Sonochemistry refers to the study of the effects of sonic (sound) waves on chemical systems, and is emerging as a relatively new topic in the field of chemistry. Before understanding the specifics of why sonic waves affect chemical systems in the way that they do, it is important to understand the basic properties of them.
Sonic waves are caused by the vibration of an object, such as a string. Vibration refers to moving back and forth repeatedly, which usually occurs very quickly. For example, consider a vertical guitar string where the endpoints of the strings are fixed. However, the middle of the string moves to the right, and then to the left, repeatedly. The air immediately to the right of the string, while the string moves to the right, forces air particles to move to the right as well. That area travels to the right as the air particles bounce into other air particles, forcing them to the right. Then, since the string is vibrating, the string moves to the left. The movement of particles causes compressions, or when particles move closer to each other, and rarefactions, or when the particles move away from each other. Compressions also cause areas of high pressure, because there is a higher concentration of particles. Rarefactions similarly cause areas of low pressure. Because of this, sound waves are known as pressure waves.
Since sound waves involve moving of particles, it must have entropy. The entropy of sound waves is known as wiener entropy, or spectral flatness, and it measures the width and the organization of the spectrum. Spectral flatness is measured in decibels, and on a zero to one scale. White noise, or a random, messy wave, approaches one, and a pure tone approaches zero. The entropy of the wave is not affected by its amplitude because if the period changes, the amplitude is not necessarily changing. Overall, the entropy of sound waves relies on how the sound is being portrayed. If the sound is uneven, the particles near the wave tend to move in abnormal motions, resulting in a higher entropy value. Pure tones result in the particles moving uniformly, which gives a lower entropy value.
The picture below is the wave of white noise, shown by “spikey” waves.
The following picture is a pure tone.
Sound waves also have free energy levels associated with them. This is an important aspect of sound waves to understand when considering sonochemistry, because the free energy of any specific wave is also the maximum amount of energy that can be harnessed by a chemical system. Quantifying the free energy value of a sound wave is complicated, but in general, it increases as the frequency of the sound wave or the amplitude of the sound wave increases.
Now that the basic properties of sonic waves have been discussed, sonochemistry can be analyzed more in detail. Specifically, sonochemistry is defined as the effect of the application of ultrasound on chemical systems, with ultrasound referring to sound waves with frequencies ranging between 20 kHz and 10 MHz. Firstly, it is important to distinguish between chemical systems with gas molecules, chemical systems with liquid molecules, and chemical systems with liquid and solid molecules, because sonic waves have very different effects on the three different states of matter.
The effect of ultrasound on gas molecules can be understood in terms of positional entropy in cohorts. When a sonic wave hits a chemical system, in accordance with positional entropy, the gas particles in the system will constantly diffuse from regions of high pressure to regions of low pressure in order to evenly dissipate the pressure. Since, as stated earlier, sonic waves are essentially alternating regions of high and low pressure, the gas particles will move back and forth. In theory, this induced vibrational movement of particles could speed up reaction rates or fulfill activation energy levels in the same way that adding heat energy would. However, it is relatively inefficient in its transformation of free energy from the sonic wave, so it is generally not used in sonochemical applications.
On the other hand, the application of ultrasound on systems with liquid molecules or on systems with both liquid and solid molecules is notably more efficient, due to a process called acoustic cavitation. Cavitation is defined as the growth and collapse of gas bubbles in a liquid, and acoustic cavitation refers to cavitation that is caused by ultrasound. The reason that ultrasound can cause cavitation to occur is that when a liquid is bombarded with these high-frequency sound waves, the pre-existing gas bubbles grow and shrink in response to the alternating pressure regions. When certain conditions are met, specifically when the bubble grows too large for the intramolecular forces to hold the bubble together, the bubble implosively collapses and a cavity is formed. Acoustic cavitation causes different effects in chemical systems with only liquid molecules than in chemical systems with both liquid and solid molecules, and we will go more into detail about these differences next time.
Sonochemistry involving acoustic cavitation has many effects on chemical reactions. It can increase chemical reactivity and speed up chemical rates by up to a million times. It can also shift the reaction’s equilibrium to the left, therefore increasing the yield of the reaction. Sometimes it can even change the entire process of the reaction by changing the reaction pathway. Overall, from the standpoint of a chemist in a lab, sonochemistry has many beneficial effects on chemical reactions, and may be put into widespread use sometime in the future.