148 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tions (2, 43). Furthermore, it has been claimed that ultrasonic waves afford control over crystal size (20) with smaller crystal size being favored in supersaturated solutions. An adequate quantitative theory has not yet been developed for the effect of ultrasonic waves on crystal nucleation. It is well known, however, that any form of mechanical motion in a liquid tends to increase to some extent the rate of nucleation. The tendency for smaller crystals to be formed can be explained in terms of the development of many centers for crystal growth simultaneously in the presence of the sound waves. The author has learned of several companies in the United States which are using ultrasonic waves on a small industrial scale to help promote and control the crystallization of organic materials. Sohochemical Reactions The term SOhochemical reactions describes those chemical reactions which can be made to take place through the action of sound waves. All SOhochemical reactions appear to be contingent on cavitation. In the absence of cavitation, ultrasonic waves even at the highest intensities currently available in the laboratory do not have sufficient energy to break ordinary chemical bonds. Typical of the SOhochemical reactions which have been studied exten- sively are the following: 1. The formation of hydrogen peroxide in water containing a dissolved gas such as a rare gas or oxygen (13, 23). 2. The formation of nitrogen compounds in water saturated with nitrogen gas (29). 3. The breakdown of various organic molecules in the liquid state to yield decomposition products which then undergo subsequent chemi- cal reactions (48, 50). 4. Ultrasonically induced polymerization (1, 10, 15). 5. Ultrasonic degradation of polymers (19, 26, 36-38, 49). Two types of mechanisms involving cavitation have been proposed to explain SOhochemical reactions. The first is contingent on the fact that the compression of gas within the cavitation bubbles during the part.of the cycle involving collapse is at least partially adiabatic. Very high instan- taneous temperatures then are to be expected with?n the bubbles (13, 47). These high temperatures (e.g., 1000øC.) in turn cause the dissociation of the various components within the cavitation bubbles. In aqueous solutions, water is dissociated to yield hydrogen and hydroxyl radicals. The over-all SOhochemical reaction is then the result of subsequent re- actions of the dissociation products. For example, according to this mechanism the SOhochemical formation of hydrogen peroxide in water during cavitation represents the recombination of the hydroxyl radicals.

CHEMICAL-APPLICATIONS FOR ULTRASONIC WAVES 149 If nitrogen gas is present, the reaction of the nitrogen with the hydroxyl radicals results in the formation of nitrites and nitrates. The second proposed mechanism involves electrification phenomena which have been postulated by various workers (11, 12, la,, 23) to be asso- ciated with the development cavitation bubbles. Electrical discharges are presumed to occur within a cavitation bubble because of charge dif- ferences between various parts of the bubble such as the atmosphere within the bubble and the surface of the bubble. Sohochemical reactions are attributed to the dissociation of various components of the cavitation bubbles as result of these discharges. Unfortunately the primary mechanism for SOhochemical reactions has not yet been completely resolved. Recent work (9) with isotopic tech- niques for the study of the SOhochemical formation of hydrogen peroxide supports the thermal dissociation mechanism far better than any electri- fication mechanism. Sohochemical reactions in general are characterized by poor yields in terms of the amount of acoustical energy required for the reactions. Furthermore, the majority of the reactions are only of minor industrial significance even if good yields were obtained. Such reactions as the formation of hydrogen peroxide, however, must be considered when studying the effects of sound waves on biological systems--for example, in the sterilization of water. The most extensively studied SOhochemical reaction is the formation of free iodine in aqueous solutions containing potassium iodide and saturated with carbon tetrachloride (13, 47, 48, 50). Starch is usually added to establish the presence of the I2. A deep blue color develops within a few seconds after the solution has been introduced into a moderately intense sound field (e.g., 5 watts/cm.2). During cavitation, the carbon tetra- chloride is dissociated to yield CI= which in turn reacts with the iodide ions to form I2. A small fraction of the I= is also the result of the oxidation of the iodide ions by SOhochemically produced hydrogen peroxide. One type of SOhochemical reac5•on which may prove of industrial sig- nificance is the initiation of polymerization with ultrasonic waves (1, 15). Free radicals formed during cavitation are capable of initiating free radical polymerization (23). The information in the literature is not sufficient to provide a basis for evaluating the full promise of this type of SOhochemi- cal effect. Ultrasonic wav=s are also capable of degrading polymers dissolved in solution. The monomer is not produced. According to some investi- gators (2a,, 26, 36), cavitation is not required for polymer degradation with ultrasonic wa'½es. These workers attribute the degradation to fractional and shear effects inherent in the sound field. In most instances, however, supporters of this theory have not taken adequate precautions to prevent