LONG-LIVED SOAP BUBBLES The Use of Sodium 9,10-Dibromostearate Solutions A. L. KUEHNER Bishop's University, Lennoxville, Quebec, Canada
INMOST sciences soap bubbles and films have found wide application both in research and in the lecture room. Lecture table demonstrations include the explosion of mixtures of inflammable gases and oxygen in soap bubbles, the demonstration of surface tension phenomena and the floating of a soap bubble in a jar half filled with carbon dioxide to illustrate the relative density of this gas. Rafts of small bubbles on the surface of a soap solution have been used to simulate the behavior of metal crystals under stress. Bubbles have served as thin-walled ionization chambers for studying the ionizing power of radioactive rays. The rate of flow of gases through tubes has been determined by first passing the gas through a soap solution and then measuring the rate of advance of the single soap films through the tube. I n all of these diverse uses and demonstrations the films and bubbles must have a reasonably long life. I n this paper a soap is described from which can be made bubbles and films far more persistent than those made from ordinary soaps.
VOLUME 35, NO. 7,JULY, 1958
Boys (1) and Lawrence (2) found that the most satisfactory solutions for long-lived bubbles were made by saponifying a highly purified oleic acid, their method of purification of the acid being a long and tedious operation. It seemed, therefore, worthwhile to search for a soap other than highly purified sodium oleate which would form films of equal or better stability. There can be little doubt that a soap film is made up of a sandwich of solution between tmo adsorbed layers ( 3 ) . If the soap solution is free from excess alkali, so that hydrolysis may occur, the surface monolayers consist of either acid soap (4) or molecules of free acid ( 5 , 6 ) . I t would be reasonable to suppose that the surface monolayers on soap films exposed to the air and subject t o the action of carbon dioxide would consist entirely of fatty acid molecules. Once this assumption is made the problem is to find a fatty acid having bulk and surface properties similar to those of oleic acid. Such an acid is 9,lO-dihromostearic acid. When purified this acid bas a melting point of 28.5"-29°C.
though, as ordinarily prepared, it is a liquid a t room temperature. The viscosity of the acid is greater than that of oleic acid and a monomolecular layer also shows a high surface viscosity. This high surface viscosity should increase the stability of soap films made from sodium 9,lO-dibromostearate solutions by decreasing the rate of drainage (7). Experiment has shown that bubbles made from the dihromo soap are superior to those made from sodium oleate. Bubbles, 20 cm. in diameter, made from a 4% solution of sodium 9,lO-dibromostearate, containing an equal volume of glycerol, had the astonishing average life of 102 minutes in the open air, the longest-lived bubble lasting for 152 minutes. By comparison, 20-cm. diameter bubbles blown from a 3% solution of purified sodium oleate, also containing an equal volume of glycerol, had an average life of only 36 minutes. The sodium oleate was made from the same purified oleic acid that was used to prepare the 9,lO-dibromostearic acid and its sodium salt. A 4% solution of the dibromo soap was compared to a 3% sodium oleate solution so as to have approximately equal molar concentrations of soap in each case. I n this work the relative stability of bubbles was determined by pouring soap solution, containing an equal volume of glycerol, into a 10-cm. diameter Petri dish and blowing a bubble 20 cm. in diameter, allowing it to rest on the rim of the dish. The bubbles were not protected in any way. The lifetime of the bubble waa recorded as the average of the life of ten or more iudividual bubbles. I n bubbles made with sodium 9,lO-dihromostearate solutions the surface monolayers seem to be in equilibrium with the solution in the interior of the film since a few moments after being blown the bubbles no longer show any turbulent motion of interference colors. That the film is still liquid is evidenced by the slow appearance of horizontal concentric rings of interference colors at the bottom of the bubble. Very interesting bubbles consisting entirely of black film, a film which is too thin to give colors by interference, may he made using the sodium 9,lO-dibromostearate solution. A layer of water to maintain a high humidity is placed in the bottom of a large jar and a Petri dish containing the soap solution is mounted above the water. A bubble which rests on the rim of the dish is then blown inside the jar and the jar covered. After several days of drainage a circular patch of black film appears at the top of the bubble. This gradually increases in size until the entire bubble is black. When completely drained, the thickness of the bubble film approximates the thickness of the two surface monolayers. There is a large variation in the life of such protected bubbles since, being so thin, they are very fragile. Nevertheless, one such bubble lasted for 48 days. Purification of Oleie Acid
