Soap and Water Interaction

The formation of soap bubbles is intriguing, because it relies on the physics of water (which is, in my opinion, even more fascinating) (4). Water bonds to itself and each water molecule connects to another through an attractive force called hydrogen bonds (Figure 1). Hydrogen bonds result from an electrostatic attraction between a proton (from a hydrogen atom) in a water molecule and an electronegative atom (oxygen) in another water molecule (5). A simple experiment to observe hydrogen bonds is to add as many drops of water possible onto a dry penny. If gentle enough, one could add over thirty drops, creating a dome shape of water on the penny. The dome will not exceed the width of the penny, but it will keep getting taller.

Figure 1. Visual of hydrogen bonds between water molecules
(http://www.beautybythegeeks.com/)

For another experiment, add a little soap to a bowl of water. It will spread across the surface forming a layer between the air and the water (4). This is because of the way soap molecules are (Figure 2). They resemble a long worm with many hydrogen and carbon atoms as its body/tail, and oxygen and sodium atoms as its head. The tail is hydrophobic, or “water-fearing,” so it is attracted to oil and bonds to it. The head is hydrophilic, or “water-loving,” so it is attracted to water and pulls the oil free from other oil molecules (6). The hydrophilic head also weaken water’s attraction to itself. This allows soap to clean most things and it’s how bubbles form.

Figure 2. Molecular structure of soap (detergent)
(https://upload.wikimedia.org/)

With soaps hydrophilic and hydrophobic ends, a “trio of layers” is created (Figure 3). It is essentially a top and bottom layer of soap molecules with water molecules trapped in between (6). This phenomenon is why soap bubbles are round and sphere shaped. Also, the “trio of layers” helps prolong the time of evaporation by preventing air from entering and escaping the bubble. However, since gravity pulls the soap molecules downwards, the bottom becomes thicker and the top thinner. This causes the air inside to rush out, bursting the bubble.

Figure 3. Visual of the "trio of layers." In the zoomed-in area of the bubble, the white part indicates
water, and the little yellow hairs pointing away from the water are the hydrophobic tails.
(http://www.clearbiology.com/)