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J. J. BIKERMAN
Vol. 56
MECHANICAL DESTRUCTION OF YOUNG AND OLD FOAM FILMS BY J. J. BIKERMAN Research Laboratories, Merck & Co., Inc., Rahway, N . J . Rscsitad Octobar #.#,I060
Single foam films (1.0-3.5 cm. in diameter) of aqueous disodium a-tocopheryl phosphate, sodium laurate, “acid” sodium laurate and of beer were broken by a stream of mercury droplets. Fresh f i l m (e. 6 sec. old) lasted in a mercury shower longer than aged films (e.g., 60 sec. old Similarly, fresh films of a “benzalkonium ctforide” solution endured more punctures b a glass needle than older films. Tkis result is in agreement with Rayleigh’s theory of foams. It indicates also that mec&mical devices for foam destruction are likely to be less efficacious the fresher the foam. The persistence of untouched single films was shorter for larger films; in several instances i t was inversely proportional t o the film area; apparently, this cannot be accounted for by the probability theory of rupture. were modelled after the well-known mercury cleaning device I. Introduction and are exemplified in Fig. 1. The bot,t,om part of the UThe striking insensitivity of foam to mechanical tube waa filled with mercury, and the rest of the longer damage is well known.2 Over 80 years ago Dupr& branch was then almost filled with the solut.ion to be tested. and later Rhumbler* dropped solid bodies such as By slightly immersing a fine pipet into the solution and in air with a rubber bulb, single foam films spanning lead granules, cork balls or iron f i g s through sin- blowing the tube were produced. Their persistence (life time) lo gle soap films or columns of soap lather without was determined. Then a fine stream of mercury dropleta any damage to the films. On the other hand, me- wm allowed to fall through the film into t,he solution and to chanical destruction of join the mercury column under the solution) until t e film burst. The exces mercury flowed through the spout into a foam is a recognized indus- container and was used for another series of experiments. trial process. The disThe diameters of the tubes along the stretch where the agreement between these films were produced, were 1.0 and 3.5 cm. in the two instrutwo sets of observations ments em loyed. The smallest mercury droplets had dimight be resolved if the ameters oPeither about 0.02 cm., or, in the later experiments, about 0.05 om., and fell from a height of about 2 probable cause of the great cm. The rate of flow of mercury was about 0.5 g./sec. or, resilience of foam lamellae in the later experiments, approximately 0.3 g./sec. The temperature of the laboratory varied between 21.2and 23.2’. is considered. It may be deduced from T o avoid fluctuations due to variations in the temperature, the foaminess of the solutions, and so on, experiments on the accepted theory of films of different ages have been performed alternately, e.g., foam stability6 that sur- in the order 5 sec., 30 sec., 90 mc., 240 sec., 5 sec., etc. face films are self-healing If the film waa tl sec. old when first hit by the mercury because removal of a part shower and if this shower continued for t 2 sec. before the film burst, then the lamella was tl + t 2 sec. old in the moment of the surface layer ex- of The relation between tn on one hand and ti or poses the bulk liquid ( t l rupture. + t ~ on ) the other hand was studied. which has a higher surface Only aqueous solutiom were used. They included: tension; because of this (a) 0.43% (w./v.) disodium cu-tocopheryl phosphate mono(b) 3.7% (w./v.) sodium laurate, prepared by greater surface tension the hydrate; mixing an acetone solution of lauric acid (C.P.) with an inexposed patch contracts sufficient volume of 5 N NaOH, boiling the acetone off, and and again brings the con- adding 5 N NaOH until the pH, determined with a n inditaminated areas in uninter- cator, reached 8.2 (it became 8.6 after dilution); (c) “acid” Fig. 1. laurate solution prepared by short heating 0.05 g. of rupted contact (“Maran- sodium lauric acid with a solution of 0.659 g. of sodium laurate in eoni effect”). This mechanism can be .exDected aqueous acetone, boiling the acetone off and diluting to 50 to operate as long as there is enough bulk-liquid cc. with water; this liquid was turbid immediately after to exhibit the higher surface tension. When the amount of solution between the surface layers in a soap lamella becomes too small, the MaranCHa /~(CHI)[C&I~CH(CH~)[CH~~,CH(CH~)ICH~~~CH( CHa)a, H2O goni effect should be depressed or even suppressed. Hence, “wet” foams should, and “dry” foams should not, pre aration, deposited flakes a day or a few days later but coupd be approximately restored by heating until incipient be resilient in respect to mechanical damage. formation; the foaminess of this liquid greatly varied This conclusion has been tested and confirmed by foam from da to day; (d) 0.32% (w./v.) “ben?alkonium chlothe experiments described below. ride’’ sorution, “benzalkonium chlonde” being the designation used b U. S. Pharmacopeia (XIVedition) for a mixture 11. Experimental Procedure of mlts [ d a s C H 2 N CHs)2R]Cl, R re resenting a mixture
6
v
,’
Tlie instruments used in the major.part of the experimeob
(1) The main results and concluaions of this paper have been presented at the 118th Meeting of the American Chemical Society, Chicago, Ill., September ?, 1950. (2) J. J. Bikerman, “Surface Chemistry for Industrial Researah.” Aoademio Press Inc., New York. N. Y.. 1947, p. 95. (3) A. Dupr4, “Thdorie meohanique de la chaleur,” Cauthier-VilIsrs, Paria, 1869. p. 352. (4) L. Rhumbler, Ergdniase Physiot., 14, 626 (1914). (5) Rayleigh, €’roc. Boy. Inrt. Qt. ErdL 18, 86 (1890)
of the alkyls from Cs(HI7 to CU Hw;(erbeer.