solution was chilled to -20°C. for eight hours by placing it in
the freezing compartment of any ordinary refrigerator. From time to time the flask containing the solution was shaken to hasten crystallization. The precipitated solid acids were then removed by suction filtration through a Buchner funnel which had been cwled to -20°C. T o recover the aleic aeid the filtrate w.rp l ~ m r c d inw ilir;till+dw t + r and the deiv arid layer rcpbnitrJ. Tlrir a h l a waslw~lz r w r d timw with hot iliitilkl w t r t . . .\iwr tlor fird zrparutim the wmxinirfi~a w t m r and w:xtrr in the olrir. acid were swept out with a stream of carbon dioxide from a cylinder of compressed gas, the acid being kept warm during the sweeping. Ten grams of solid acids and 182 g. of purified oleic acid were recovered. Preparation of 9,lO-Dibromostearic Acid Purified oleic acid (28.2 g.) was placed in a beaker cooled in a mixture of ice and salt. By means of a fine-tipped dropper, 16 g. of bromine was then added st such a rate that the temperature did not exceed 0 % During the addition of the bromine the contents of the beaker were stirred vigorously with a motor-driven stirrer. The last few drops of bromine added reacted quite slowly indicating saturation of the oleic aeid. The dibromo acid was then dissolved in a mixture of 80 ml. acetone and 5 ml. of distilled water and chilled t o -20°C. for six hours. A further small amount of solid acids crystallized which wa8 removed by filtration. The acetone solution was then poured into hot distilled water and the heavy dibromo acid was separated and washed twice with hot distilled water. After the final separation the acid was warmed and swept free of acetone and water with a stream of carbon dioxide, and 41.2 g. of 9,lO-dibromostearic acid was obtained. During all of these operations grest care was taken to avoid contamination with grease. All equipment was washed with acetone before use. No stopcock grease was used to lubricate the stoppers of the separatory funnel. Preparation of Sodium 9.10-Dibromostearate Solution Twenty grams of 9,lOdibromostearic acid and 470 ml. of distilled water were placed in a beaker equipped with s n efficient stirring device. T o follow the course of the saponification the electrodes from a aR meter were immersed in the solnt,ion. Sndium hydroxide solution was sddedsliwli until nearly all of the acid had reacted and dissolved. The solution was then allowed to stand for a day to permit the small amount of unreacted aeid to become thoroughly dispersed. The addition of sodium hydroxide was then continued a t a very slow rate until the solution became clear. This occurs a t a pH of 10-10.1. Addition of excess alkali a t this point is fatal, presumably became excess alkali represses the hydrolysis of the soap so that, in bubbles farmed from the solution, the formation of the surface monolayers of free acid is interfered with. Just before use one volume of glycerol is added to an equal volume of soap solution. Under conditions of very low humidity the ratio of glycerol to soap solution may be increased. Sodium oleate solution is prflrunvl in tlw ti.$me way, t h solu~ rim l*.mning rlcbr at pll 99-10. If 11.e .mlutiunr do not Imome y i~ indirated. vlcw, iwffiomt rvmovnl of r>tursttd f ~ t f acids ~
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LITERATURE CITED (1) BOYS,C. V., "Soap Bubbles, Their Colours and the Forces Which Mold Them," The Macmillan Company, Ner*.York, 1074
(2) LAWRENCE, A. S. C., "Soap Films," G. Bell and Sans, Ltd., London, 1929. Ind. Eng. Chcm., 21, 815 (1929). (3) FOULK,C. W., (4) . . LAING.M. E.. J. W. MCBAIN.AND E. W. HARRISON."6th Colioid ~y&posiumMonog&h," Chemical ~ a t d o i ~ o m pany, New York, 1928, p. 63. (5) NUTTING,G. C., F. A. LONG,AND W. D. HARKINS,J. Am. C h a . Soe., 62, 1503 (1940). (6) NUTTING,G. C., AND F. A. LONO,J. Am. Chem. Soc., 63, 84 (1941). (7) CAMP,M., A N D K. DURHAM,J. Phys. C h a . , 59, 993 (1955). (8) Ind. Eng.Chem., 39, 126 (1947).
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