Experiments with oil on water

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JOURNAL OF CHEMICAL EDUCATION

MAY.1931

EXPERIMENTS WITH OIL ON WATER*j E D I ~ RNOTE: 's The following is a stenographic report from a n educational talking motion picture in which Dr. Langmuir accompanies his talk with close-up views of his experiments. Necessarily, there are sections of the text which are ambiguous since in theFlm Dr. Langmuir could point sfiecifically to parts of the demonstrations. Parenthetical insertions have been made occasionully i n the text to clarify certain points, and at other places the points huve been illustrated with enlargements from the &lm. The sound track appenrs to the left of the pictures as a jagged, black line. The film, made with RCA-Photophone eouipment, was produced by the Motion Picture Department of the General Electric Company. Dr. Lnngmuir did not talk from a prepared manuscript.

During the ten-year period from 1890 to 1900, the late Lord Rayleigh carried on many investigations of the phenomena of surface tension and the spreading of oil films on water. He found, however, that the physicists in general were not much interested in these phenomena, so that gradually he went on to other lines of investigation; but I think in these days nearly every one is more deeply interested in the phenomenon of surface tension than they were at that time. I want to tell you of various experiments that I have made along these lines. A film of oil on water produces many phenomena besides the traditional one of quieting troubled waters. For example, many beautiful iridescent colors are formed, although if we see such colors on our favorite swimming pool they do not appear very beautiful to us. Visible films

* This manuscript has been preparedat the editor's request by MR. GW BARTLETT of the General Electric Company. t This talking motion picture is that shown before the Division of Chemical Education at the Columbus meeting of the A. C. S., May 1, 1929.

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EXPERIMENTS WITH OIL ON WATER

of this character are comparable in thickness to a wave-length of light. I n fact, i t is the interference phenomenon that produces these colors. About 0.00001 of an inch, or up to 0.0001 of an inch, is the usual thickness of these visible films; but I want to talk to you particularly of films that are very much thinner-invisible filmsfilms that are in thickness about 0.00000001 of an inch. The experiments that I plan to show you with these films will be made with very simple apparatus, apparatus such as a photographic tray, talc, and strips of paper (Figure 1). I can show you some of the effects of these films made with what, as a boy, we used to call camphor boats. These boats were propelled over the surface of the water by motion set up in water by pieces of camphor, which alter the surface tension. Many years later, I found that the late Lord Rayleigh had spent-several years in the study of the properties of camphor film phenomena and their motions on water. In fact, many generations of physicists before him had been puzzled by these phenomena. To show the motion of this camphor boat, I am going to pour some water into this hard rubber, photographer's tray, but I want to show the motion of the surface of the water that will have this invisible film on it. For that purpose I will make up a little

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FICUKE I-'' WIIH V B R Y SIMPLE A,'PARATUS, APPARATUS s u m AS A PHOTOGRAPHER'S TRAY, TALC. A N D STRIPS OP PAPER."

SOAPSTONE I N A I.I'L.TLC "A(:.''

FIGURE3 . "TRESIILI.ITLE

P I I K I S OP CAM. PHOR ARE I N A CONSTANT STATE O F

MOTION."

