Aharon Gedanken
Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Sonochemistry is the research area in which molecules undergo chemical reaction due to the application of powerful ultrasound radiation (20 KHz - 1 MHz). The physical phenomenon responsible for the sonochemical process is acoustic cavitation. A number of theories have been developed in order to explain how 20k Hz sonic waves can break chemical bonds. They all agree that the main event in sonochemistry is the creation, growth, and collapse of a bubble that is formed in the liquid. The stage leading to the growth of the bubble, occurs through the diffusion of solute vapor into the volume of the bubble. The last stage is the collapse of the bubble, which occurs when the bubble size reaches its maximum value. We will adopt the hot spot mechanism, one of the theories that explain why, upon the collapse of a bubble, chemical bonds are broken. The theory claims that very high temperatures (5,000-25,000 K) are obtained upon the collapse of the bubble. Since this collapse occurs in less than a nanosecond, very high cooling rates, in excess of 1011 K/sec, are also obtained. This high cooling rate hinders the organization and crystallization of the products. The results of the many reactions carried out by my group and by others show that the products were having always nanometric size, but so are 40-50 other methods by which nanomaterials are prepared. The question is therefore asked what are the advantages of using sonochemistry. I will mention and discuss four such advantages. 1) Preparation of amorphous products. 2) Insertion of nanomaterials into mesoporous materials. Ultrasonic waves are used for the insertion of amorphous nanosized catalysts into the mesopores. A detailed study demonstrates that the nanoparticles are deposited as a smooth layer on the inner mesopores walls, without blocking them. When compared to the other methods such as impregnation or thermal spreading sonochemistry shows better properties. 3) Deposition of nanoparticles on flat and curved surfaces. We have immobilized a large variety of nanomaterials on metals, ceramics, polymers, glass, textiles, and paper, imparting a large variety of properties such as magnetic , conductive, antibacterial, antiviral, antibiofilm and more to the substrates. The adherence of the |nanomaterials to the substrate was demonstrated when cotton coated by CuO nanoparticles were washed 65 cycles in Hospital washing machines (92 deg. cent.) and the fabric was found highly antibacterial after this long washing. 4) The formation of micro and nanospheres of proteins, DNA, RNA, starch, chitosan and more. The encapsulation of drugs in the spheres will be demonstrated. The use of the DNA nanospheres for gene silencing will be discussed.
