WHAT HAPPENS WHEN A DROP of oil or some other liquid is placed on the surface of water? If it is soluble in water, it disappears quickly into the bulk liquid. If it is insoluble, as paraffin oil is, it remains where it is placed. If it has an intermediate solubility, as olive oil has, it may spread over the surface of the water, eventually becoming invisibly thin; it will pool or bead on the surface if more drops are added. What determines the behavior of the drop?

The answer to the question remains somewhat uncertain. Part of it is that the behavior of a drop placed on a water surface is determined by the complex interactions (mostly electrical) of the molecules at the interface. Recently Frode Wissing of the Royal Dental College in Denmark sent me a manuscript about his experiments on the solubility of oleic acid (a major component of olive oil) and paraffin oil in water. His work, which was designed as a classroom demonstration, reveals several curious pieces of the solubility puzzle.

The solubility of a chemical compound in water depends on the extent of the bonding between the molecules of the compound and those of the water. The degree of solubility results from a competition between the bonds that hold each molecule together and the alternative bonding opportunities offered by the other substance.

Organic compounds range widely in their solubility in water. Life on the earth would not exist without this variability. The insoluble organic compounds have component groups of atoms that form few bonds (and in some cases none) with water molecules. Such groups are said to be hydrophobic, as is a molecule that is insoluble because of such groups. The term is misleading in that it implies a repulsion between the molecule (or a group) and the water. The effect arises not from repulsion but from the fact the bonding is so weak that the cohesion of the water keeps the hydrophobic compound out.

Paraffin oil is an example of a hydrophobic compound. It consists entirely of hydrophobic groups of carbon and hydrogen atoms. Since hydrocarbon groups form few bonds or none with water molecules, a drop of paraffin oil stays intact when it is placed on water. The drop pushes down on the water surface because of its weight, but it cannot dissolve in the water or even spread out over the surface.

Many other organic molecules are partially soluble in water because at least some of their atomic groups bond with water. The more such groups a compound has, the more soluble it is. For example, the sugar glucose dissolves readily in water because it has six hydroxyl (OH) groups that bond well to water molecules.

Oleic acid is intermediate to paraffin and glucose. One end of the molecule has a COOH group that bonds well to water. Such a group is said to be hydrophilic. The rest of the molecule, which is a chain of hydrocarbon groups, is hydrophobic. This dual nature of the oleic acid molecule is responsible for its ability to spread over the water in a layer only one molecule thick.

Wissing set out to illustrate the differences in solubility of paraffin oil (strictly hydrophobic) and oleic acid (partly hydrophilic). He did his experiments in petri dishes that were nine centimeters in diameter and absolutely clean. (Any contamination, even from a fingerprint, deposits a monolayer on the water surface and may alter the results.) A dish was placed on an overhead projector and surrounded by dark paper to make the light pass only through the dish. The arrangement yielded a magnification of about 10 for the events in the dish, more than enough for a class to follow as Wissing added drops of organic fluids to water in the dish. I repeated his experiments in my kitchen satisfactorily even though I substituted pure olive oil for the oleic acid.

Wissing first demonstrated the insolubility of paraffin oil by placing a drop of it on distilled water. Since the oil is insoluble, it merely rested on the water in a lenslike shape. As more drops were put on the water some coalesced to form a larger lens, but none spread over the surface or dissolved.

Wissing then did three similar experiments with dishes containing other solutions: hydrochloric acid, sodium hydroxide and ammonium hydroxide each in the dilute concentration of .1 molar. In each dish the oil formed a lens, neither dissolving in the solution nor spreading over its surface.

Repeating the demonstrations with a drop of oleic acid, Wissing got more dramatic results. He had dyed the oleic acid with a small amount of Sudan III Red so that he could see it better. When he put a drop of oleic acid on pure water, the water surface developed a shock wave, as if someone had thumped the dish. The drop spread quickly over the surface, leaving a film so thin that even the red dye did not show up. Some of the acid also formed into a large lenslike drop and several smaller ones.

In my experiments I deployed a tiny drop of olive oil by flicking it from a straightened paper clip. (I had previously cleaned the clip with detergent and then heated one end to red heat.) The drop immediately disappeared as it spread over the surface, forming a layer much too thin to be observed directly or even to generate the kind of optical interference seen when an oil slick lies on water. The surface must have been almost completely covered with the olive oil, because a second drop held together for several minutes before spreading gradually into a layer that was quite visible in some places and at least thick enough in others for optical interference to create colored bands on it.

