations can create extreme physical and chemical conditions in otherwise cold liquids.

With liquids containing solids, similar phenomena may occur with exposure to ultrasound. Once cavitation occurs near an extended solid surface, cavity collapse is nonsphereical and drives high-speed jets of liquid to the surface. These jets and associated shock waves can damage the now highly heated surface. Liquid-powder suspensions produce high velocity interparticle collisions. These collisions can change the surface morphology, composition, and reactivity.

Three classes of sonochemical reations exist: homogeneous sonochemistry of liquids, heterogeneous sonochemistry of liquid-liquid or solid-liquid systmes, and, overlapping with the aforementioned, sonocatalysis. Sonoluminessence is typically regarded as a special case of homogeneous sonochemistry. The chemical enhancement of reactions by ultrasound has been explored and has beneficial applications in mixed phase synthesis, materials chemistry, and biomedical uses. Because cavitation can only occur in liquids, chemical reactions are not seen in the ultrasonic irradiation of solids or solid-gas systems.

For example, in chemical kinetics, it has been observed that ultrasound can greatly enhance chemical reactivity in a number of systems by as much as a million-fold ; effectively acting as a catalyst by exciting the atomic and molecular modes of the system (such as the vibrational, rotational, and translational modes). In addition, in reactions that use solids, ultrasound breaks up the solid pieces from the energy released from the bubbles created by cavitation collapsing through them. This gives the solid reactant a larger surface area for the reaction to proceed over, increasing the observed rate of reaction.

While the application of ultrasound often generates mixtures of products, a paper published in 2007 in the journal Nature described the use of ultrasound to selectively effect a certain cyclobutane ring-opening reaction.

Sonochemistry can be performed by using a bath (usually used for ultrasonic cleaning) or with a high power probe.