Chemistry for Everyone edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz
Kutztown University Kutztown, PA 19530
Stilling Waves with Ordered Molecular Monolayers submitted by:
checked by:
Ed Vitz Department of Physical Sciences, Kutztown University, Kutztown, PA 19530;
[email protected] James H. Maynard Department of Chemistry, University of Wisconsin–Madison, Madison, WI 53706
This is hardly a new demonstration. It was first done by Lord Rayleigh around 1900 in a public lecture at the Royal Institution (1) in which he used an electric fan (replacing the organ bellows used in his research) and a six foot trough. The demonstration was based on Benjamin Franklin’s1 observation in 1770 that a teaspoon of vegetable oil stilled waves on half an acre of pond surface (2). Observation of the behavior of oil droplets on the surface of water dates back to Babylonian lecanomancy (divination by observation of the shapes taken by oil on water) in the 18th century B.C.E (3)! It is probably time to bring this demonstration up-to-date. There have been several attempts to develop a practical application for stilling waves in this manner. Franklin supervised a large-scale trial to see whether it could be used to calm violent surf near Portsmouth (England), but the experiment failed; oil has no effect on large sea swells (4). Use on individual boats, at one time encouraged by the United States Hydrographic Office in Washington, may have yielded some success (5). A recent article in this Journal (6) summarizes most of the chemically relevant information in a book by Tanford that discusses the history of the experiment in depth. Tanford observes (7) that the experiment is an exception to the dogma that science
about 5 ft to screen
fish tank aerator
8” t 12” Pyrex dish
laser beam reflected here
3” 5 i.d., ” o.d. 16 16
latex hose tip cut from 1 mL Beral pipet
Figure 1. Top view of apparatus.
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~20” laser, about 8” higher than water surface
proceeds as new technology allows more sophisticated experiments. The same investigation, with essentially the same simple apparatus, was repeated by three Nobel laureates (Rayleigh, Langmuir, and Roentgen) and one amateur scientist Agnes Pockels (8), who may have made the most important experimental contribution (9) after Ben Franklin’s original report. Lord Rayleigh (10) used the data to deduce the size of a molecule, while Langmuir (11) related the effect to molecular properties. Franklin did neither, although he is sometimes credited with these insights when the “dimensions of a molecule” experiment (12–15) is repeated in countless educational laboratories. Demonstration It has been a challenge to repeat Rayleigh’s 1900 demonstration in a dramatic and convincing manner. We tried using spinbars and mechanical vibrators as well as wind from a hairdryer to generate waves but could not reliably reproduce the effect. Oil is ineffective at quelling low-frequency swells, so “choppiness” is the desired effect. Swells with frequencies less than about 10 Hz are called gravity waves, because the restoring force is mostly gravitational, while waves of higher frequencies (up to several kHz) are called capillary waves. For capillary waves, gravitational restoring forces are insignificant compared to the effect of surface tension. The most reliable method we found for easily generating high-frequency waves relies on an air stream from a fish tank aerator2 directed at the water surface. We developed two methods of demonstrating the effect: method 1 is reliable and beautiful, but method 2, which requires an overhead projector, has some advantages in that it encourages performance of several related demonstrations. Either method should be practiced beforehand. Method 1 Place an 8 in. × 12 in. × 1 in. dish on the lecture table, and mount a laser pointer in a three-finger clamp on a stand so that the beam is reflected from the surface of water and hits a screen on the front wall of the lecture hall (Figure 1). Some laser pointers have a power switch that can be held in the “on” position by the clamp. Use a thermometer clamp on another ring stand to hold the cut-off barrel of a 1 mL plastic Beral pipet so that it is just above the surface about 1–2 in. to the side of the spot where the laser beam is reflected. Use 3/16 in. i.d. × 5/16 in. o.d. latex hose to connect the pipet barrel to the fish tank aerator. Alternatively, the aerator can be clamped in a large three-finger clamp, and a short length of rubber tubing can be used to connect the pipet barrel so that the position of the tip can be adjusted carefully.
