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CHEMICAL SOUNDS
21/05/1995

The application of ultrasound to chemical dissolution can allow to reach, safely and relatively simple methods, localized temperatures as high as those of the Sun's surface and pressures as intense as those in the deepest of the depths underwater.

half-life of this phenomenon is measured in microseconds and often can increase the chemical reactivity by factors of over one million times. In the last few years, scientists are learning to understand and master this new technique, the sonochemical, which can result in varied and beneficial consequences. ULTRASOUND

. The sounds that humans perceive are nothing special waves that can be captured by our auditory system. They are understanding and expansion waves, which have spread material means, ie through solids, liquids or gases, and must have a frequency range between 20 hertz and 16 kilohertz. One hertz means one cycle of understanding or expansion for each second. Sound reaches an ultrasound classification if its frequency exceeds 16 kilohertz, so it will be, in principle, although the intensity was inaudible higher than a jet engine.

In our world, the practical use of ultrasound is fairly widespread. Everyday examples are: ultrasonic welders and drillers, sonars used in fishing and navigation and a review of industrial materials by ecopulsos; emulsified cosmetics and foods, different types of medical scans such as those used in pregnant women or diagnostic tests, use of ultrasound to break up kidney stones or gallstones, ultrasonic cleaning baths, such as those used in jewelry, cell disruptors in the laboratory, and even small household tools such as mosquito repellents and other animals, burglar alarms or the signal transmitters. However, what is covered in this article something else. These are the effects of high intensity ultrasound, mainly on liquid media, producers of the phenomenon known as cavitation, which entails interesting consequences in chemistry.

The phenomenon of cavitation was first observed about a hundred years ago when the first began to develop powerful torpedo boats. Sir John Isaac Thornycroft, a naval architect, builder of those first torpedo boats for the Royal Navy found an interesting fact. Due to the high speed rotation of the propellers of torpedoes were formed large bubbles or cavities, whose subsequent collapse was accompanied by enormous turbulence, heat and pressure, which were transmitted to the surface of torpedo propellants causing its rapid erosion. CAVITATION

. An ultrasound source usually consists of a piezoelectric tip moves at high speed using electric power for it. In 1927 it was discovered that ultrasound sources with sufficient intensity, applied to liquids also produced the phenomenon of cavitation. The consequence of this chemical, sonochemistry, only started about ten years ago, when it became available in the laboratories of efficient generators of high intensity ultrasound. Ultrasounds have wave lengths between 10 inches and hundredths of a millimeter. This means that its size is much higher than molecules, so their effects can not be exercised through a direct physical interaction between waves and molecules that are hit by them. What happens is that the ultrasound waves pass into the liquid where the expansion cycle causes a negative pressure on the liquid so that makes locally to separate the liquid molecules, creating there a true cavity.

Usually this takes place at sites previously contaminated the solution, there are those in which small particles or microbubbles. The cavities thus formed absorb energy from the ultrasonic waves and grow more or less quickly, which depends on several parameters and circumstances known to reach a limit. At that time it rapidly compresses the gas cavity, it collapses, it produces its implosion, which generates a large amount of heat that can reach 5,000 ° C, with a lot of pressure, which can reach 1000 atmospheres, in a process lasting almost instantly. This is therefore a mechanism that serves to concentrate in the form of useful chemical energy, diffuse energy carried by the ultrasonic wave.

UTILITIES . What might be useful that energy?. There have been many investigations using different kinds of systems: liquids with suspended solids, homogeneous liquid mixtures, polymers in solution and heterogeneous mixtures on homogeneous and heterogeneous catalytic systems, and so on. The range of possibilities that is opening is wide, including the possible industrial use next to some of them. To cite some of the most promising in liquid systems containing metal particles in suspension can be made to hit one another at speeds of 2,000 mph, with temperatures such that it is possible to merge separate. This results in significant changes in surface texture and reactivity.

The action of ultrasound on certain mixtures of hydrocarbons has led to results similar to those obtained by the complex systems of high-temperature pyrolysis, which form the basis of the current petrochemical complex. Several polymer molecules have been fragmented and modified. Of great interest in drug and medical for dispensing medicines, is considered the collection of small proteinaceous microspheres which may contain within them various substances, including water-insoluble liquid. Other achievements relate to the relatively easy implementation of complex organic and organometallic reactions. Be obtained in heterogeneous media, amorphous metal powders have technological implications since, for example, the amorphous iron powder has proven to be an excellent catalyst for the synthesis of liquid fuels from carbon monoxide and hydrogen from coal treatment. Another important area is to improve the properties of metal catalysts and the development of new heterogeneous catalysts, some of great value. For example, catalytic converters for automobiles, obtained by conventional techniques, in which some of the components will cost more than 3,000 pesetas per gram.

In summary, ultrasound and are commonly used in industrial processing of liquids, as well as emulsification, degassing and dispersion of solids. Also in the processing of solids: cutting, merging, cleaning, or precipitation. Which now opens a new facet of the sonochemical, in which it is hoped that the silent waves of ultrasound to make any changes and chemical modifications are difficult to achieve so far, modeling the reactivity of different surfaces, achieving more effective catalysts, and so on.