Dancing Crystals: A Dramatic Illustration of Intermolecular Forces.

Intermolecular forces can come to life for students when they are used to generate motion. These forces are being studied for use in various industrial applications (1) and as a source of motion for nanomotors (2, 3). In this demonstration, crystals of naphthalene form in a solution of acetone and dance about in an animated fashion. Flows within the solutions can be visualized by various means. Some previous demonstrations of surface motion are also explored and compared with the dancing crystals. This demonstration can be used for discussion of intermolecular forces, properties of liquids, surface tension, Marangoni effects, crystallization, solubility, and heat of solution. Video presentations are also available in Supplemental Materials.

Figure 1. Dancing Crystals show as a blur of motion in this photograph taken with a one second shutter speed. Crystallization begins on the outer part of solution followed by sheets of naphthalene. Small crystals form in solution and start to move randomly.

Dry a sample of acetone by adding sodium sulfate until the crystals no longer clump together. Add enough crystals to cover the bottom of a 200 ml bottle about one centimeter thick. Wait fifteen minutes stirring occasionally for the drying process to complete itself. Preparations for two solutions are described below using the dried acetone. Solution A works well in low ambient humidity and Solution B works well in high ambient humidity but both work well over a wide range of humidity.

Dissolve 7.25 g of Naphthalene in 20.0 ml of dried acetone decanted from the bottle. Swirl this occasionally in a closed screw capped bottle. It may take from 30 minutes to an hour to completely dissolve. After the solid dissolves add 16 drops of distilled water from a dropper which delivers 20 drops per ml. (0.8 ml). A solid will precipitate. Swirling the solution should redissolve the solid. Keep this solution in a tightly capped bottle. The solution will rapidly evaporate acetone and change concentration so it must be closed when not in use. A 1.5 – 2 ml sample on a watch glass should show the dancing crystals upon evaporation. You may need to adjust the number of drops of water to the humidity conditions. (See procedure below)

Dissolve 8.8 g of Naphthalene in 20.0 ml of dried acetone decanted from the bottle. Swirl this occasionally in a closed screw capped bottle. This may take overnight to dissolve. Keep this solution in a tightly capped bottle. A 1.5 – 2 ml sample on a watch glass should show the dancing crystals upon evaporation. (See procedure below)

When placed on a watch glass the solution on the surface becomes saturated as acetone rapidly evaporates. In about 1-2 minutes, crystals of naphthalene will form and begin to move about. After allowing them to dance for a while use a fine-tipped plastic pipette to puff air at a point on the surface, the crystals will move to that point and remain relatively motionless. Please note that crystals may not dance for every trial and results may depend on humidity. Although this demonstration can be shown on the overhead it is best to use a video camera or have the students observe in small groups. Background music also adds to the entertainment value. I have used the Nutcracker Suite.

Some Observations about the Process

Some students may be surprised that naphthalene, a nonpolar hydrocarbon, would dissolve in the polar compound acetone. In fact, naphthalene is not very soluble in water. However the naphthalene aromatic pi electrons, which are polarizable, can participate in a dipole-induced dipole interaction with acetone. These forces allow naphthalene to dissolve. The

Pi electrons and high molecular weight also provide strong London forces as evidenced by the face that naphthalene is a solid at room temperature. In spite of the presence of a polar carbonyl group, the surface tension of acetone is relatively low (surface tension can depend on a complex set of factors). Counting drops per 0.5 ml with a 1ml syringe gives 42 drops for acetone and 33 drops for the solution. Larger drop size indicates a higher surface tension for the solution. We can conclude that forming a solution with naphthalene would lead to stronger intermolecular forces and therefore to an increase in the surface tension. A few simple observations can illustrate this higher surface tension and show that motion can be generated by this effect. If lycopodium powder is placed on the surface of the naphthalene-acetone solution, touching a drop of the original solution to the surface will have little effect. But touching a drop of pure acetone will propel the powder away from the drop. Acetone has a lower surface tension than the solution and the surface moves in the direction of the higher surface tension away from the acetone. Adding the naphthalene-acetone solution near the powder on acetone temporarily breaks up the powder due to solution flow. The higher surface tension of the solution then draws some of the powder towards the drop. It will eventually return to the rest of the powder. The observations above suggest that surface tension is higher with naphthalene in acetone.

If lycopodium powder is spread on the surface of pure acetone, touching solid Naphthalene to the surface causes only slight movement. The Naphthalene will eventually dissolve and draw the powder towards the solid. Naphthalene solid is a weak surface coating agent on acetone but changes the surface tension upon dissolving. Naphthalene does not vaporize significantly during this demonstration. The weight of the residue remaining upon complete evaporation of the acetone is not significantly different from the amount originally dissolved in the solution.

Naphthalene dissolves endothermically in acetone. The temperature drops to 15 ^oC upon dissolution to make a nearly saturated solution in a closed container with minimal evaporation occurring. This is consistent with strong London forces favoring naphthalene solid. Dissolution must be driven by an entropy increase.