Journal of Chemical Education • Vol. 85 No. 8 August 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
Chemistry for Everyone
Adjust the height and angle of the pipet barrel so that the laser beam is deflected by about 16 in. on the screen (beautiful two dimensional patterns resembling a flame are sometimes observed). The room must be darkened. Add a drop or more of 0.1% oleic acid in ethanol and observe the dramatic decrease in the deflection of the laser beam by the water surface. A few drops is enough to create a monolayer of oleic acid on the surface, which can be visualized (see below) by several methods. Ethanol has no effect, as can be demonstrated either before or after the experiment suggested above. The original Rayleigh experiment may be simulated by using a drop or two of pure olive oil. Several drops of pure light mineral oil3 does not show the effect because pure mineral oil disperses to a monolayer very slowly. A few drops of a solution of mineral oil in acetone4 also has no effect because it does not form a surface monolayer (see below), while diesel fuel does show the effect. Arm and Hammer detergent (dermatologist tested) does still the waves because of the amphiphilic detergents it contains. Since the dish must be cleaned thoroughly between uses, it may be a good idea to have more than one available so that the demonstration can be easily repeated in the lecture hall. Method 2 Place a flat-bottomed glass dish on an overhead projector and add an inch of clean water. An 8 in. × 8 in. Pyrex baking dish works well. Attach a 4 in. length of 3/16 in. i.d. by 5/16 in. o.d. latex rubber hose to the outlet of the fish tank aerator, affix the aerator in a large three-finger clamp, and adjust its position above the tray of water so that the end of the latex hose is partially submerged in the water, causing bubbling and ripples. The hose should not be totally submerged, but interaction with the water waves will cause intermittent blockage of the hose with resultant “gurgling”. When the waves are damped by oil, the hose is no longer intermittently submerged in the waves, reducing further wave generation and stilling the surface. Observe the image caused by the rippled surface, adjusting the projector to focus near the surface of the water (not the glass bottom of the dish). Add a milliliter of 0.1% oleic acid in ethanol to the water near the hose outlet, and observe the change in the image. Repeat with other liquids.5 Related Demonstrations on an Overhead Projector To emphasize the incredibly small volume of oil required to produce a molecular monolayer, prepare the 0.1% oleic acid as part of the demonstration by adding 1 drop (~ 1/20 mL) of oleic acid to 50 mL of ethanol. Show that a monolayer is produced by placing the dish on the overhead projector, adding pepper or lycopodium powder to the surface, and adding a drop of the solution. The powder is immediately cleared from a large area of the surface, then contracts slightly as ethanol dissolves in the water, leaving only the oleic acid monolayer on the surface. The most amazing demonstration of the dimensions of a surface monolayer may be done by scratching one’s scalp with a finger, then touching the water surface with powder on it. Typically, a circle of 2–6 cm diameter will be cleared by the sebum (scalp oil composed of free fatty acids, wax monoesters, and triglycerides with antifungal or antibacterial properties). From the area of the cleared surface and estimates of the area covered by a single molecule (on the order of 1 nm2, depending on how tightly the molecules are packed) it can be easily calculated that,
in a fraction of a second, a number of molecules orders of magnitude higher than the U.S. population of humans (300 million) leaves the finger and coats the water surface. For the wave damping demonstrations, the surface of the water must initially be oil and detergent-free, and the previous paragraph suggests that this may not be a trivial accomplishment. The surface can be checked by sprinkling a few small crystals of camphor on it. If the “camphor dance” (10, 16) is observed, the surface is clean; if not, it is coated with oil and the dish must be washed with detergent and rinsed thoroughly with warm water. This test is worthy of discussion in itself and may be done as part of the demonstration. The resistance to spreading of mineral oil3 may be demonstrated by adding a drop of the oil to a water surface sprinkled with pepper or lycopodium powder. This may be contrasted with the result when a drop of olive oil is added to the powdercovered surface. We thought that a solution of mineral oil in petroleum ether or acetone might spread on the surface, but it does not. A drop of petroleum ether6 on a powdered water surface spreads very little, and it