CHEMICAL APPLICATIONS FOR ULTRASONIC WAVES 147 the interfaces. In the case of a liquid phase, this boundary layer cannot be reduced to less than 10 -3 cm. with ordinary agitation. Sound waves are capable of disrupting these boundary layers and the associated diffusion gradients in the case of both gases and liquids. Research (52, 53) at Western Reserve University has shown that cavita- tion is the primary agency for the disruption of concentration gradients within liquids. A Schlieren microscope has been used to prove that ultra- sonically produced cavitation reduces the effective thickness of the bound- ary layer at a solid-liquid interface to 10 -4 cm. as compared with the value of 10 -3 cm. stated above. The oscillating cavitation bubbles near the interface produces micro-agitation which disrupts the gradient. In processes which are limited by diffusion in a liquid phase despite extensive stirring, ultrasonic waves offer promise as a means for increasing the reaction ratks. This is particularly important for reactions involving a liquid phase within a porous solid. Typical of the heterogeneous processes, the rates of which can be substantially increased with ultrasonic waves, are extraction processes, dialysis (31), dissolution of sparingly soluble materials, electrodeposition (52, 53) and dyeing of fabrics (5). The effects of ultrasonic waves in promoting the cleaning of metal sur- faces with organic solvents as well as the cleaning of fabrics with detergents is in part the result of improved mass transport within the liquid phase. In addition, however, cavitation helps to disperse material adsorbed on the solid surfaces. Industrial cleaning of metal surfaces with ultrasonic waves at the present time is probably the most important processing application for sound waves. Sound waves are also capable of reducing concentration gradients within gas phases. Unfortunately information is not generally available in the literature as to the magnitude of the effects or the practicality of using sound waves for promoting mass transport in gases on a large scale. Inasmuch as sound waves expedite mass transport within fluids, heat transfer should also be increased. The mechanisms as well as the magni- tude of the effects should be the same for both cases. The use of sound waves in heat exchangers involving either gases or liquids might permit an appreciable reduction in size. The generation of the sound waves within the heat exchanger can be accomplished readily by incorporating a flow-type generating unit in the feed line to the exchanger. Such devices operate off the flow of fluid through them and will be described later in this paper. Crystallization Effects Ultrasonic waves have been reported to increase the probability of nucleation for the formation of crystals in a number of cases (3) including organic solutions (21) which are difficult to crystallize, e.g., sucrose solu-

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.