The complete explanation for the motion of the crystals may not be fully resolved. A variety of effects can be responsible. Density gradients and surface tension gradients can occur at the same time creating convection currents and motions within the whole liquid layer. Such gradients can be generated in different ways, due to local changes in concentration and also in temperature, due to evaporative cooling. These different effects will be discussed below.

One possible explanation involves surface effects upon evaporation of the solvent. At random sites on the surface, rapid evaporation of acetone causes a multitude of changes; an increase in surface tension, an increase in concentration, an increase in density and a decrease in temperature. The surface tension draws the surrounding surface towards (5) this site and higher density solution at the site will sink. This creates a local current on the surface and below. Concentration and temperature gradients would also facilitate these flows. A real time observation of the flow patterns can be obtained by adding excess phenol red powder to the solution, swirling, and watching the flows develop as shown in the Supplemental Materials. Similar patterns of motion also form with acetone without naphthalene. Both the solution and pure acetone show the tears of wine effect characteristic of Marangoni flow (6). The observation that crystals move to a puff of air blown on the surface suggests evaporation may be creating a site of high surface tension or low temperature which draws the crystals to that site. This strongly suggests that surface tension flows (Marangoni flows) or convection currents already formed on the surface are a contributing source for the motion. If we imagine the naphthalene crystal blocking evaporation while high surface tension sites form around it, it is clear how chaotic motion can develop.

Temperature effects are also significant. The evaporating solution on the watch glass develops a temperature of about 14 ^oC during the demonstration due to evaporative cooling and warms a degree or two when crystals begin to form. Temperature on the surface may be colder but could vary with position according to flows and evaporation patterns. In fact when crystals form we expect to release the heat of solution leading to local warming. These temperature differences could lead to surface tension gradients or convection currents which could also explain the patterns observed above and motion of the crystals (7, 8).

Several other substances exhibit motion on liquid surfaces. Many of these substances are capable of coating the surface with molecular layers (9, 10). These molecules on the surface can reduce the surface tension and create surface tension gradients (11-13). The "camphor dance", "dancing ghosts", and "thunderstorm in a dish" are described below, and illustrated with pictures or videos in the Supplemental Materials.

Camphor dance: Camphor, when added to warm water, moves on the surface until it disappears (14-17).  The motion of the camphor is decidedly different from the naphthalene. The camphor pieces move constantly and swirl or spin in a regular motion. In contrast, the dancing crystals move more like a butterfly, changing direction and speed randomly and they sometimes change motion at the same time as if responding to flows. A drop of a surface active agent (such as soap or several drops of stearic acid/hexane solution) on the surface will stop the motion of camphor. Even lycopodium powder on the surface will stop the motion. With the stearic acid, swirling the dish will create some surface space for the camphor and motion will start again in that small space. Also air blown with a pipette has only a slight effect on the motion of the camphor and only when close to the camphor piece. This suggests that the camphor motion is due to localized surface effects brought about by camphor itself (15, 16). Since camphor is not soluble in water there should be no heat of solution effects or concentration gradients.

The surface flow of camphor can be visualized by creating a coating of naphthalene crystals on the surface before adding the camphor. A few drops of 3.12 g of naphthalene in 20 ml of dried acetone will coat the surface. Breaking this up with the tip of a plastic pipette will form a layer of loose crystals. When the camphor is added, the surface action of the camphor pushes aside the naphthalene and produces a moving open space. This is illustrated in Figure 2.

Figure 2. A moving camphor crystal pushes aside naphthalene as it coats the surface of water.

Dancing Ghosts: Another surface motion can be observed when drops of 1-pentanol (n-allyl alcohol) are added to a water surface (18, 19). Several drops must be added to saturate the water with the alcohol. Surface tension disturbances can be seen during this process but eventually droplets form that contort and move with a ghostly appearance. The motion can be similar to dancing crystals. After 2-3 minutes a more rapid straight line motion is observed. The droplets change shape like the wake of a boat when they go in motion, as shown in the Supplementary Materials. The importance of surface phenomena is shown if a drop is added quickly through the surface. It will stay at the bottom motionless.

Thunderstorm in a dish: Long range surface tension disturbances giving the appearance of a thunderstorm can be seen if ethyl acetate is added to the surface of water as described by Ahmad (20) and shown in the Supplemental Materials.

The surface of a liquid can provide a varied dance floor on which intermolecular forces can be showcased. I can imagine the dancing crystals responding to small differences on the surface to shift from one flow to another. The other forms of surface motion presented here are more active motions which depend on a stronger coating interaction of the substance with the surface. The surface coating activity, the volatility and the difference in surface tension between the substance and the surface all combine to generate the motion. The relative balance among these factors can provide a wide variety of motion.

Naphthalene, although used in mothballs, is potentially toxic and should not be inhaled. Prolonged breathing should be avoided and it should be used only in well ventilated areas (21). As noted earlier very little naphthalene vaporizes during the experiment and temperatures are low in the solution so it should be safe for short term exposure. However it should be covered and placed in a hood or a screw capped bottle after the demonstration. Gloves should be worn during the demonstration to prevent skin absorption. Acetone is a flammable, volatile liquid and should not be used near flames or ignition sources (22).

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