Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
8. Chemistry in the World
Why is lotion so oily?
Why is oil so oily?
Oil lubricates things. A lubricant is a film of liquid that prevents parts from touching, so they slide easily past one another.
When you get a lubricant on your fingers, one finger can’t feel the other—they only feel the lubricant. They slide past one another, and we say they feel oily, since most oils make good lubricants.
Lubricants generally work because their molecules bind more tightly to the parts that would otherwise rub together than they do to one another. The oil added to machines sticks to the metal parts, so it stays on the metal as a coating. But the coatings themselves do not stick to one another and instead just glide by.
Oil on your fingers works the same way to make your fingers slip past one another and thus feel slippery. Soap is also a good lubricant for fingers because one end binds to the skin and the other end is an oil, which does not bind to the oil ends of other soap molecules.
Many lotions contain oils. This is to prevent moisture loss from the skin, which a coating of oil will do. What people don’t like is a greasy feeling. Grease is a thick oil or fat that is only a lubricant when under pressure. It is thicker and does not allow the skin surfaces to easily slide past one another.
Unsaturated oils can oxidize, which can make them form a plastic film that feels sticky and unclean. Lotions fight this in two ways: by adding ingredients that prevent oxidation, like vitamin E (tocopheryl acetate) and vitamin C (ascorbic acid, or ascorbyl palmitate), or by using saturated oils like mineral oil, which stays liquid and does not oxidize.
Most moisturizing lotions actually don’t add much moisture to the skin. Instead, they use oils and fats to prevent moisture loss. After all, it is easy to moisturize your skin in a shower or a bath. The problem is keeping it moist after you leave the water.