Experiment 5
| Surface Tension and Soap Bubbles |  |
Background
Soaps and Detergents
Soaps are compounds which are made by heating fats or oils, from animal or vegetable sources, with lye, a strongly basic compound. A typical soap molecule has the formula:
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CO2-1 Na+1
A detergent is a similar kind of molecule, that is made from petroleum products. A typical formula is:
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2OSO3-1 Na+1
These compounds belong to a class of chemicals known as surfactants, from surface active agents. These molecules have some special properties which make them very useful for cleaning and forming bubbles and foam. In particular, the long hydrocarbon ends of the molecules are very nonpolar and do not form intermolecular bonds to water molecules. This end is hydrophobic (water fearing). On the other hand the ionic ends are very soluble in water and form rather strong ion-dipole intermolecular bonds with the very polar water molecules.
In order to simplify the description of the properties of these in more detail, the molecules will be drawn schematically as:
where the jagged black line represents the nonpolar, hydrocarbon end of the molecule and the blue circle represents the charged, polar end of the molecule.
Surface Tension
| An
idealized 2-dimensional view of the structure of a sample
of water is shown on the right. In this drawing the gray
lines represent the hydrogen bonds between the individual
molecules. An important point to note is that each of the
interior molecules have four hydrogen bonds but, on the
surface, the molecule are only hydrogen bonded to two
other molecules. Before reading on, count the number of water molecules on the surface. |
 |
| If we were to place an object with a low density on the surface of the water, the surface would be distorted as is shown on the right. Now count the number of surface molecules again. There are obviously more of them. Since no additional molecules were added, the new ones on the surface must have come from the interior. There are now fewer molecules with four hydrogen bonds and more with two hydrogen bonds. This means that there has been a net breaking of bonds. This costs energy. It costs energy to distort the surface of the water. This resistance to distortion is called the surface tension. |  |
When a water droplet is formed its spherical shape is the result of the surface tension. If it were to have any other shape, there would be more surface molecules and so would cost energy. The size of the droplet is also controlled by the surface tension so by measuring the number of drops in the same volume of different liquids, we can compare the surface tension of the liquids. The lower the surface tension, the smaller the drop and so the more drops in the same volume.
Surfactants and Surface Tension
| On the right is seen a schematic view of a detergent or soap in water. The surfactant part, in this example, has a negative charge at its polar end and the positive counter-ions (green) are distributed throughout the solution. Because the nonpolar ends of the surfactant molecules are so much unlike the polar water molecules and the negatively charged ends are attracted to the water, surfactant molecules form a layer on the surface with the non polar ends pointing away from the water. Not all of these molecules can fit on the surface and so some are forced to be on the water. These molecules would have a lower energy if they were on the surface. |  |
| If the surface of the water is distorted there is room for the surfactant molecules to get to the surface and out of the interior of the water. They are now in a lower energy environment and the interior structure of the water has more hydrogen bonding. This means that the water surface will be easier to distort and the surface tension is reduced. |  |
Surfactants and Bubbles
| The
structure of the surfactant is also responsible for the
formation of bubbles. A cross section, again idealized,
of a bubble formed by a soap or detergent is shown on the
right. By forming two layers, one on the inside, and one
on the outside, separated by water molecules, the
surfactant can maximize the bonding between the nonpolar
regions and also between the polar regions. This
stabilizes the bubble structure. An interesting question arises. Is there a connection between surface tension and bubble stability? |
|
Two good references to bubble literature are:
1. Scientific American, May 1969, p. 128.
2. Soap Bubbles and the Forces Which Mould Them, C.V. Boyd, Doubleday, 1959.
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