evaporates to leave no bare surface. A solution of mineral oil in petroleum ether leaves a film of mineral oil with about the same area that the solvent itself covered, an effect dramatically different from what is observed when an oleic acid solution is applied to the surface. Hazards Chemical Laboratory Information Profiles have been published in this Journal for oleic acid (17), which is innocuous; petroleum ether (18), which is toxic and flammable; and acetone (19), which is flammable. Ethanol has low toxicity and is flammable, and mineral oil is nontoxic. Care must be taken not to direct laser pointers at the eye (20). Discussion This demonstration reveals several properties of water surfaces that are extraordinary and generate discussion in chemistry courses at any level. Even though it appears that the monolayer is the locus of wonder, the properties of water should be the focus because, as Loren Eiseley (21) said, “If there is magic on this planet, it is contained in water.” If the water molecule were not (i) small enough to differentiate the hydrophilic and hydrophobic domains of an amphiphile, (ii) polar enough to interact strongly with the hydrophobic domains, and (iii) prone to organization around hydrophobic domains (preventing dissolution of amphiphiles entropically, see below), the monolayer would not form. There would be no calming of waves and possibly no observers to document the whitecaps. Biogenesis may well depend on formation of amphiphile layers as cell membranes. Before the advent of life, early oceans may have been covered with an oily layer, which could be converted photochemically to amphiphilic molecules (22). Amphiphilic phospholipids in water under ultrasound yield liposomes with similarities in structure to protocells (23). The biomimetic structures imparted to amphiphiles by water are modeled by “magic sand”, where again the magic is in the water, not in the sand (24). How do these demonstrations reveal the magic in water? First, the fact that nonpolar mineral oil does not spread quickly indicates that spreading must be due to molecular properties of water and of vegetable oils (or the fatty acids obtained by
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 8 August 2008 • Journal of Chemical Education
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Chemistry for Everyone
their hydrolysis). The polar molecules of vegetable oils bond to the surface of water, naturally promoting spreading. Bonding reduces the surface tension of water in a localized region, further enhancing spreading by Marangoni effects (25). The hydrophobic hydrocarbon chains prevent the oils from dissolving in the water. Even though petroleum oils bond exothermically to water, they do not dissolve because dissolution leads to a reduction of entropy (the hydrophobic effect). Alkane-based petroleum oils lack the polar domains found in vegetable oils that lead to rapid spreading on water surfaces. The spreading of mineral oil on the water surface is spontaneous (due to the exothermicity of the bonding), but slow, unless aided by wind. The mechanism for calming of water waves has been explored in a recent article (26). The authors claim that bulk surface tension effects could only explain 15% of the damping, while 85% results from the Gibbs–Marangoni effect. The general idea is that there are increased shear forces in the water below the surface, induced by surface tension gradients at the surface, which in turn are caused by longitudinal compression and rarefaction of the oil monolayer, caused by the water waves. The details of the explanation are technical and would be appropriate only in a physical chemistry course where surface phenomena are studied extensively. The effect depends on a nonzero surfacedilatational elasticity (Gibbs surface elasticity), εd, that is defined as εd = ‒Γ(dσ/dΓ), where Γ is the excess surfactant concentration in the longitudinally compressed regions of the monolayer and α is the surface tension. Interested readers are referred to the classic text by Horace Lamb (27) for historical perspective, or a more recent review by Scott (28). Notes
1. In a letter to William Brownrigg in 1773, Franklin wrote: In these experiments, one circumstance struck me with particular surprise. This was the sudden, wide, and forcible spreading of a drop of oil on the face of the water, which I do not know that any body has hitherto considered. If a drop of oil is put on a polished marble table, or on a looking-glass that lies horizontally, the drop remains in place, spreading very little. But when put on water it spreads instantly many feet around, becoming so thin as to produce the prismatic colors, for a considerable space, and beyond them so much thinner as to be invisible, except in its effect of smoothing the waves.
