Module 4.2 - pH-reactive cosmetics

Some dyes are colorless, then they’ll turn into a color upon contact with your skin. Why?

cuz the dyes are acid-base indicators (take on different colors at different pH levels).

i.e., it’s reacting with the pH of your skin.

pH causes color changes, but so does moisture. why?

cuz these are structural changes that affect how LIGHT is absorbed —> thus how color is perceived!

The pH scale is 1 to 14. But this only is accurate at what temperature?

At 25C

Pure water at 25C has a pH value of ____.

It’s where [H+] = [OH-], i.e.,

Pure water at 25C has a pH value of 7.

It’s where [H+] = [OH-], i.e., hydronium concentration = hydroxide concentration

*hydrogen interchangeable with hydronium

pH stands for…

pH = potential of hydrogen

pH is on a logarithmic scale. What does this mean?

that a change of 1 pH unit is actually a 10-fold change in hydrogen concentration.

that’s why pH is SOOO critical in our products!

Concentration is in units of…

mol/L aka molarity M!

Recall 1 mole = Avagardo’s # =

6.022E23

Why is hydronium interchangeable with hydrogen?

cuz realistically a bare hydrogen ion cannot exist on its own in water. it automatically binds with water nearby.

H + H2O —> H3O+

What is the formula for calculating pH from H+ concentration and vice versa?

don’t forget the negative sign!

pH ___________ are found in cosmetics to design and maintain the makeup’s functional pH.

pH buffers or adjusters are found in cosmetics to design and maintain the makeup’s functional pH.

Rem. log is base 10. So tip: if you see [H+] = 10^-3 aka 0.001 mol/L

How can you calculate pH quickly?

Cancel out the base and argument!

Let’s define acids and bases:

• acids: generate hydronium ions when dissolved in water; i.e. donates H+ to other species to generate hydronium

• bases: generate hydroxide ions when dissolved in water; i.e. accepts H+ to generate OH-

pH adjusters found in cosmetics are typically made up of citric acid, lactic acid, and _______________.

pH adjusters found in cosmetics are typically made up of citric acid, lactic acid, and triethanolamine.

mnemonic: CLT sandwich (chicken lettuce tomato)

How to make your perfect match? GET A COLOR-CHANGING PRODUCT!

How does it work?

1. Starts colorless in the tube. cuz the dye is in its dry (anhydrous) form (protons are removed).

2. Apply to skin.

3. …

4. …

1. Starts colorless in the tube. cuz the dye is in its dry (anhydrous) form (protons are removed).

2. Apply to skin.

3. Moisture and natural acidity returns protons.

4. This makes the molecule protonated and thus colored.

Slight pH differences in people’s skin case subtle shade variations, giving the illusion of bespoke color.

But what does this also mean?

a cosmetic on your friend may look different on YOU, my dear!

Example of a colorless anhydrous pigment in the tube: _____ 27

Red 27!

colorless when dry, turns pink (usually; cuz most ppl’s skin falls within a similar pH range) when protonated

Shade intensity can vary depending on the person’s skin’s:

• moisture

• pH

• skin tones (underpainting)

• part of face (e.g., lips vs cheeks have different local pH)