In chemistry, the study of sonochemistry is concerned with understanding the effect of ultrasound in forming acoustic cavitation in liquids, resulting in the initiation or enhancement of the chemical activity in the solution. Therefore, the chemical effects of ultrasound do not come from a direct interaction of the ultrasonic sound wave with the molecules in the solution. The simplest explanation for this is that sound waves propagating through a liquid at ultrasonic frequencies do so with a wavelength that is significantly longer than that of the bond length between atoms in the molecule. Therefore, the sound wave cannot affect that vibrational energy of the bond, and can therefore not directly increase the internal energy of a molecule. Instead, sonochemistry arises from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid. The collapse of these bubbles is an almost adiabatic process, thereby resulting in the massive build-up of energy inside the bubble, resulting in extremely high temperatures and pressures in a microscopic region of the sonicated liquid. The high temperatures and pressures result in the chemical excitation of any matter that was inside of, or in the immediate surroundings of the bubble as it rapidly imploded. A broad variety of outcomes can result from acoustic cavitation, including sonoluminescence, increased chemical activity in the solution due to the formation of primary and secondary radical reactions, and increase chemical activity through the formation of new, relatively stable chemical species that can diffuse further into the solution to create chemical effects (for example, the formation of hydrogen peroxide from the combination of two hydroxyl radicals formed following the dissociation of water vapor inside the collapsing bubbles what water is exposed to ultrasound.
The influence of sonic waves traveling through liquids was first reported by Robert Williams Wood (1868–1955) and Alfred Lee Loomis (1887–1975) in 1927. The experiment was about the frequency of the energy that it took for sonic waves to "penetrate" the barrier of water. He came to the conclusion that sound does travel faster in water, but because of the water's density compared to our earth's atmosphere it was incredibly hard to get the sonic waves into the water. After lots of research they decided that the best way to disperse sound into the water was to make loud noises into the water by creating bubbles that were made at the same time as the sound. One of the easier ways that they put sound into the water was they simply yelled. But another road block they ran into was the ratio of the amount of time it took for the lower frequency waves to penetrate the bubbles walls and access the water around the bubble, and then time from that point to the point on the other end of the body of water. But despite the revolutionary ideas of this article it 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 sonoluminescence. 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 nonspherical 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 reactions exist: homogeneous sonochemistry of liquids, heterogeneous sonochemistry of liquid-liquid or solid–liquid systems, and, overlapping with the aforementioned, sonocatalysis. Sonoluminescence 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 affect a certain cyclobutane ring-opening reaction. Atul Kumar, has reported multicomponent reaction Hantzsch ester synthesis in Aqueous Micelles using ultrasound.
Some water pollutants, especially chlorinated organic compounds, can be destroyed sonochemically.
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- Gonzalez-Garcia, J, Saez, V., Tudela, I., Diez-Garcia, M.I., Esclapez, M.D., Louisnard, O., (2010) Sonochemical Treatment of Water Polluted by Chlorinated Organocompounds. A Review. Water 2(1), 28–74. http://www.mdpi.com/2073-4441/2/1/28/