When Wissing added a drop of oleic acid to a dish of dilute hydrochloric acid, it again spread over the entire surface and formed a large drop. This time the lens was flatter, nearly filling the dish. Oleic acid added to a solution of sodium hydroxide formed an irregular lens. The acid flowed slowly outward from the drop in thin streams. After 20 minutes the lens had disappeared, leaving a turbid solution in the dish.

A drop added to the ammonium hydroxide zipped over the surface in small streams, each one bordered with clear zones, and then disappeared: The surface became calm after the acid stopped reacting with the ammonium hydroxide. Reaction products lay on the surface, but the bulk liquid showed no turbidity.

Wissing set himself the task of explaining those variations in terms of the forces acting between the oleic acid and the different solutions. The simplest force is gravity, which pulls a drop of oleic acid down into the solution, tending to spread it over the surface. Another force, arising from the displacement of the solution, is buoyancy.

More complex (and interesting) are the many electrical interactions that can take place between the molecules in the drop and those in the solution. I shall discuss the simple quantum-mechanical models of these forces and then follow Wissing's application of the models to his experiments. Bear in mind, however, that all the models are flawed by the lack of detailed understanding of the quantum mechanics involved.

When a molecule has sites with a net electric charge, it is able to bond through electrostatic interactions. For example, a molecule that has a site of net positive charge attracts a molecule that has a site of net negative charge in what is termed an ionic bond. A second type of electrostatic attraction develops when one or both molecules have an electric moment. (This term describes the distribution of charge in a nonspherical molecule.) Although the molecule may be neutral, the center of its negative charge (from the electrons) does not coincide with the center of its positive charge (from the protons). Such a state sets up an electric field surrounding the molecule. When two such molecules sample each other's electric field, they are attracted.

A third possible attractive force arises when a molecule that has a strong electric moment is near another molecule that has no moment. The electric field from the first molecule shifts the charge distribution in the second molecule, creating an attraction between them. For example, the first molecule might shift the electron orbits in the second one so that the center of the negative charge is then farther away than the center of the positive charge.

A virtually ubiquitous interaction between molecules with permanent electric moments is the hydrogen bond. Here attention is directed to a hydrogen atom lying between the molecules and serving to hold them together. Water is cohesive because of the hydrogen bond. A water molecule consists of one oxygen atom and two hydrogen atoms, together forming a wide V with the oxygen at the apex. Although the molecule has no net charge, the electron associated with each hydrogen atom is strongly attracted to the large oxygen nucleus. The molecule thus has an internal charge distribution that creates an electric field around it.

A simpler description is that the hydrogen ends of the V are left positive and the oxygen apex is left negative. One should resist the oversimplification of stating that the hydrogen atoms have been stripped of their electrons. The charge distribution is not that extreme. Often the charge distribution of a water molecule is said to be polar to indicate the presence of an electric moment.

Imagine two adjacent water molecules aligned as is shown in Figure 5. The molecule at the left presents one of its hydrogen atoms to the oxygen in the molecule at the right, thereby forming a hydrogen bond. The attraction is an electrical one in which the positive end of one molecule is pulled toward the negative apex of the other. The hydrogen atom is not lost by the molecule at the left (which is called the hydrogen donor) or gained by the molecule at the right (the hydrogen acceptor). The intermediate hydrogen is about a third of the way between one oxygen and the next and is said to be shared by the two molecules, even though it is still more strongly held by the donor.

Although this picture of a hydrogen bond serves well, I should point out that no mathematical model yet devised has proved to be completely successful in explaining the bonding. The interaction of two water molecules is truly not the simple static one I have described. In bulk water the stablest arrangement of the molecules arises when each molecule has hydrogen bonds with four adjacent molecules. To two of them the central molecule acts as a hydrogen donor. To the other two it acts as an acceptor. Although the arrangement is the stablest one, the hydrogen bonds can still be stretched, rotated and broken. Without this flexibility water could not flow. Indeed, the arrangement of molecules is not static even when water lies undisturbed in a container. Bonds are constantly being broken and re-formed.

Water frequently dissolves a compound because the water molecule is small enough to move close to the compound's molecules. If the compound has sites with a net charge, the water forms ionic bonds through an electrostatic attraction as it presents either its oxygen atom or a hydrogen atom to the charged site. At places on the molecule where there are polar groups the water forms hydrogen bonds with the compound.

In general only oxygen and nitrogen atoms participate in hydrogen bonding. A carbon atom cannot do so because it tends to surround itself with hydrogen (to form a hydrocarbon group) and does not leave an exposed end to which a water molecule could bond. Moreover it does not set up an electric moment to form a polar group. This difference in bonding capabilities between carbon and oxygen is the main reason the hydrocarbon groups in oleic acid are hydrophobic whereas the COOH group at the end of the molecule is hydrophilic.

