1. Why Water and Oil Separate Even After Shaking
If you put water and oil in a container and shake it hard, the oil breaks into tiny particles and looks like it has mixed temporarily. But if you leave it alone, it separates into two layers again [4].
When you add a surfactant like soap and shake, the mixture stays cloudy and mixed [4]. Grease and oil stains can be washed away with water because soap creates this "mixed state." But what shape are the soap molecules that make this possible?
2. Soap Molecules Have a Water-Loving Head and an Oil-Loving Tail
Soap is a mixture of sodium or potassium salts of fatty acids, which look like long chains [2]. One end of the molecule has ionic properties, so it likes water. The long hydrocarbon part has no electrical bias; it repels water and likes oil [1].
The side that likes water is called the hydrophilic group. The side that likes oil is called the lipophilic (or hydrophobic) group. Substances that have both are called surfactants. Because they can "talk to" both water and oil, they can make these two incompatible substances live together [1].
3. How Soap Molecules Clump into Spheres Called Micelles
As the amount of soap in water increases, the molecules gather. They arrange themselves with their oil-loving tails on the inside and their water-loving heads on the outside, facing the water. This forms a sphere-like particle [3]. This structure is called a micelle.
It is said that forming a sphere with heads on the outside makes it harder for the tails to touch the water, making the structure more stable than when the molecules are scattered [1].
The concentration at which micelles begin to form is called the critical micelle concentration [3]. Some explanations say that unique functions of surfactants, like emulsification and cleaning, appear after passing this concentration. However, the specific concentration changes depending on the type of soap and conditions, so no specific numbers are given here.
4. How Micelles Wrap Oil Inside Through Solubilization
Inside a micelle, the oil-loving tails gather. This area is friendly to oil, so it can take in oily substances that do not dissolve in water. This phenomenon is called solubilization, and it is considered one of the important functions in cleaning [3].
Chemistry textbooks explain that grease and oil droplets are surrounded by the hydrophobic tails of soap in the center of the micelle and dissolved in water [2]. The reason it can be washed away with water is not that the oil itself dissolved in water, but that it became a shape that likes water by being surrounded by the tails.
5. Why Soapy Liquid Can Look Clear or White
A liquid where oil has entered the micelle (solubilization) looks clear. On the other hand, in emulsification, oil droplets become larger particles than micelles, and the liquid looks white and cloudy [4]. It is said that the water in laundry or dishwashing becomes whitish because it is in the state of emulsification.
Solid particles, like dirt, have surfactant molecules attaching to their surroundings. They scatter and become stable in the water. This is called dispersion [4].
Soap also weakens surface tension, making it easier for water to soak into cloth (penetration) [5]. Dirt comes off because emulsification, dispersion, and penetration work together.
Note: From the materials used here, it is not clear what percentage of cleaning is done by solubilization versus emulsification in actual laundry or dishwashing.
6. Why Soap Works Less in Hard Water and Makes Soap Scum
Hard water contains a lot of magnesium and calcium. When soap meets these, it changes into a salt that does not dissolve in water [2]. The ring-like soap scum that forms in bathtubs is explained as the result of this change.
This reaction happens before the soap can attach to dirt. The more metal soap that forms, the less soap is left to clean the dirt [6]. It is explained that more soap is needed in water with high hardness.
7. A Simple Experiment to Try with Water, Oil, and Soap
What you can do today: Put water and a small amount of salad oil in a clear plastic bottle. Close the lid tightly and shake it. Measure the time it takes for the oil to separate. Then, add a few drops of soapy water, shake it again, and compare the cloudiness and how it separates. Make sure the lid is closed tightly, and wipe up any spilled liquid.
If you want to investigate more: Look up the difference between "emulsification" and "solubilization" using examples like mayonnaise or milk. You will find the same mechanisms in everyday things.