A Bit More on Sonochemistry

In the previous four blog posts, we discussed sonochemistry in detail, but only from a purely theoretical point of view. In this blog post we would like to shift our discussion to sonochemistry in practice, in the lab.  In order to do this, we will be referencing an email conversation that we had with a college-level professor with knowledge in the field of sonochemistry.  We contacted this professor during the construction of the four previous blog posts, and asked a series of questions pertaining to sonochemistry in the lab.  Shown below are the questions that we asked and the corresponding answers that the professor gave us.  Note: the professor requested to be nameless, so we will be referring to the professor as “the professor” throughout the entirety of this blog post.

1) What are some of the applications of sonochemistry in the lab?

Ultrasound is particularly useful for any “mixed-phase” reaction: for example, a reaction between something dissolved in a liquid with something else that is a solid.  As a specific example, Grignard reactions (organic halides reaction with magnesium metal) are driven by sonication much faster than without.

2) Does the cavitation involved with sonochemistry affect different types of molecules differently? (i.e. organic vs. inorganic)

Not really, but there are two kinds of sonochemical reactions:  those that happen inside the bubble (which involves mostly only volatile compounds) and those that happen at surfaces due to bubble collapse and microjet formation at the solid surface.  So there can be a difference between volatile compounds and non-volatile compounds and between homogenous solution vs. heterogeneous (i.e., solutions containing solids or powders).

3) What equipment is used in the lab when dealing with sonochemistry?

Either a cleaning bath (which barely works and only for heterogeneous reactions) or a high intensity ultrasonic horn (also called a cell disrupter because biochemists use them to break open living cells.)

4) What are the advantages of using sonochemistry in the lab?

Faster rates, sometimes different kinds of reactions.

5) Would you be able to discuss an experiment where sonochemistry was used?

Tri-iodide formation in 1 M KI aqueous solutions would be an easy test, which can be more sensitive if you add a small amount of a starch solution (either before or after the sonication).  I3-  is formed from the H2O2 created during cavitation in water.

6) Generally, how large are the cavitation bubbles?

Cavitation bubble sizes depend on frequency.   At 20 kHz, average bubble size is a few microns, with biggest size during expansion of ~50 um and smallest size < 1 um.

Much of the information touched upon above by the professor was discussed in our previous blog posts, while much of it was not discussed in the previous posts.  For the rest of this blog post we will discuss the above information that was not mentioned in our previous blog posts.

As aforesaid by the professor, using sonochemistry in the lab instead of classical heat-based chemistry has two major advantages.  The first major advantage is that sonochemical reactions can occur faster than heat-based reactions. For example, mixed phase reactions, which are reactions between a solid reactant an aqueous reactant, generally occur several times faster when activated using sonochemistry, as opposed to heat-based chemistry.  Thus, it is highly advantageous to use sonochemistry for mixed phase reactions in the lab.  An example mixed phase reaction that is typically considered a model sonochemical reaction is the organic halide reaction with magnesium metal.  The second major advantage of sonochemistry is that it can completely alter the mechanisms of reactions.  There are even a few sonochemical reactions whose mechanisms are so different from heat-based reactions that they can not even be induced to occur by adding heat.  An example of such a reaction whose sonochemical mechanism is different from its heat-based mechanism is the formation of triiodide in an aqueous KI solution, which is also a very cheap and easy reaction to conduct in the lab.  When conducting sonochemical experiments in the lab, ultrasonic waves are usually generated with either a cleaning bath or an ultrasonic horn.  Both have their own unique advantages and disadvantages.  Lastly, sonochemical reactions conducted in the lab can generally be judged for success by observing the cavitation bubbles.  Cavitation bubbles are generally only a few micrometers wide at their equilibrium position at 20 kHz, which is a typical ultrasound frequency as used in the lab.  Thus, if cavitation bubbles are observed at significantly different sizes, something must be going wrong in the experiment.  If anybody reading this blog post is interested in conducting lab-based research on sonochemistry, we hope that this blog post offers a substantial gateway from sonochemistry in theory to sonochemistry in practice.

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