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

150 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS cavitation in their experimental work and have made mathematical assumptions in their theoretical treatments which do not appear to be justified. The majority of the workers (19, 30, 33, 49) believe that cavita- tion is required for ultrasonic degradation. Recent measurements in the author's laboratory (33) indicate that degradation does not occur with solutions of polymers such as polystyrene in the absence of cavitation even at an intensity of 1000 watts/cm. •', a value some 30 fold higher than used by previous workers. The mechanical effects associated with res- onating cavitation bubbles are far greater than those associated with the sound waves which give rise to the cavitation. The extreme rates of shear near the surface of the cavitation bubbles probably lead to the rupture of the polymer molecules. The graph in Fig. 6 indicates (33) the extent of the degradation of poly- styrene dissolved in toluene as a function of acoustical intensity and frequency under the conditions stated in the legend. The relative viscos- ities of the polymer solutions are represented on the left ordinate while the viscosity-average molecular weights are represented on the right. The initial molecular weight was 3.3 X 106. The threshold intensity for degra- dation in Fig. 6 correlates with the threshold for cavitation. While the rate of degradation increases with increasing ultrasonic in tensity, the ultimate degree of degradation is not a function of intensity (19, 28, 30, 35) provided there is cavitation. This is in accord with cavitation theory (16, 28, 35). A molecular weight of approximately 30,000 has been reported 3.0 • q3 o- 300 kc. - •.• ,• Undegraded -• •-- 800 kc. x •Qx •. -O 0-1 rnc. : 2.5 • -•' •'-2 rnc. - 2.75 o 2.0 - 1,65 x n., 1.5 - 0.83 -- 1.o , I , I , I , I , I , I , I [ o.oo o 1 2 $ 4 5 6 7 8 INTENSITY, WATTS PER CM,' Figure 6.--Uhrasonic degradation of polystyrene dissolved in toluene. Concentration of polymer: 0.5% exposure time: 10 min. temperature: 25øC. Viscosity average molecu- lax weight of original polymer: 3.3 X 106.