A. L. KUEHNER Bishops University, ~ennoxville,Quebec, Canada
Boys1 AND L A ~ R B N C Ewere ~ able to prepare, from specially purified sodium oleate, films which had under controlled conditions a lifetime of several years. Thin films would seem to possess all of the characteristics necessary for immortality. They resemble the characters in George Bernard Shawls play "Back to Methuselah" who, living in an age in which man has achieved immortality, wander about through the centuries waiting for a fatal accident to happen. If dust and carbon dioxide are excluded and evaporation is prevented, a sodium oleate film may live for a long time before meeting with a fatal accident. In the open air its lifetime is relatively short. Ordinary soap and commercially available sodium oleate produce short-lived films, because of the presence of sodium stearate and palmitate which form solid monolavers on the surface of the film, with the result that the film is unable to withstand 'sudden stresses. Purification of oleic acid from which sodium oleate is prepared is a long and tedious process. The addition of glycerol greatly increases the life of soap films, possibly because the surface monolayers are more liquid and because drying out is prevented. Cooka has pointed out the fact that addition of small amounts of tannin greatly improves the toughness of soap films. He also recommends the use of triethanolamine oleate solution for bubbles with a sufficiently long life for ordinary purposes. With so many new types of surface-active compounds available4 the investigator of tough films and bubbles is no longer limited to soap films, which are susceptible to carbon dioxide and other acid fumes as well as to particles of heavy metal salts. These substances are no doubt largely responsible for the fatal accidents which befall soap films. It occurred to the writer that it should be possible to make long-lived films using surface-active compounds not affected by small amounts of acid and other destructive agencies. The commercial product Aerosol O.T.,6 the surface activity of which was popularized by its ability to sink a duck, was selected for experimental purposes. Aerosol O.T. is the dioctyl ester of sodium sulfosuccinate. Solutions of Aerosol O.T. of different concentrations 'BOYS,C. V., "Soap Bubbles, Their Colours and the Forces Which Mold Them," The Maemillan Company, New York, 1924~ -~ - -.
l L h ~ A. ~8. C., ~ "Soap ~ ~ Films," , G. Bell & Sons, Ltd., London, 1929. ' COOK,G . A,, J. Chem. Edue., 15, 161 (1938). ' Ind. Eng. Chem., 35,126 (1943). 6 Manufactured by the American Cyanamid and Chemical Corporation, Stamford, Connecticut.
were prepared. To test film strength a ring of heavy platimum wire, 8 cm. in diameter, was const,ructed. One end of the platimum wire was bent up over the center of the ring and served as a handle and as a means of hanging the ring in a horizontal position. The ring was dipped horizontally into the solution to be tested and then slowly withdrawn leaving a film stretched across the ring. The ring was then suspended horizontally on a glass rod clamped to a retort st,and. The lifetime of the film was measured with a stop watch. Average lifetimes recorded are in all cases t,he average of ten or more trials. Solutions containing only Aerosol formed very weak films. The average lifetime of 8-cm. films made from 2 per cent Aerosol solution was -only 2.2 seconds. When 5 per cent Aerosol solution was used the lifetime of similar films increased to only 5.0 seconds. Since it is possible that the increase in the toughness of soap films by the addition of glycerol may he due in part to an increase in the viscosity of the films, it was decided to determine the effect. of sucrose on films formed from Aerosol solution. Sucrose has a pronounced effect on the viscosity of,the solution with no large decrease in the vapor tension. As shown in Table 1 an increase in the viscosity of the films by the addition of sugar greatly increased their average lifetime. An entirely unexpected result was also obtained. Films made from solutions containing 10 to 14 g. of sugar in 25 ml. of 2 per cent Aerosol had a sufficiently long life to allow the film to dry out. After d r j i n g out, 11
JOURNAL OF CHEMICAL EDUCATION