Additional bonding can be provided (although rarely) by the natural dissociation of water into H+ and OH- ions. They are attracted to sites on the compound that have an electric moment or a net charge. Finally, the compound might be able to dissociate to provide a hydrogen atom for a water molecule. Then the negative site left on the compound attracts the positive water ion.

When two spherical and neutral molecules are close, there should be no electrical interaction. Because they are spheres, the center of each molecule's negative charge coincides with the center of its positive charge and so they lack electric moments. Ionic attraction is absent. Still, they can attract each other through a curious circumstance.

In the simple quantum-mechanical picture of the molecules the orbits of the electrons become synchronized, creating instantaneous electric moments that set up fleeting electric fields. The fields give rise to an attraction between the molecules. The attraction is usually termed the van der Waals force.

In a simple picture of the van der Waals force one first imagines the electrons in orbit in adjacent molecules. Each molecule should "see" the other as electrically neutral because the positive and negative charges within the molecules are equal. When the molecules are close enough, however, the two groups of electrons interact and develop synchronized orbiting.

One can then imagine that at a given instant a molecule has a momentary separation of charge, giving it an electric moment. The same is true of the other molecule. For that instant the two molecules attract each other. The attraction may fluctuate as time passes, but over time the average of the attraction is not zero; indeed, it is strong enough to hold the molecules together. Paraffin oil is held together by van der Waals forces.

Armed with this handful of models for intermolecular forces, Wissing was able to explain his experiments and to propose new ones. When he put a drop of paraffin oil on water, there was essentially no bonding between the hydrocarbons in the molecule and the water molecules. Hydrogen bonding is impossible between the two types of molecules. Other types of bonding are too uncommon to compete against the hydrogen bonding maintaining the cohesion of the water or the van der Waals force maintaining the cohesion of the oil drop. Hence the oil drop is merely pulled downward into the water surface slightly by gravity. When the drop is placed on the other solutions, the same thing happens because there is still little chance of bonding at the interface.

The story behind the behavior of the drops of oleic acid now becomes clearer. When a drop of it is deposited on pure water, bonding immediately begins at the interface. The hydrophilic end of the oleic acid molecule is attracted to the water, but the opposite end and much of the length of the molecule are not. The hydrophilic end is commonly referred to as the head of the molecule and the opposite end is called the tail. The molecules of oleic acid at the interface rotate so that the heads line up toward the underlying water surface, leaving the tails upward.

Two types of bonding develop between the head and the water. Since the head and a water molecule are both polar, their electric moments end up in a configuration that leads to attraction. (More properly the attraction should be described as hydrogen bonding. A water molecule presents one of its hydrogen atoms to the exposed oxygen atom in the head in order to form the bond.)

The other kind of bonding involves the dissociation of the head group, which loses a hydrogen ion (a proton) to a water molecule. The loss leaves the head negatively charged (COO-) and the newly formed water ion positively charged (H₃O+). The two attract each other with ionic bonds because of their opposite electric charges.

Further bonding then begins along the interface. When the oleic acid molecules become oriented with their heads toward the water, they bond to each other in an orderly way. Adjacent heads form a hydrogen bond: the OH in one head aligns with the oxygen in a neighboring head. More bonds form between the hydrocarbons in the chain that makes up the rest of the molecule. At some sites the adjacent lengths of hydrocarbon attract each other through the van der Waals force.

The oleic acid at the edge of the drop bonds to the water as the acid molecules rotate into formation. Through the van der Waals force their motion pulls fresh oleic acid molecules onto the water, where they bond and then pull still more molecules from the bulk of the drop. The drop thins as more of its liquid moves to- the edge and into molecular alignment. Soon it becomes an invisible sheet one molecule thick. The sheet cannot be stretched beyond this stage because of the bonding between its adjacent, aligned oleic acid molecules.

When a small amount of oleic acid is deposited on water, only a portion of the water surface is covered by the monolayer. With excess oleic acid the monolayer forms and the excess is left in a lenslike drop (or in several drops) on the surface. When the surface of the water has become covered with oleic acid molecules, no more of them form bonds with the water. They are left with only their van der Waals forces and the other electrical interactions that make them cohere. These surplus molecules bond themselves into the leftover drops.

When a drop of oleic acid is put on dilute hydrochloric acid, the process is much the same except for one thing. This time the abundance of hydrogen and chlorine ions in the bulk liquid precludes any systematic dissociation at the head of the oleic acid molecules. The molecules still rotate into alignment with heads down and tails up, but now their bonds with the water are almost all hydrogen bonds. Again the drop of oleic acid spreads over the surface.

