How Does Soap Dissolve Grease?

Why Oil Doesn't Dissolve in Plain Water

Substances can be classed as polar or nonpolar. Imagine you had a supply of little bar magnets the size and weight of toothpicks and a supply of actual toothpicks. If you tried mixing all these little bars by shaking them in a bag, when you opened the bag you would find that all the magnets had stuck together, while the toothpicks were excluded.

Molecules can have an electric polarity similar to the magnetic polarity of the little bars, or they can have no such polarity, depending on what substance they're molecules of. Water is a highly polar substance, and oils are nonpolar. So oil and water don't ordinarily mix because the water molecules are drawn to each other, to the exclusion of the oil molecules. Oil is thus said to be "hydrophobic."

The Relevant Property of Soap

Soap molecules are hydrophobic at one end---their nonpolar end---but are hydrophilic (attracted to water because of being polar) at the other end. Obviously, a soap molecule can't be both dissolved in and excluded from water at the same time, so in water soap molecules orient themselves as their best compromise to satisfy those opposite tendencies.

Soap + Oil + Water

When oil and water are both present and mixed with soap, the hydrophobic portion of the soap molecules mixes with the oil, while the hydrophilic portion of the soap molecule mixes with the water. The attraction of the polar end of the soap molecules is strong enough to keep pulling the molecules into the water, so the oil molecules associated with the nonpolar ends are pulled into the water along with the soap molecules. The final arrangement of the soap-oil combination is in the form of micelles, little balls with oil in their middle, coated with soap. The micelles are what actually disperse in the water.

What Keeps the Process from Reversing

In addition, soaps are salts. The soap molecule's polar end is composed of a negatively charged ion and a positively charged ion. In water, the positively charged ion is very mobile and separates easily from association with any particular soap molecule. This means that the coat of soap on the surface of a micelle is negatively charged, and it gives the entire micelle a negative electric charge. Because like charges repel each other, the micelles tend not to join together, so the soap-oil mixture stays dispersed in the water.