the films were essentially solid and there was no longer any movement within them though they retained the iridescent color patterns characteristic of thin liquid films. I n the solid form the films lasted almost indefinitely though they were quit,e fragile. TABLE 1 G. sucrose added to $5 ml. of B per e a t Aerosol solution
Average lifetime of 8 - m . film, secads
5.9 7.5 9.4 12.2 36.7. 50.0 Film solidified Film solidified Film solidified
When a wire loop 3 t o 4 cm. in diameter, attached to a handle, was dipped into the solution containing 12 g. of sucrose in 25 ml. 2 per cent Aerosol solution and the loop then waved through the air, large numbers of bubbles were formed. If these bubbles were kept suspended in the air for about one minute they dried out and would then roll along the floor without breaking, all the time retaining the beauty of a soap bubble. The effect of glycerol on the lifetime of films made from Aerosol solution is shown in Table 2. TABLE 2 G. gl~eeroladded to .25 ml. of B per cent Aerosol solution
Average l~fetzmeof 8-cm. film, seconds
I t will be noted that the effect of glycerol is greater than that of sugar on the lifetime of these films since glycerol not only increases viscosity but also prevents drying out. These Aerosol-glycerol solutions were, however, of no value for blowing bubbles since the bubbles burst soon after they began to expand. The films, though stable, were unable to adjust themselves rapidly to stretching. Addition of both sugar and glycerol t o Aerosol solution should produce a nondrying film of high viscosity. As shown in Table 3, the lifetime of films containing both of these substmces is increased enormously, the increase heing greater than the'sum of the separate effects of sugar and glycerol. Here is a nice example of synergy.
While the average lifetime of 10-cm. bubbles made from solution No. 3 was 19.1 minut.es, the longest-lived bubble lasted for 45 minutes. The average lifetime of TABLE 3 M1. of B per cent G. of Solution Aerosol sucrose No. solution added 1 2 3
25 25 25
14 14 20
glycerol added
Life of 8-cm. film, minutes
5.0 10.2 13.5
6.1 21.4 141 0
M l . of
.
Life of 10-cm. bubble, minutes
... ... 19.1
19.1 minutes for a 10-cm. bubble compares favorably with the average lifetime of 10 minutes for bubbles of equal diameter blown with triethanolamine solution as reported by C o ~ k . ~ Solutions 2 and 3 (Table 3) containing sugar and glycerol produced films which were extremely elastic. Bubbles floating in the air could be largely deformed by sudden puffs of air. It was possible also to blow very large bubbles from these solutions. If a wire loop 16 cm. in diameter was dipped into a tray of solution and withdrawn so as to contain a film, floating bubbles 15 to 18 inches in diameter were produced by waving the loop through the air. Substitution of dextrose for sucrose had no significant effect on the lifetime of the resulting films or bubbles though the films were, if anything, more elastic. When determining the lifetime of films in the open air, results vary from day to day depending in part on t,he relat,ivehumidity of the air. An at,tempt was made t,o study the lifet,irneof films under controlled humidity conditions hy suspending the films under a bell jar standing in saturated solutions of various salts or in water. Such conditions, however, approach those required for film immortality. For instance, an 8-cm. film which had an average lifetime of 2 minutes in open air lasted for more than three days in the quiet saturated air under a bell jar. Under these conditions it was next t,o impossible to determine difference in the toughness of films. Preparation qf Solutions. To make 2 per cent Aerosol solution, 2 g. of the waxy solid Aerosol O.T. 100 per cent is added to 98 g. of distilled water in a flask. The flask is shaken from time to time over a period of twenty-four hours. The excess Aerosol (about 0.5 g.) over that required for saturation remains suspended in the solution. When sugar or glycerol is added t o the solution the suspended Aerosol dissolves. The best solution for making solid bubbles contains 12 g. of sugar in 25 ml. of 2 per cent Aerosol solution. For very elastic films and large bubbles 20 g. of either sucrose or dextrose and 13.5 ml. of glycerol are added to 25 ml. of 2 per cent Aerosol solution. Gentle warming is necessary to bring the sugar into solution within a reasonable time.