2. The aerator is model 82341, 120 V, 4W, from Hartz Mountain Corporation, Harrison, NJ 07029. 3. Light mineral oil (NF/FCC, Fisher 0121-1) was purchased from Fisher Chemical. Mineral oil sold as a laxative at drug stores may spread on a water surface. 4. In a new, 20 mL glass liquid scintillation vial, 1 drop (0.016 g) of light mineral oil (Fisher 0121-1) was dissolved in 14.337 g of acetone (Fisher HPLC grade, A949-1). 5. The checker obtained similar results with a pattern generated with a 12 mm “ball end point source” on a wave generator operated 27 or 40 Hz, with better results at the higher frequency. The wave
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generator is Cenco Digital Electronic, model CP33687-00, from Sargent Welch, 1-800-727-4368; http://www.sargentwelch.com/product. asp_Q_pn_E_CP33687-00_EA_A_Wave+Generator%2C+Digital+ Electronic%2C+CENCO_E_ (accessed Mar 2008). 6. Petroleum ether, pesticide grade (Fisher P480-4) was purchased from Fisher Chemical.
Literature Cited 1. Tanford, C. Ben Franklin Stilled the Waves; Duke University Press: Durham, NC, 1989; p 117. 2. Mertens, J. Phys. Today 2006, 59, 36–41. 3. Tabor, D. J. Colloid Interface Sci. 1980, 75, 240–245. 4. Tanford, C. Ben Franklin Stilled the Waves; Duke University Press: Durham, NC, 1989; pp 76–77. 5. Tanford, C. Ben Franklin Stilled the Waves; Duke University Press: Durham, NC, 1989; pp 100–102. 6. Gugliotti, M. J. Chem. Educ. 2007, 84, 941. 7. Tanford, C. Ben Franklin Stilled the Waves; Duke University Press: Durham, NC, 1989; p 2. 8. Agnes Pockels–Making History at the Kitchen Sink. http://home. frognet.net/~ejcov/pockels.html (accessed Mar 2008). 9. Giles, C. H.; Forrester, S. D. Chem. Ind. (London) 1971, 43. 10. Lord Rayleigh. Proc. Royal Soc. London 1889–1890, 47, 364– 367. 11. Langmuir, I. J. Amer. Chem. Soc. 1917, 39, 1848–1906. 12. McNaught, I. J.; Peckham, G. D. J. Chem. Educ. 1985, 62, 795. 13. Lane, C. A.; Burton, D. E.; Crabb, C. C. J. Chem. Educ. 1984, 61, 815. 14. King, C.; Neilsen, E. K. J. Chem. Educ. 1958, 35, 198–200. 15. Solomon, S.; Hur, C. J. Chem. Educ. 1993, 70, 252–253. 16. Mundel, D. J. Chem. Educ. 2007, 84, 1773–1775. 17. Young, J. A. J. Chem. Educ. 2002, 79, 24. 18. Young, J. A. J. Chem. Educ. 2001, 78, 1588. 19. Young, J. A. J. Chem. Educ. 2001, 78, 1175. 20. Vitz, E. J. Chem. Educ. 2003, 80, 30. 21. Eiseley, L. The Flow of the River. In The Immense Journey; Random House: New York, 1957; p 15. 22. Lasaga, A. C. Science 1971, 175, 53. 23. deDuve, C. Life Evolving: Molecules, Mind, and Meaning; Oxford University Press: Oxford, 2002; pp 71, 89. 24. Vitz, E. J. Chem. Educ. 1990, 67, 512. 25. Gugliotti, M.; Baptista, M. S.; Politi, M. J. J. Chem. Educ. 2004, 81, 824. 26. Behroozi, P.; Cordray, K.; Griffin, W.; Behroozi, F. Am. J. Phys. 2007, 75, 407. 27. Lamb, H. Hydrodynamics, 2nd ed.; Cambridge University Press: Cambridge, 1895; pp 552–555; The last edition is the 5th edition (1930). 28. Scott, J. C. Nature 1989, 340, 601–602.
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Journal of Chemical Education • Vol. 85 No. 8 August 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
