The Science
of Cosmetics
Meet the layers of your skin, find out why skin is slightly acidic and what soap does to it, see how emulsifiers hold oil and water together, learn how surfactants clean, and discover how sunscreens block UV light.
- 00First Principlesp.2
- 01Skinp.4
- 02pHp.5
- 03Emulsionsp.6
- 04Cleansingp.7
- 05Sun & colourp.8
- ✦Self-check · Make it · 5 Questions · Side Questp.9
You'll need: jars with lids, water, cooking oil, mustard, dish soap, a red cabbage leaf, a timer, a magnifier and an adult partner.
Principles
A first principle is a basic truth you can build on. The science of cosmetics rests on four: everything is made of chemicals, skin is a barrier, oil and water need a go-between and dose and skin type matter.
What do I already know?
What is necessary?
Everything in this module is built from four bricks. Rate yourself on each one, then check the example to see if your rating holds up.
What is the objective?
By the end, you’ll be able to describe the layers of the skin and its barrier, explain the skin’s acidity and why harsh cleansing can upset it, explain how emulsifiers make creams and lotions, describe how surfactants lift dirt and oil, and compare mineral and chemical sunscreen filters.
- 01Name 3 skin layers.
- 02Give skin’s pH range.
- 03Make a cabbage indicator.
- 04Explain an emulsifier.
- 05Draw a micelle.
- 06Compare UVA and UVB.
The list on the left is ours. Now make it yours. What do you want out of this?
How much do you know about the science of cosmetics right now? Shade the boxes in pencil. You'll come back at the end (p.9) and shade it again in pen.
Why start this way? When you know what you already have (1), what you need (2) and where you're going (3), you learn faster and you notice the gaps. Every module on the Cosmetics Path starts with these same three questions.
Skin
Skin is a barrier of cells and lipids.
Your largest organ
- The epidermis is the thin outer layer. Its top, the stratum corneum, is made of flat dead cells.
- The dermis holds collagen, blood vessels, sweat glands and oil glands.
- The hypodermis (subcutaneous layer) is mostly fat that cushions and insulates.
Bricks and mortar
Scientists picture the outer skin as a wall: dead cells are the bricks and fatty lipids are the mortar. The wall keeps water in and keeps many germs and irritants out.
Oil glands add sebum, which helps keep skin soft.
Seal and soak
Most moisturizers combine three kinds of ingredients: humectants like glycerin pull water in, emollients like plant oils smooth the surface, and occlusives like petrolatum or beeswax form a thin layer that slows water loss.
- Which part of the skin barrier surprised you most?
- Why might skin get drier in an Alberta winter?
Skin up close
With a magnifier, look at the skin on your fingertip, palm and the back of your hand. Draw what you see. Which area has the deepest lines? Why might that be?What is the outer layer of skin called?
pH and skin
Skin is slightly acidic; soap is basic.
The acid mantle
Healthy skin’s surface is slightly acidic, usually about pH 4.5 to 5.5. This mild acidity helps the barrier repair itself and helps friendly microbes live there.
Pure water is pH 7 (Lens 1).
Basic vs balanced
True soap is made by reacting fats with a strong base (lye), so bar soaps are usually basic, around pH 9 to 10. Many face and body washes are “syndets” (synthetic detergents) made closer to skin’s pH.
Making soap from lye is for trained adults only: lye burns.
A natural indicator
Red cabbage contains pigments called anthocyanins that change colour with pH: red or pink in acids, purple near neutral, and blue-green in bases. Scientists use indicators like this to test pH without special equipment.
- Why might a soap that’s great for dishes be too harsh for faces?
- What else at home could you test with cabbage juice?
Cabbage test
With an adult, blend a red cabbage leaf with tap water and strain it (no heat needed). Add a spoon of the juice to small cups of vinegar, water, baking soda solution, shampoo and hand soap. Record each colour.Is healthy skin’s surface acidic or basic?
Emulsions
Emulsifiers keep tiny droplets from joining up.
They won’t mix
Water molecules stick tightly to each other and push oil away, so oil floats in a layer. Shaking breaks the oil into droplets, but they soon join up again.
An emulsion is one liquid spread through another as tiny droplets.
The go-betweens
An emulsifier molecule has a water-loving head and an oil-loving tail. It sits around each droplet, so droplets can’t join.
- Oil-in-water: oil droplets in water. Most lotions; feels light.
- Water-in-oil: water droplets in oil. Thick barrier creams; feels richer.