It is a different story when a drop of oleic acid is placed on a dilute solution of sodium hydroxide. The solution neutralizes the acid, yielding sodium oleate. At the concentration in Wissing's experiment the oleate molecules form into tiny drops called micelles. The molecules at the interface between a micelle and the surrounding water have their polar heads outward and their hydrocarbon tails inward. The heads bond with the surrounding water through hydrogen bonds and sometimes also through an electrostatic attraction created when a head group loses a hydrogen ion to a water molecule.

The formation of micelles can be seen clearly in Wissing's experiment. Soon after a drop of oleic acid falls onto the surface of the sodium hydroxide the drop sends out surface streams that gradually disappear. At the same time micelles form just below the streams and the drop. The bulk solution gets turbid as the micelles obstruct the passage of light through the solution.

Ammonium hydroxide also neutralizes oleic acid, but no micelles form. As the oleic acid is neutralized it is vigorously expelled from the drop in streams. The drop and the streams move rapidly over the surface. I do not understand why the acid is expelled.

The next set of experiments done by Wissing fascinates me. He began with clean water to which three or four drops of oleic acid were added. The monolayer again formed and the excess oleic acid was left in a large drop. In the photograph at the top left in Figure 1 the drop rests on the water, which is still shimmering after the sudden formation of the monolayer.

Away from the drop of oleic acid Wissing deposited five drops of paraffin oil. When the first one was added, the drop of oleic acid developed a broad extension. After the fifth drop of paraffin oil was put into the dish the oleic acid drop sent a stream over to the nearest paraffin drop and began to consume it. For several minutes the oleic acid slid over to the drop of oil, climbing it and eventually dissolving into it, turning the oil pink. The oleic acid drop was then fatter and more spread out.

The photograph at the middle left in Figure 1 shows a second drop of paraffin oil just before it was consumed by the oleic acid drop. Traces of the first drop (already consumed) can still be seen on one side of the oleic acid drop. The photograph at the bottom left was made after a second and third drop had been eaten by the oleic acid drop. The rest of the experiment continues in the photographs running from top to bottom at the right.

Neither Wissing nor I can fully explain this demonstration. He suggests that the presence of the paraffin oil offers a means whereby the hydrocarbon groups of the excess molecules of oleic acid can bond with the hydrocarbon groups in the oil. My guess is that initially the molecules in the oil drop tug on the tails of the oleic acid molecules in the surrounding monolayer. Since the molecules of the monolayer are held together, the tug is transmitted to the oleic acid drop, with the result that part of the drop is pulled toward the oil drop.

I am greatly puzzled by this experiment. Why does one drop of oil cause the drop of oleic acid to extend whereas five oil drops make the oleic acid start consuming the paraffin? I think the reason has to do with the horizontal force on the monolayer from the drops of paraffin. Suppose that before the first drop of paraffin is put in place the dish is covered with only a monolayer and the drop of excess oleic acid. Because the liquid in the drop of acid tends to slide off to the sides, it exerts a horizontal force on the monolayer.

When the first oil drop is added, it creates more force on the monolayer. Four additional drops of oil create even more force. By now an oil drop has begun to bond with the nearby oleic acid molecules surrounding it.

From which direction will a fresh batch of oleic acid come? Preferentially from the direction of the oleic acid drop because of the force with which it pushes on the monolayer. The drop of oleic acid droops onto the monolayer. This extension is then pulled by the monolayer (because of the bonding at the oil drop) and pushed by the hydrostatic pressure of the fluid in the rest of the oleic acid drop. Eventually the extension reaches and then devours the drop of oil.

Wissing sometimes changes the experiment while the oil drops are being gobbled up by the oleic acid. He pours one or two milliliters of concentrated ammonium hydroxide near one side of the oleic acid drop. The drop begins to dance wildly as the new compound neutralizes it. Micelles begin to form. The scene on the classroom screen is vivid with red drops and streams in crazy motion until the abundance of micelles blocks the passage of light through the petri dish.

Wissing's final experiments are just as entertaining. In one of them he deposits enough oleic acid on water so that a drop of the acid remains. He puts on the middle of it a drop of Triton X-100, a commercial detergent. When the detergent has had a chance to penetrate the acid drop and reach the water, the surface of the solution is suddenly cleared of the oleic acid.