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MAY,1931

bag of talc, simply by tying up some talc or powdered soapstone in a little bag (Figure 2). I will use this to dust over the surface of the water so as to make the movements of the surface visible. Instead of using talc, I can also use sulfur in a similar bag. I will put some water in this tray and then put on the water a few little pieces of camphor. I am not ~ o i n gto take any special precautions to keep these surfaces clean; it is not essential. These little pieces of camphor are in a constant state of motion (Figure 3). Small ones cause rapid movement of the water; larger ones cause slower movement. I will now take a strip of paper and out of i t cut a little boat, but in order to make the boat behave rightly in this tray I will leave a little rudder-not the same as most rudders, as it FIGURE4.-"I rrcK ur ONE OF THESE will be on the wrong side. PIECES O F CAMPHOR AND PUT IT IN A LITTLE RECESS AT THE RACK OP THE B O A T . . . ." Now, you see, I have made my boat and I have a little space here (at the stem) in which I can put a piece of camphor. I pick up one of these pieces of camphor and put it in the little recess a t the back of the boat (Figure 4) and, you see, the boat immediately starts to move. The rudder action is pretty strong, so it goes in a circle (Figure 5 ) . The reason for this movement is a change in the surface tension of the water. FIGURE 5.-"THE KUDDER AC.rION IS PRETTY STRONG. SO IT GOES I N A C I R C L ~ " There is a decrease in tension right back of the boat, so the water pulls the boat ahead more than it pulls it back; for instance, you can see that right back of the boat are rather vigorous currents. I sprinkle a little talc over the surface of the water and, as the boat advances, the water pushes the talc away. The talc is pushed away from the rear. showing that motions or currents are set up and go one way while the boat moves in the opposite direction. This motion of camphor takes place only upon water that is relatively clean.

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If I put on the water a small amount of oleic acid from a globule on the end of a wire (Figure €9, there is, first of all, a spreading action that starts off from the drop of oil that has been added, and the action of the camphor is entirely stopped. You have iust seen how the oleic acid placed on the surface of this tray of water spreads out, setting up sutface currents until the whole surface is contaminated. Camphor does the same thing; that is, it contaminates the surface and it spreads out over the surface, setting up currents in the water. The reaction to those currents produces this motion of the boat, but if the surface is already covered with camphor there is nowhere for the oleic acid to spread to, as the camphor spreads a good deal more strongly than the acid does. There are many interesting features connected with t h i s spreading. To illustrate some of these, I will show you some further experiments. I will take a smaller sized tray and will make some paper barriers to put on this tray by taking some large sheets of paper and folding them so as to make some long strips. Then I fold them a t the ends so that they are just the width of the tray. I will fill this tray with water and then I will put a little sulfur powder, which is equivalent to FICUKB (j.-''. . . . I IWT O N T H E W A T E R A talc, on the surface of the water SMALL AMOUNT OF O L E l C ACID FROM A C L U B to render the movements of the ULE ON T H E E N D O F A W I R E . . . . . ." surface visible. If I blow on the snrface, it is possible to blow the sulfur about two-thirds of the way down the tray, but not much farther; that shows that there is enough contamination on the surface to cover about one-third of the surface of the tray.

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If I'blow that contaminated part away from me and then move the paper, I can get a better measurement of the area that is contaminated because now, you see, blowing causes circulation of only that surface; it does not cause the contamination to move away from the paper barrier which I am moving. If I move the paper barrier all the way to the end, I am able to get rid of the contamination. If I do this a couple of times, I get a surface which is clean. I can remove this barrier, and on its wet surface most of the contamination sticks. I get a surface which is not absolutely clean, but about 95% clean. The contaminated part has been blown to one end. On this clean surface of water, sulfur is very mobile. I can push it in any direction with the utmost ease. If I now place on the surface an extremely small amount of oleic acid-taking even more pains than before to get every bit of excess acid off the wire, rendering the surface a little more visible by adding a little more sulfur, and then putting the end of the wire on the surface of the water so as to get a very small amount of the oleic acidyou see the sulfur disappear just as if I had blown there. Now notice the difference. By blowing on the water I can push the sulfur back only a short distance; for example, that circle represents the area from which I can blow this film of oleic acid Froune 7.-" T H E A R E A FROM WHICH I (Figure 7). I will put on more CAN BLOW T H E FILM OP OLEIC ACID." oleic acid; in fact, I will put on all I can. There is a limit to the amount of oleic acid that will go on. The surface is now a completely contaminated one. To show that it will hold very little more, I will put on a larger globule of oleic acid. Notice i t practically does not spread any more. This surface is in equilibrium with a globule of oleic acid which I can see on the surface. These floating pieces of sulfur on the surface give us a way of telling whether or not the surface is contaminated. Free mobility of the surface means freedom from oil film, or any film that is non-volatile and nonsoluble. A contaminated surface, on the other hand, resists compression. I t is just that action of resisting compression which makes the effect of oil in quieting wave action. I will show you another experiment to illustrate the magnitude of these forces on the surface of the water. I will take this tray full of clean water,

Vor..

