What Is Shampoo, What Does It Do? Formula and How It Works
Other names: hair shampoo
Shampoo is a surfactant-based cleanser that removes oil, sweat and product residue from the scalp and hair. One end of these substances attaches to oil, the other to water; dirt is enveloped in micelles and washed away by rinsing. Foam is not a measure of cleansing; it only helps the product spread over the hair and scalp.
See products for youAt a glance
- Type
- Surfactant-based hair and scalp cleanser
- Typical content
- 2-4 surfactants after water, care polymer, fragrance, preservative
- What it does
- Makes oil, sweat and product residue rinsable
- Concept often confused
- The amount of foam does not show cleansing power
- Caution
- If itching or a rash persists, see a dermatologist rather than changing the product
What's on this page?
The difficulty of cleansing hair lies not in the amount but in the surface area. The total surface of an average woman's hair is calculated at 4-8 square metres. Because the oil on this surface does not dissolve in water, water alone cannot carry it away. This is the reason shampoo exists: it contains substances that can slip between water and oil and carry both in the same flow.
How do surfactants remove dirt?
A surfactant molecule has two different ends. One end loves water, the other loves oil. Above a certain concentration in water (the critical micelle concentration) they form small clusters called micelles, with the oil-loving ends pointing inward and the water-loving ends pointing outward.
- Wetting: The surfactant allows water to spread more easily over the hair and scalp.
- Lifting: The oil-loving ends attach to sebum, dirt mixed with sweat, and product residue.
- Carrying: Oil is drawn into the oil-loving core of the micelle.
- Rinsing: Because the outer surface loves water, the micelle is washed away with the water.
That is why shampoo's real target is the scalp. Sebum is produced there, and as the foam is rinsed it passes down the lengths and cleans them. You can find the washing steps in the how to wash hair article.
What does foam show, and what does it not show?
It is a common belief that a shampoo that foams a lot cleans better, but foam only helps the product spread. Cleansing is done by micelles and surfactants. Two observations explain this:
- Oil suppresses foam. When there is a lot of sebum on the hair, foam decreases. This is also why foam increases on a second wash: the first wash has already removed part of the oil.
- A shampoo with little foam also cleans. Low-foaming formulas are known to clean adequately. Foam boosters are added to give the formula a greater sense of spreading and use.
Which surfactants are in the formula?
Modern shampoos usually combine 2-4 surfactants. The surfactant listed first on the label is the primary cleanser; the second makes up for its shortcomings.
| Class | Charge | Example on the label | Role in the shampoo |
|---|---|---|---|
| Anionic | Negative | Sodium Laureth Sulfate, Sodium C14-16 Olefin Sulfonate | Main cleansing and foam. Removes oil well, may leave the strand rough |
| Amphoteric | Depends on pH | Cocamidopropyl Betaine | Auxiliary cleanser; makes the formula gentler and supports foam. Common in baby shampoos |
| Nonionic | Uncharged | Decyl Glucoside, Cocamide DEA | The gentlest group; auxiliary cleanser and support for consistency |
| Cationic | Positive | Cetrimonium Chloride | More conditioning than cleansing; reduces static |
The first item on the label shows the character of the formula, but does not by itself say how much it will dry out the hair. The proportions and how the substances are used together also matter. This is also why the sulfate debate is a separate topic from this page.
Shampoo formula types
| Type | Formula logic | Intended for |
|---|---|---|
| Normal and oily hair | Strong anionic cleanser, little conditioning agent | Quickly greasy scalp |
| Dry or processed hair | Gentle cleanser and conditioning agents such as silicones | Chemically processed and dry hair |
| Both together (2 with 1) | Silicone droplets and cationic polymers suspended in the liquid | People who want cleansing and conditioning in one product |
| Clarifying shampoo | Stronger cleanser, little conditioning agent | Product build-up, infrequent use |
| Baby shampoo | Mostly amphoteric, less stinging to the eyes | Babies |
| Active-ingredient shampoo | Actives such as Piroctone Olamine, salicylic acid | Dandruff, seborrheic dermatitis (if needed, on a doctor's recommendation) |
| Dry shampoo | No water or micelles; absorbs oil like powder | Temporary between two washes |
Conditioning shampoos work in an interesting way. Cationic polymers and silicones stay suspended in the shampoo, but during rinsing, as the mixture dilutes, they deposit onto the hair. A significant portion of the conditioning agent that remains on the hair is released at the moment of this dilution. Which points to check when choosing a shampoo are covered in the how to choose a shampoo article.
Why is shampoo's pH discussed?
Because the isoelectric point of hair strands is low, as pH rises the negative charge of the surface increases; the strand rubs and tangles more. For this reason dermatology reviews recommend that shampoo pH not exceed about 5,5, the pH of the scalp. Labels often do not state pH; substances such as Citric Acid adjust the acidity of the formula. Even so, you cannot infer how much a shampoo will dry out from a single pH value; the whole formula also matters.
This page is for general information only and does not replace medical diagnosis or treatment. If you have an ongoing skin or scalp problem, consult a dermatologist.
