As discussed in the previous blog post, the most practical use for sonochemistry in the lab is for reactions involving liquids and solids, because of the acoustic cavitation process. Again, acoustic cavitation refers to the ultrasound-induced implosive collapse of a gas bubble in a liquid, and it occurs when the alternating regions of high pressure and low pressure in the sound wave cause the bubble to becomes too large for the intermolecular forces to hold it together, as pictured below.
Acoustic cavitation occurs both when a homogeneous liquid solution is exposed to ultrasound and when a heterogeneous solution with solids is exposed to ultrasound. However, the specific effects of this process are significantly different for each of the two phases, so acoustic cavitation in homogeneous liquid solutions must be distinguished from acoustic cavitation in heterogeneous solutions with liquid-solid interfaces.
For acoustic cavitation in homogeneous liquid solutions, the bubble collapse produces extremely large amounts of energy by converting sound energy to kinetic energy of the liquid molecules, and then to heat energy. The site of the bubble collapse becomes a localized high energy spot in the solution, having temperatures of about 5200 K and pressures of hundreds of atmospheres, according to experiments done by Dr. Kenneth S. Suslick, a chemistry professor at the University of Illinois. These extreme conditions inside the cavities induce many effects in the rest of the system, one notable effect being chemical reactions. The heat energy in the cavities can be used to overcome the activation energy barrier, and the unequal distribution of pressure as a result of the high pressure cavities spontaneously mixes the solution, which of course, causes the reaction to occur at a faster rate. Therefore, sonochemical reactions involving homogeneous liquid solutions occur in the same way as traditional reactions (reactions that are induced simply by directly adding heat) just at faster rates. Another effect that the localized high energy cavities can have on homogeneous liquid solutions, under certain conditions, is sonoluminescence. Sonoluminescencerefers to the ultrasound-induced emission of light from imploding bubbles. The exact mechanism of sonoluminescence is uncertain, but it occurs when various atoms present in the cavity become ionized because of the extremely high temperatures, and then recombine with the removed electrons and release photons. Here is an image of sonoluminescence reproduced in a lab.
For acoustic cavitation in heterogeneous solutions with a liquid-solid interface, the bubbles still collapse and create local high energy spots, but one major difference is that the collapses occur in irregular shapes, as opposed to the spherical shapes of the collapses in homogeneous liquid solutions. The cavities occur in irregular shapes because of the uneven distribution of solid particles around the bubble, which of course restricts the bubbles’ spacial arrangements. It is partly because of these irregularly shaped cavities that sonoluminescence does not generally occur in heterogeneous solutions with a liquid-solid interface; sonoluminescence is a process that only occurs in homogeneous liquid solutions. Another major difference is that the extreme conditions of the local high energy cavities have different effects on liquid-solid interfaces than they do on just liquid interfaces. In liquid interfaces, the extreme conditions really only result in the liquids mixing with each other, but in liquid-solid interfaces, the extreme conditions can generate jets of high speed liquid, as shown in the image below.
These jets of high – speed liquid can reach speeds of up to 100 m/s can accelerate the solid particles in the solution, if they come into contact with them, which often results in high velocity collisions between solid particles (see video above starting at 2:05). These collisions can cause significant damage to the solid particles, including changing the surface morphology and composition. There are also high-pressure shock waves associated with the high-speed jets, that can have pressures as large as 10^4 atm. These shockwaves themselves can also cause deformation of the solid particles. These effects on the solids are significant for the chemical system as a whole, because they can drastically change the mechanisms of the chemical reactions that occur, or even cause completely different reactions to occur, reactions that would never happen if only heat was added to the system.