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and will render the surface visible with a little sulfur powder. Then I will put a barrier, a strip of paper, dividing the tray into halves. I want to show you with how much energy this paper is forced forward by the oleic acid film. I will cut this paper a little shorter than the width of the tray so it will move along the sides. I will now place on the surface a little oleic acid (Figure 8), and you see that the paper - . is immediately forced back to the end of the tray (Figure 9). You have noticed that a small amount of oleic acid will spread out only to a certain extent, or over a certain area, on water. This phenomenon was apparently first noticed, or a t least first published, by Miss Pockels in i891. She found that the surface tension of water was not appreciably affected by small amounts of oil; only when you increase the amount to a certain definite value F I G U R E 8 . ~ ' '. . A L.II..ILL< O L B I C A C I D . . . . " does the surface tension begin to change. This was given a very simple explanation in 1898 when Lord Rayleigh assumed that an oil film consists of a single layer of oil molecules-what we now call a monomolecular film. If this is so, it explains immediately these fundamental properties of oil films-their tendency to spread out over certain distances and not beyond those distances. FIGURX 9.-". . . . . .me PAPER IS IMMEDIYou can get a more definite ATELY ~ O R C B DBACK r0 THE END OF TEE idea of how molecules behave on these surfaces by a simple illustration that I can make with steel ball bearings I am going to place in this tray. Let us imagine that these steel balls are molecules of oleic acid and that this is the surface of the water. If I put just a few molecules on the surface it corresponds to a water surface containing a very little oleic acid. A motion of this kind then corresponds to the thermal motion or agitation of molecules (Figure 10). Tipping the tray like this would correspond to blowing on the surface of the water

I 1

.,

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MAY.1931

(Figure ll), forcing the oleic acid molecules in a certain direction. If I put a large number of balls over the surface, it corresponds to a large amount of oleic acid. By a force exerted from a strip of this sort I will move all the balls to one end of the tray, corresponding again to what I can do with my paper bamer in the case of oleic acid (Figure 12). An understanding of those monomolecular films explains their behaviorthe behavior we have seen in these experiments. Some oils spread, and others do not. Oleic acid, olive oil, common vegetable and animal oils spread readily on the surface of water; but mineral oils such as Russian mineral oil, used for medicinal purposes, or Nujol, of the same general character, do not spread a t all on the surface of the water. I will now show you that action by taking this tray and putting water in it, cleaning the surface as usual, and then, by means of this pipet, transferring a certain amount of Nujol to the water. I am going to ask you to look over my shoulder at the reflection of the window, and in that way you will be able to see the surface motion more plainly. I will put some of the Nujol on the water, and you will notice that it does not spread a t all (Figure 13). You can see that FIcmr: 10.-". . . . . .CORRESPONDS TU THE the water is clean by the ease THERMAL MOTION OR AGITATION OP MOLEwith which the sulfur moves over CULES." the surface. If I now take some oleic acid and put it in the center of the Nujol also, nothing happens. The reason is that the oleic acid has not yet penetrated through the Nujol. If I move the wire down a little deeper (Figure 14) so that it penetrates through the Nujol, and the oleic acid comes in contact with the surface of the water, then there is a spreading of the oleic acid, carrying the Nujol with it. You will notice that the drops of Nujol move over the surface without any change. If I put on some more Nujol it does not spread;

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l3XPERIMENTS WITH OIL ON WATER