III. Results Figure 2 shows the decrease of reailience (expressed as time tz sec.) when the age at the start of mercury shower (tl sec.) or at rupture (ti tp) increases, for the disodium u-tocopheryl phosphate solution. The 1.0-cm. tube and the bigger mercury droplets were employed. Each 6 value is the
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Feb., 1952
MECHANICAL DESTRUCTION OF YOUNG AND OLDFOAM FILMS
I 1 mean of 50 determinations. The persistence (to) of untouched films was not properly determined but was approximately 7 minutes. E.g., of eight successive untouched films,six did not burst within 300 sec. In another series, three out of six successive films did not burst within 420 sec. T h e other results in the l.0-cm. tube are exL5 30I pressed below as averages of (usually) ten deter90 240 minations. Time, sec. Sodium laurate solution, to = 120 to 400 sec.; Fig. 2.-upper curve, age at rupture ( t l + k ) ; lower curve, t2 = 16 to 20 sec. after tl = 5 sec., and t2 = 3 to 9 age at start ( t i ) ; both in sec.; ordinate, tz sec. sec. after tl = 60 sec. Both large and small mercompositions. In fact, it appears that a foam film cury droplets have been used. “Acid” sodium laurate solution, 4 = 200 to 450 bursts “spontaneously’’ when its resilience reaches a very low value, near zero; then even the slightest sec. ; fs not measured. Fresh beer, to = 20 to 60 sec.; t2 = 2 to 3 sec. damage caused, e.g., by dust particles or air moveafter tl = 5 sec., and ta was less than 1sec. after tl = ment is incurable. The hypothesis advanced in the Introduction 20 sec. (small mercury droplets). Beer, 5 weeks old, to = 90 to 140 sec.; tn = 9 would have been confirmed even better, if the sec, after tl = 5 sec., and tz = 3.5 sec. after tl = 30 drainage (ie., the decrease of the water content of the film) were determined for different times tl, and sec. (bigger mercury droplets). Beer, 7 weeks old, 4 = 390 sec.; t2 not measured. it could be shown that the decrease of t 2 with inThe results in the 3.5-cm. tube are summarized in creasing tl parallels the decrease in the water conTable I. Each figure represents the average of tent. It had been hoped to estimate the volume of (usually) ten single determinations. Small mercury liquid in the films from their interference colors. droplets have been used for the “acid” sodium lau- Unfortunately, the films-also of disodium a-torate solution, and bigger droplets for the other solu- copheryl phosphate which is chemically the most homogeneous of the substances tested-had no tions. uniform thickness and at any one time exhibited TABLE I patches of different colors. PERSISTENCE OF U N T O U ~ E FILMS D (to) AND DURATION (tt) OF t1 SEC. Om FILMSIN A MERCURY SHOWER,FILMDIAM- The main observation reported in this paper, namely that fresh foams withstand more mechanical ETEB 3.5 cab. damage than do aged foams, should be of impork. -tt,aeo.Solution 880. t1 = 6 880. ti = 2Oseo. tance for the problem of foam destruction. It may N&a-tocopherylphosphate 30to39 4.5; 5.3 2.1; 2.5 be expected that all the mechanical agents-from Na laurate 13to32 3.7; 6 . 1 2.4; 2.8 rotating blades to supersonic waves-would have no effect or only a very limited effect on a fresh “Acid” Na laurate 35to90 13to27 6 t o 1 2 foam, and that their eflicacy will regularly increase Beer, 7 weeks old 18 to 51 Not determined with the foam age. As mercury droplets in traversing the films may Comparison of the persistences (to) of untouched become coated with the surface-active agent and, lamellae in 1.0- and in 3.5cm. tubes shows what therefore, may destroy the films also by reducing kind of relation exists between to and the diameter the amount of this agent in the foam rather than by of a film. The ratio of to for the two tubes was purely mechanical means, another procedure was about 12 for disodium a-tocopheryl phosphate, employed to which this objection does not apply. from 9 to 13 for sodium laurate, 13 for seven-week Foam lamellae were produced in a test-tube (diam- old beer, and about 5 for “acid” sodium laurate. eter, 1.05 cm.) and then destroyed by pricking The film area in the wider tube was about 12 times with a glass fiber the end of which, by heating, was that in the narrower tube. Thus, it appears that, made to a ball 0.056 cm. in diameter. The lamel- in some instances at least, tois inversely proportional lae were pricked every second. If the number of to the film area. punctures required for bursting is n, then n - 1 In analogy to the probability theory of tenseems a justifiable measure of resilience. The sile strength, the greater persistence of a smaller “benzalkonium chloride’’ solution (“d” of Section film might be thought of as due to the smaller like11) was used for these tests. After tl = 5 sec., n - lihood of a weak spot being present within a smaller 1 was 3.94, and after tl = 60 sec. n - 1was 0.08, area. Following the reasoning applied to adhesive each figure being the mean of 50 determinations. joints6 and glass threads,? the least persistence in a The rapid decrease of resilience on aging is shown batch of 12 small films should be equal to the averalso by this method. The persistence of the age persistence of large films. This conclusion was “benzalkonium chloride” films was 128-137 sec. not supported by the few experiments performed to (averages for 10 films). test it. Apparently, small lamellae differ from large ones not only because they contain less weak IV. Discussion spots. Such was also the opinion of Plateau.8 The above results by two methods confirm the (6) J. J. Bikerman, J . 800.Clem. Id., 60, 23 (1941). expected decrease of the resilience of the film during (7) J. J. Bikerman and G. H. Passmore, UZars Ind., 49, 144 (1948). its aging for films of four very different chemical (8) J, Plateau, Acad. roy. Belo.. MSm., 37 (1869), p. 8 of the rnelnoir. 1
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