Mayonnaise is an emulsion
Mayonnaise is oil droplets in vinegar and lemon juice, held together by lecithin and proteins in egg yolk. Milk is a natural emulsion of fat droplets in water. Cosmetic chemists use the same science, with emulsifiers chosen to be gentle on skin.
- Which foods and products at home are emulsions?
- Why do you think lotions are usually white?
Shake test
Put equal amounts of water and oil in 2 jars. Add half a teaspoon of mustard to one. Close the lids tightly and shake both for 30 seconds. Time how long each takes to separate.What does an emulsifier do?
Cleansing
Surfactants trap oil in micelles that rinse away.
Surface-active
Surfactants are molecules with a water-loving end and an oil-loving end (like emulsifiers). In water, they surround oil and dirt in tiny balls called micelles, which rinse away.
Shampoo, body wash and “micellar water” all use them.
Over-cleansing
Surfactants can’t tell skin’s own lipids from dirt. Washing very often with strong cleansers, or with very hot water, can strip the “mortar” from the skin barrier and leave skin dry and itchy.
Gentle and lukewarm is usually enough.
A myth
Many people think more bubbles means more cleaning. Foam mostly shows that air is trapped in the liquid; some gentle cleansers barely foam at all but still lift oil. Companies often add foaming ingredients because people expect bubbles.
- Do you think more foam means cleaner? Why?
- How often do you think people really need to wash their hair?
Pepper scatter
Fill a plate with water and sprinkle ground pepper on top. Touch the centre with a dry fingertip, then with a fingertip dipped in dish soap. What happens? Why might soap change the water’s surface?What is a micelle?
Sun & colour
Sunscreens absorb or scatter UV before it reaches skin.
Two kinds that reach us
- UVB mostly burns the outer skin. SPF measures UVB protection (Lens 1).
- UVA goes deeper and adds to skin ageing.
- Both can damage skin cells and raise skin cancer risk.
“Broad spectrum” on a label means it protects against UVA as well.
Two families
- Mineral filters (zinc oxide, titanium dioxide) mostly absorb UV, and scatter some.
- Organic (chemical) filters, like avobenzone, absorb UV and release it as a little heat.
In Canada, sunscreens are drugs or natural health products, not cosmetics (Lens 6).
Pigments
Makeup colours come from pigments: iron oxides make reds, yellows, browns and blacks; titanium dioxide makes white; mica adds shimmer. Eye and lip products need extra care, because they can get into eyes and mouths.
- Why might people skip sunscreen, even when they know the risks?
- Would you choose a mineral or chemical sunscreen, and why?
UV bead test
With an adult, put UV-sensitive beads (from a craft or science shop) under clear plastic wrap in sunlight. Smear sunscreen on part of the wrap. Which beads change colour? Never look at the sun.Which kind of UV mostly causes sunburn?
Quick self-check
Six questions, covering every stop. Show your thinking. Answers are upside down at the bottom; no peeking till you're done.
1 · Name the three layers of skin.
2 · About what pH is healthy skin’s surface?
3 · Why is bar soap basic?
4 · What does an emulsifier do?
5 · What is a micelle?
6 · Name a mineral sunscreen filter.
Re-shade your knowledge meter
How much do you know about the science of cosmetics now? Flip back to p.3 and shade the meter again in pen (or shade this one). Did any of the four bricks on p.2 move from "Not yet" to "Got it"?
The emulsion test
Which go-between holds oil and water longest?
1 · Plan
Fill 4 identical jars with 50 mL of water and 50 mL of cooking oil. Add nothing to jar 1, half a teaspoon of mustard to jar 2, a few drops of dish soap to jar 3 and a pinch of salt to jar 4. Close tightly, shake each for 30 seconds and time how long each takes to separate.
- Prediction first
- Same amounts
- Same shaking
- Only one change per jar
2 · Results
| Jar | Added | Separates (min) |
|---|---|---|
| 1 | nothing | |
| 2 | mustard | |
| 3 | dish soap | |
| 4 | salt |
3 · Must-haves
- A fair test
- A bar graph
- Best emulsifier
- Which was a control?
4 · What I found
This is a kitchen science test, not a cosmetic: don’t put the mixtures on your skin or eat them. Keep dish soap away from eyes, wipe up oil spills so no one slips, and pour the oil into the garbage, not down the drain.
5 Questions
I Still Have
This module didn't answer everything. Good. Write 5 new questions it didn't answer, one of each type below. There are no wrong questions, only ones nobody's asked yet.
★ Star the one question you'd most like answered. You'll need it on the next page.
1What I'll find out
Break your big question into 2 or 3 smaller ones you could actually answer.
2Where I'll look
3How I'll share it
Who I'll share it with:
Done by: