CHEMICAL EFFECTS O F ULTRASONIC WAVES
1139
A means of identifying the relative positions of the amino and carboxyl groups in the amino acids has been suggested. This involves the ability of nonadjacent groups to form rheological structures which are in contrast to those formed with amino acids having adjacent groups. REFERENCES
(1) ALEXANDER, JEROME: Colloid Chemia!ry, Vol. VII, pp. 431-42. Reinhold Publishing Corporation, Xew York (1950). (2) BRADLEY, W. F.: J. Am. Chem. SOC.67, 975 (1945). (3) GIESEKING, J. E.:Soil Sci. 47, 1-13 (1939). W . G., A N D CUTHBERT, F. L . : J. Am. Ceram. SOC. SO, 137-42 (4) GRIM,R. E., ALLWAY, (1947). (5) HAWSER, E. A., A N D LEGCETT, M. B.: J. Am. Chem. SOC.62, 1811-14 (1940). S. B.:J. Phys. Chem. 46, 68-81 (1941). (6) HENURICKS, JOHNW.:J. Phys. & Colloid Chem. 63, 294-305 (1919). (7) JORDAN, (8)JORDAN, JOHNW., HOOK,B. J., ANI) FINLAYSON, C.M.:J. Phys. & Colloid Chem. 64, 11~~-1208'(1950). (9) SLABAVCH, w.H., A N D C:L;LBERTBON, J. L.': J. Phys. & Colloid Chem. 66, 744 (1951). (10) SMITH,C . R.: J. Am. Chem. SOC. 66, 1.56-3 (1934).
THE CHEMICAL EFFECTS PRODUCED BY ULTRASONIC WAVES OLLE LIKDSTROM
AND
OLE LAMM
Division of Physical Chemistry, The Royal Institute of Technology, Stockholm, Sweden Received February 6 , 1060
Ultrasonic treatment of a water solution causes, as is well known, some chemical effects depending on the phenomenon of cavitation. Several investigators have devoted attention to these chemical effects, being principally effects of oxidation and reduction, and have tried to explain them in various ways. In general it is assumed that dissolved oxygen in some way is activated in the course of cavitation (3, 9, 11, 12). However, there are reasons indicating that the water molecule itself may be decomposed by ultrasonic treatment and that the effects of oxidation, though to a less extent, are noticeable even when oxygen is absent (7). In this way the ultrasonic waves have effects similar to those of the ionizing radiation from radioactive preparations. The chemical effects of ionizing radiation have been subject to much investigation (1, 4,5 , 6, 8, 10) ; the application of this knowledge to the corresponding conditions associated with ultrasonics seems simple, in addition to being of considerable value. Schematically and very briefly, it may be stated that certain chemical effects in water solutions of the ultrasonic waves-or, more correctly, of cavitation-
1140
OLLE LINDSTR6M AND OLE LAMM
may be explained in the following way:' According to our experimental results we assume as the primary characteristic reaction:
+
H*O--tH OH (1) This reaction is regarded as the fundamental and typical reaction produced by electric discharge in water vapor and by ionizing radiation on mater vapor and water solutions. What then happens with the primarily produced free radicals is of a complicated and not yet fully established nature. Experiments show a production mainly of hydrogen peroxide, hydrogen, and oxygen. If now it were possible to establish the presence of free radicals in water solutions subjected to ultrasonic waves, the analogy between cavitation-chemical and radiochemical effects could be stated more conclusively. Investigating radiochemical reactions in water solutions, Dainton has studied the polymerization of acrylonitrile in water solution. The polymerization was initiated by the free radicals produced by ionizing radiation from strong radioactive preparations (5). We therefore,thought it suitable to investigate whether the ultrasonic waves might bring about a similar effect. DISCUSSION AND RESULTS
Experiments concerning the polymerization of acrylonitrile in water solution induced by ultrasonic treatment have given positive results. In support of the opinion that the polymerization is initiated in the way indicated, the authors desire to adduce the following: (1) Some possible effect of dissolved oxygen is eliminated in these experiments. The solutions were degassed by boiling under a reflux condenser. In some experiments the solutions were degassed in vacuum and the ultrasonic treatment performed in an atmosphere of nitrogen. Furthermore, the ultrasonic waves themselves have a strong degassing action. Oxygen is a strong inhibitor in the polymerization of acrylonitrile. The positive result of these experiments is therefore an indication of the absence of oxygen. (2) The possibility of the reaction being induced by some catalyst present as an impurity is gainsaid by the fact that not even boiling for a meek could bring about polymerization or any other change of the acrylonitrile solution used in the experiments. ( 3 ) It is conceivable that the ultrasonic waves by oxidation or in some other way destroy any inhibitor substances. Any catalysts present will then start the polymerization. If this assumption were correct the polymerization, after being initiated by the ultrasonic waves, would continue even after the ultrasonic treatment has been interrupted. This, however, is not the case. (4) Some catalyzing effect of the material in the reaction vessel is not probable. The reaction is the same, whether the oscillating parts of the reaction vessel are made of platinum, copper, or glass. 1 After this paper had been sent to the Editor the authors noticed an article by R.0. Prudhomme and P . Grrtbar (J. chim. phys. 46,323 (1949)), who suggest the same mechanism.
CHEMICAL EFFECTS OF ULTRASONIC WAVES
1141
According to the hypothesis of the authors some of the chemical effects of the ultrasonic waves can be explained in a simple way. What happens with the free radicals after the primary dissociation of water evidently depends on the many, often very intricate factors that influence the chain reactions where the chain carriers are free radicals. However, the predominant reactions are probably the back-reactions: H OH -+ Hz0 (2)
+
H+H-+HZ (3) H2 OH H20 H OH OH -+ HZ02 (4) H Hz0z -+ H20 OH When water free from impurities is treated with ultrasonic waves the net result will thus be zero, since the equilibrium concentrations of hydrogen peroxide and hydrogen may be too small to be detected. The presence of dissolved substances in the water will increase the possibilities of other reaction series than the proper back-reactions. Especially compounds with a high vapor pressure, which may occur in a higher concentration in the cavities, will take part in the secondary reactions. Possible reactions in the presence of dissolved oxygen are:
+ + +
+ +
-+
Oz+HtHO2 H0z H HzOz
+
(5)
-+
Hydrogen atoms are to some extent withdrawn from the back-reactions 2, 3, and 4. Simultaneously the corresponding amount of hydroxyl radicals will react with other oxidizable substances. This is in agreement with the fact, established by several investigators in this field, that dissolved oxygen greatly increases the oxidation effects of ultrasonic waves and that the amount of hydrogen peroxide observed is too small to explain these oxidation effects. These circumstances are generally explained by stating that oxygen is directly activated by the ultrasonic waves according to the equation:
o,io+o
(6)
Such a reaction is obviously possible and will occur simultaneously with reaction 1 . However, the concentration of oxygen is small compared with that of water, even in the cavities. We must therefore conceive of reaction 1 as the predominant reaction. For that reason the influence of dissolved oxygen in this connection is best represented by equation 5, whereas equation 6 is of secondary importance. In this way it is possible to explain most of the chemical reactions produced by ultrasonic treatment of water solutions, e.g., oxidation of potassium iodide, hydrogen sulfide, organic dyestuffs, and hydrocarbon halides such as chloroform; reduction of potassium permanganate and mercuric chloride; production of KHa, XHZOH, NOT, and KO; in the presence of dissolved nitrogen. Quantitative investigations of the matter discussed above are difficult to carry
1142
OLLE L I N D S T R ~ MAND OLE LAMM
out. Reactions where free radicals are essential components are not easy to survey. In addition, the reproducibility is not good, since estremely small amounts of impurities may have a considerable influence on the course of the reaction series. A further complication, compared with the corresponding radiochemical investigations, is the insufficient reproducibility of the cavitation as such. THEORETICAL DETAILS
The kinetics of the ultrasound-induced polymerization of acrylonitrile in dilute aqueous solutions may be explained in the following way: The free radicals (1) primarily produced are involved in various secondary reactions, some of which have been indicated above. At steady-state conditions a constant fraction of the primarily and secondarily produced free radicals is available to induce the polymerization of the present monomer. However, the production of these free radicals is not the only cause of the polymerization reaction. An analysis of the experimental results will show that in addition there is a kind of autocatalytic effect. This effect can be ascribed to the well-known depolymerizing effects of the ultrasonic waves. Thread molecules of colloidal size, subjected to strong ultrasonic vibration, suffer such strain that C-C bonds may be broken The end-groups at the fracture may be unsaturated and of a radical nature like the active end-group in a growing polymer molec$e. The fragments obtained in the splitting of a polymer molecule are thus capable of continued polymerization. The following abbreviations are introduced for the subsequent treatment. In the chemical equations they refer to chemical individuals and in the mathematical equations to concentrations of these species in moles per liter.
M = monomer, M O= initial concentration, R = radical groups of the growing polymer, P = polymer, n = the average number of monomers in the polymer, p = fraction polymerized, and t = time in hours. As above, it is assumed that a constant fraction of the radicals produced in the cavitation is available for participation in the polymerization reaction. Cavitation H, OH (participating in the polymerization reaction) : H , O H + 2 M ka 2 R (7) ---f
The depolymerizing reaction can be written:
P
k
2R
The polymerization is assumed to proceed thus (5, 13): k + M -4 R (chain reaction) R + R 2% P (end reaction)
R
(9) (10)
CHEMICAL EFFECTS OF ULTRASONIC WAVES
1143
It can easily be shown that the possible end reaction
R
+ H, OH
--$
P
is of no importance in connection with this. Approximately
The quotient R/M must certainly be extremely small. Furthermore, at steady state
Inserting
we obtain:
Examining this quotient we find it also to be extremely small. We therefore mu& conclude that reaction 11 can be ignored. The usual steady-state approximation gives
R
+ k,P)/k,
= ~'(k,
and
Introducing p and n gives
In these experiments p is I0.02 and thus (1 - p ) this approximation and integrating we obtain
1. After introducing
or t
- to
= Cl(d1
+ c*p - 1)
In our experiments to, the induction period, amounts to -2.5
hr. After deter-
1144
mining the constants periment :
OLLE LINDSTROV .WD c1
OLE LAMM
and c2 for a typical run we obtain for this special ext = 10.7 4 1
+ 8 0 9 0 ~- 8.2
In figure 1 this function is arawn, together with the corresponding experimental points. Evidently the suggested mechanism of initiating the polymerization is satisfied by the experimental results. A qualitative picture of how the polymerization velocity depends on the acoustic efficiency is given by figure 2. The course is representative of effects depending on the phenomenon of cavitation. I t is necessary that the acoustic efficiency exceed a definite limit value in order to obtain cavitation and the chemical effects depending on it,
fiz I
5
5
OO Acoust. eff., Wlcm?
FIG.1 FIG. 2 FIG.1. Twenty-five milliliters of 0.60 m acrylonitrilc polymerized at 30°C. & l o ,700 kc./sec., and an acoustic efficiency of 8 Wlcrn.* 0, experimental values; --, t = 10.74- 8.2. FIG.2. Twenty-five milliliters of 0.60 m acrylonitrile polymerized for 48 hr. a t 25'C. f2' and 700 kc./sec. EXPERIMESTAL DETAILS
The acrylonitrile used in the experiments was purified from added inhibitor substances by a simple distillation performed at constant temperature within 0.1"C. The middle fraction of the distillate was dissolved in twice-distilled water to give a concentration of 0.60 m. This solution, which was used in the polymerization experiments, has proved to be completely stable for at least 6 months. The ultrasonic treatment mas made with a Crystalab Ultrasonorator, model SL 520, giving the frequencies of 400, 700, 1000, and 1500 kc./sec. As a reaction vessel we used a glass retort with a bottom consisting of a 0.02-mm. platinum membrane, through which the ultrasonic waves were transferred (figure 3). The course of the polymerization was followed by measurements of light absorptip. These measurements were made with a Pulfrich Stufenphotometer at 4300 A. The colorimetric method was checked by gravimetric determinations after vacuum evaporation of the monomer and the solvent.
CHEMICAL EFFECTS OF ULTRASOSIC WAVES
1145
The acoustic efficiency, W'/cm.?, has been determined by measuring the acoustic pressure. The acoustic pressure in the oil above the pieeo-oscillator was determined with the aid of a conical metal reflector in order to diminish the influence of stationary waves. A linear connection is obtained between the acoustic output in the oil and the electric input in the ultrasonorator. Applying the formula concerning transmission of sound through thin plates, given by Rayleigh ( 2 ) , it is then possible to calculate the actual acoustic efficiency in the reaction vessel.
CONCLUSION
An analogy between radiochemical and cavitation-chemical effects is observed. The primary reaction accompanying cavitation in water solution is assumed to be H20 + H OH. Considering this, it is easy to explain the effects of oxidation and reduction in water solutions treated with ultrasonic waves. I t is helieved that the increase in the oxidation effects obtained in the presence of dissolved oxygen does not depend so much on a direct activation of oxygen in the cavitation as on the effert exerted by molecular oxygen ,on the secondary reactions in which the radicals OH and H take part.
+
1146
PAUL DELAHAY AND GERALD PERKINS
Experimental evidence is given by the polymerization of acrylonitrile in aqueous solutions that is induced by ultrasonic waves. The authors are indebted to the State Council of Technical Research for furnishing the ultrasonorator and to AB Bofors Nobelkrut, Bofors, for supplying the very pure acrylonitrile that was used in the experiments. REFERENCES (1) ALLEN,A. 0.: J. Phys. & Colloid Chem. 62, 479 (1948). (2) BERGYANN, L.: Der Ultraschall und seine Anwendung i n Wissanschaft und Technik, p. 78. VDI-Verlag, G.rn.b.H., Berlin (1942). (3) BEUTHE,H.: Z. physik. Chem. A l a , 161 (1933). (4) BONET-MAURY, P., A N D LEFORT,M.: Nature 162, 381 (1948). (5) DAINTON, F. S.: J. Phys. & Colloid Chem. 62, 490 (1948). (6) FRICKE, H., HART,E. J., A N D SMITH,H. P.: J. Chem. Phys. 6, 229 (1938). (7) GRABAR, P., AND PRUDHOMME, R. 0.:Compt. rend. 226. 1821 (1948). (8) HIRSCHFELDER, J. 0.:J. Phys. & Colloid Chem. 62, 447 (1948). (9) KLING,A., A N D KLINO,R.: Compt. rend. 223, 33 (1946). (10) LANNING, F. C., AND LIND,S. C.: J. Phys. Chem. 42, 1229 (1938). (11) Lru, H., AND Wu,S.: J. Am. Chern. SOC.66,1005 (1934). (12) LOISELEUR, J.: Compt. rend. 218. 876 (1944). (13) NOZAKI, K., AND BARTLETT, P . D.: J. Am. Chem. SOC.88, 23i7 (1946).
VARIATIONS OF T H E WAVE HEIGHT WITH THE RATE OF POTENTIAL CHANGE I N OSCILLOGRAPHIC POLAROGRAPHY SINGLESSEEPMETHODus. MULTISWEEPMETHOD PAUL DELAHAY
AND
GERALD P E R K I S S
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana Received July 6 , 1060
In a previous paper (3) it was reported that discrepancies between the height of experimental oscillographic waves and the height calculated by the RandlesSevcik equation (8,9) are due to the irreversibility of the cathodic process taking place a t the dropping mercury electrode. Moreover, when several waves are recorded during the life of a single mercury drop (multi-sweep method), the concentration of reducible substance at the surface of the drop, before the recording of each new wave, decreases as the drop grows. This decrease in concentration is caused by the incompleteness of the anodic process occurring during the quiescent period preceding the wave recording. This is an additional cause of departure 1 This paper is part of a thesis submitted by Gerald Perkins t o the Graduate School of Louisiana State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Aukust, 1950.
