It's a basic rule in a general or organic chemistry class. What we mean is that a solute (the substance you dissolve) only dissolves in a solvent with similar chemical properties. We're talking about polarity.
Atoms have electrons. These electrons are involved in bonding, which creates molecules - groups of atoms. The chemical polarity of a molecule depends on those electrons, which are negatively charged. When atoms bond, they don't always share their pair of electrons evenly, which distributes the charge unevenly along the molecule, creating an electric dipole.
|δ+ and δ- mean partial positive and partial negative, respectively. δ is a lower case "delta" (Greek alphabet).|
It's like, say, two teams at tug-of-war (a loose analogy, but good enough). One team is stronger and pulls more of the rope. The other team has less of it. Just like one atom has a greater electron affinity and pulls more of the electron charge toward it, becoming more negative, and making the other atom more positive.
Why does this matter? Well, molecules with dipoles are polar. Molecules without them, or with dipoles that are arranged symmetrically (effectively "canceling" them all out) are non-polar. Polar dissolves polar, and non-polar dissolves non-polar.
|Two layers in salad dressing: oil, and water.|
|Example of triglycerides; these are non-polar and are a major component of skin oils.|
So, let's think about your face, which is, indeed, covered in sebum - a kind of oil secreted by glands in the skin. Throughout the day, dirt and bits can get into your pores and on your skin. The idea is that cleansing oils can break down all of this oil buildup and grime and makeup better than water can, because oils are non-polar and water is polar, and like dissolves like.
Stay tuned for #6: Soap!
For previous Beauty of Science installments, check these posts:
BoS #1: A Primer and Some Food for Thought
BoS #2: Antioxidants and Free Radicals
BoS #3: UV Radiation, Visible Light, Color
BoS #4: Sunscreens
*I have to make it absolutely clear that I am only a chemist, not a health practitioner. I can't recommend any courses of action or do any consulting, and everything I'm sharing is information that is widely available, simply compiled in one post.
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