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it merely collects as large drops on the surface. This pure hydrocarbon oil does not spread a t all on the water, but a very small trace of oleic acid in it will cause it to spread. Only 0.00001 part of oleic acid will cause the Nujol to spread gradually over the surface. What is the differencebetween the hydrocarbon oil and oleic acid that is responsible for the FIGURE I I .-". . . . C o R R l l s P n N D S lo nr.ow1NG entire difference in behavior of O N THR SURPACI OP 1.HE WATER." these substances when spread on water? I can illustrate this by giving you the chemical composition of oleic acid. Oleic acid consists of long molecules that have eighteen carbon atoms in a row. There are two carbon atoms in the center that are attached by a double bond, and the rest of them are connected by single bonds. Hydrogen atoms are attached to all the carbon atoms, until here FIGURE 12.-'' . . . . c o n n r < s r o ~ m ~AGAIN r; a t the end there is an oxygen WHAT 1 CAN DO WLTH MY PAPER B A R R I E R atom tied by a double bond and TO m THE CASE OF OLP-IC ACID." an oxygen-hydrogen group held by a single bond (Figure 1.5). This is the chemical structure of an oleic acid molecule, a very long molecule with eighteen carbon atoms, surrounded on all sides by hydrogen, and a t the end of the molecule what chemists call a carboxyl group consisting of two oxygen atoms, a hydrogen atom and one carbon atom, at the end of this chain. This carboxyl is a group that is characteristic of all fatty acid F I G u R ? ~13.-". . . . . I T DOES ROT SPREAD A T ALL." molecules. An oil like Nujol

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or a pure hydrocarbon oil has no such group as this. It consists simply of a long row of carbon and hydrogen atoms and we must, therefore, look upon this carboxyl group as being responsible for the spreading of oleic acid on water. In fact, if we make experiments with a large number of different substances, some having such groups and others not, we find that the presence of such a group as that carboxyl group is really the criterion for the spreading of the oil on the surface of the water. Why does this group cause spreading of oil on water? It is not hard to guess, a t any rate. This hydrocarbon part of the molecule has properties of Nujol. It does not mix with water, nor dissolve. I t does not even spread on water. This carboxyl group is characteristic of oleic acid, stearic acid, acetic acid, etc. All of these acids contain this group. In all it increases the solubility of the acid in water. Acetic acid, for example, mixes with water in all proportions. We see that this group is responsible for the spreading on the surface of the water, and it takes place undoubtedly in the following way. If this (a horizontal line on the blackboard) represents the surface of the water, and we have a molecule of this kind in contact with it, then we have this hydrocarbon part, hydrogen and carbon, not in contact with the water, and the active end, FIGUKE 14.-"IF 1 MOVE T H E W I R E DOWN II or carboxyl group, surrounding LITTLE D R B P E K , . . . . . '' itself with water just as if it went into solution (Figure 16). This carboxyl end dissolves in water, but is not able to drag this hydrocarbon part into solution. In that case the water becomes covered with a set of molecules which are placed beside each other. The whole surface of the water is covered by the single layer of molecules, packed tightly together side by side with their heads, or active ends, all in intimate contact with the water. If a globule of oleic

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acid is placed on the water it sr~rradsso that the affiniti. ior the water becomes satisfied, and yet the tendency of these hydrocarbon molecules to mix with one another is also satisfied. We then see how by this conception we explain the presence of monomolecular films of oil on water. Measurements such as I have described enable us t o do something more than was possible for Rayleigh. I t enables us to make measurements of the shapes of the molecules. Lord Rayleigh found that the thickness of the film was about one hundred millionth of an inch. We now can get cross-sections of the molecules and see if they are spherical or elongated. That is done in this way. The chemist knows, largely from the work of the physicist, just how much every m o l e c u l e weighs. He knows, for example, that a molecule of oleic acid weighs 4.6 X gram. I n a given experiment we can tell how much oleic acid is put on the surface of the water. T h a t can best be done by taking a very small amount of acid, putting i t in a large, measured volume of benzol, shaking and mixing well, and taking out a single drop It is dropped by this dropping pipet, which has been calibrated, and which transfers t o the surface of the oil a very minute hut accurately known amount of oleic acid. Knowing the weight of each molecule we can tell how many

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Experiments with oil on water

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