In another experiment Wissing deposits a tiny drop of oleic acid on water and adds enough paraffin oil to make an oil drop about three centimeters in diameter. Then additional oleic acid placed near the paraffin oil encircles it, forming . a complicated pattern of drops that continue to move for a long time. In a separate demonstration Wissing shows how a drop of oleic acid on water can be maneuvered when a cotton swab wet with concentrated ammonium hydroxide is brought nearby. With a sudden jab 51 of the swab he makes the drop of oleic acid jerk across the water surface. When he corrals it near the side of the dish, the drop oscillates in the vapor given off by the swab.

The formation of a monolayer by partially soluble organic compounds played a major role in the early work on determining the size of molecules. In 1890 Lord Rayleigh employed a minute amount of olive oil to estimate the size of its molecules. A fine platinum wire was dipped in the oil and then weighed on a sensitive balance. Some of the oil was released onto a circular water surface 84 centimeters in diameter and the wire was reweighed. The difference in the two measurements was the weight of the oil then covering the water surface.

To monitor the extent of spreading by the monolayer Rayleigh sprinkled the water with fine grains of camphor. As camphor dissolves in water on one side of a grain, it decreases the surface tension of the water. The larger surface tension on the other side jerks the grain. When Rayleigh added olive oil, the surface tension was too low for the camphor dance. He could therefore monitor the extent of the oil layer by watching the camphor grains. Adding just enough oil to stop all the grains from dancing, he then had the water surface covered evenly with a monolayer. From the surface area and the weight of the drop he computed the thickness of the layer as being about 17 angstrom units. Rayleigh thought this number was also the length of the molecules forming the monolayer.

Richard E. Crandall and Jean F. Delord of Reed College have developed a modified form of Rayleigh's experiment for their students in introductory physics. In the exercise a student calculates the length of the oleic acid molecule, the average bond length along the molecule and the mass of the carbon atom. The first step is to sprinkle a water surface with lycopodium powder. When a drop of oleic acid is added, the monolayer pushes the powder outward into a circle and then contracts into an irregular perimeter. Before the circle contracts the student sketches its boundary on graph paper. Later the sketch is measured to ascertain the area of the circle.

The student is given several facts. Th solution of oleic acid is prepared with 200 parts per million of acid in alcohol, measured in terms of volume. The size of a drop of acid is approximately .05 cubic centimeter. When the molecules of acid are aligned with heads down and tails up, a head occupies an area that can be approximated as a square with a side .1 the length of the molecule. The masses of O, C and H are in the ratio of 16:12:1. The density of oleic acid is about .895 gram per cubic centimeter.

By multiplying the concentration of the oleic acid and the volume of the drop the student calculates the volume of oleic acid that spreads over the water Dividing the volume of the acid by the surface area of the monolayer yields the height of the layer. That height is also the length of the oleic acid molecule.

The next step is to estimate the bond length, that is, the distance between two adjacent carbon atoms along the length of the molecule. For simplicity you can consider the double bond at the center of the molecule as a single bond; the rest of the bonds along the length of the hydrocarbon chain are true single bonds. You also might ignore the nonlinear structure at the head of the molecule. In this way you are dealing with 17 bonds between carbon atoms along the molecule, or 18 if the hydrogen at the tail is included. Assume that the bonds lie along a straight line. When their number is divided into the length of the molecule, you find that the bond length is on the order of one angstrom.

Multiplying the volume of oleic acid by the density of the acid gives the mass of the monolayer. Find the mass of each molecule by means of the estimate that the head of the molecule is a square with a side .1 the length of the molecule. Since the molecule's length is known by now, the area of the square of a head is easy to compute.

The number of oleic acid molecules is computed by dividing the area of the monolayer by the area of each head. Dividing the mass of the layer by the number of molecules yields the mass of each molecule. Finally, the mass of a single carbon atom is determined by counting the number of each type of atom in the molecule and employing the known ratios of their masses.

Molecules are notoriously difficult to picture because of their smallness and complexity. Yet from Rayleigh's experiments one can estimate their size. From Crandall and Delord's laboratory exercise one can determine bond length. With Wissing's colorful demonstrations one can even imagine the ordering of the molecules along the interface between water and an organic compound and can make sense of why some organic compounds spread over water and others do not.

Bibliography

THE HYDROPHOBIC EFFECT: FORMATION OF MICELLES AND BIOLOGICAL MEMBRANES. Charles Tanford. John Wiley & Sons, Inc., 1980.

INTERATOMIC AND INTERMOLECULAR FORCES. John P. McTague in Encyclopedia of Physics, edited by Rita G. Lerner and George L. Trigg. Addison-Wesley Publishing Company, Inc., 1981.

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