Inthe recent times, ultrasound has found extensive use in synthetic chemistry. Application of ultrasound can enhance rates of chemical reactions, can eliminate the need for catalysts , and can even produce entirely new products.
Ultrasound does not cause chemical reactions directly.
Small pockets of gas and vapor ( cavitation bubbles) exist in all liquids. The cavitation bubbles grow and collapse violently under the influence of the rarefaction and compression pressure cycles of ultrasound. Collapsing cavities attain high temperatures and pressures. In the vicinity of another bubble or a solid, the collapse of a bubble is asymmetric and high speed microjets are created under such conditions.
The high temperatures produced in the cavities can produce highly reactive intermediates in the bubbles. The reactive intermediates diffuse out into the surrounding liquid and cause reactions. This is one mechanism by which ultrasound affects the course and rates of chemical reactions. High speed microjets created during asymmetric collapse cause intense convection next to solid surfaces. This is another mechanism by which heterogeneous diffusion controlled reactions are speeded up.
Apart from these main effects, the microjets can clean passivated surfaces, cause fusion of particles and in this process amorphous phases, and can fragment particles creating new surface area. All these can have profound effects on catalytic chemical reactions.
There are several observed effects of ultrasound on many reactions whcih are not still understood . Besides, there are many engineering issues (e.g., scale up, variation of sound field in reactors etc) still to be resolved before ultrasound can be put to use on a commercial scale.
Our work concerns the modelling of the effect of sonication on the rates of chemical reactions since it is so essential for scale up and design. With homogeneous reactions, our apporach is as follows. The first step is to calculate the extreme conditions generated in the bubble from its dynamics. The second step is to predict the formation of reactive intermediates and other products from the extreme conditons. The last step is to follow the reactions undergone by the intermediates when they are released into the surrounding liquid. We have succeeded in predicting the rates of sonochemical oxidation of aqueous iodide ion to iodine (Naidu et al.), and sonochemical decomposition of carbon tetrachloride in water (Rajan et al.). We were also able to predict (Banerjee et al.) the enhancement in the rates hydrogen liberation in electroless deposition of nickel on zinc due to the application of ultrasound.
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