How shampoo works

Key takeaways

• Shampoo is mostly water plus a blend of surfactants. A surfactant molecule has a water-loving head and an oil-loving tail. The tails grab sebum and oily soil, and the heads let water carry it away.
• Almost every shampoo pairs a "primary" cleanser with one or more "secondary" surfactants. The primary (usually an anionic such as sodium laureth sulfate) does most of the cleaning at up to about 10% of the formula. The secondary (usually an amphoteric such as cocamidopropyl betaine, at about 1-5%) boosts foam and thickness and makes the blend gentler (Thompson 2023).
• Mixing surfactants makes them milder. Adding an amphoteric or non-ionic surfactant to an anionic one measurably reduces how much of the anionic gets into skin and how irritating it is (Moore 2003; Morris 2021; Lee 1994). "Contains sulfates" says little about mildness on its own.
• Lather does not equal cleaning. A review of shampoo physics states that foaming does not affect the surfactant's cleaning role (Thompson 2023). Sebum suppresses foam, which is why a second lather foams more than the first (D'Souza 2015).
• One wash removes essentially all surface oil from hair. What differs between surfactants is how much internal hair lipid they also pull out (Shaw 1979).
• "Mildness" is measured mostly on skin and in the lab, not on hair, and the tests do not always agree with each other or with real life (Goffin 1995; Chen 2026).
• A common claim is wrong: non-ionic surfactants are not the harshest oil strippers. They are generally the mildest and the weakest cleansers.

How surfactants lift soil

A surfactant ("surface-active agent") has a hydrocarbon tail that avoids water and a polar or charged head that likes it. In water, the molecules first line up at surfaces and lower surface tension. Above a threshold concentration called the critical micelle concentration they also cluster into micelles, tiny spheres or worm-like shapes with the tails inside and the heads outside.

A review from the University of Sheffield with Unilever co-authors describes three overlapping ways surfactants lift soil off hair: roll-up (oil droplets detach from the hair), emulsification (oil breaks into fine droplets) and solubilization (oil dissolves into the cores of micelles). Below the micelle threshold the first two dominate, and above it solubilization takes over (Thompson 2023).

The four surfactant classes

Class	Typical role in shampoo	Examples	Cleaning power	Irritation potential
Anionic (negative head)	Primary cleanser and foamer	Sodium lauryl sulfate, sodium laureth sulfate, ammonium lauryl sulfate, ammonium laureth sulfate, sodium C14-16 olefin sulfonate, sodium cocoyl isethionate, sodium lauroyl methyl isethionate, disodium laureth sulfosuccinate, sodium cocoyl glutamate, sodium lauroyl sarcosinate, sodium cocoyl glycinate	Highest	Highest for small-headed sulfates such as SLS; much lower for isethionates and amino-acid types
Amphoteric (both charges)	Secondary surfactant: foam, thickening, mildness	Cocamidopropyl betaine, sodium cocoamphoacetate, sodium lauroamphoacetate	Moderate	Low; they reduce the irritation of anionics in mixtures
Non-ionic (no charge)	Foam booster, thickener, mild primary in gentle products	Decyl glucoside, coco-glucoside, lauryl glucoside, cocamide MEA and DEA	Lowest	Lowest, though glucosides and cocamide DEA are contact allergens
Cationic (positive head)	Conditioners and co-washes, not cleansing	Behentrimonium chloride, cetrimonium chloride	Poor	Disinfectant cationics such as benzalkonium chloride are known irritants (Presley 2021)

Two classic over-simplifications are worth correcting.

• "Non-ionics strip the most." In a 1979 Unilever study of hair lipid extraction, anionic surfactants were significantly more efficient than the non-ionic Tween 20 (Shaw 1979). Reviews consistently call non-ionics the mildest and least effective cleansers (Thompson 2023; D'Souza 2015).
• "Cationics are the most irritating." The irritant cationics are disinfectants. The cationic conditioning agents in rinse-off conditioners are used differently and at different levels, so the comparison is not like for like.

Primary and secondary surfactants

A typical shampoo has 10-30 ingredients and a blend of two to four surfactants. The first surfactant listed is usually the primary cleanser. According to Thompson 2023:

• The primary is usually anionic, at up to about 10% of the formula, because a negatively charged head gives the strongest cleaning. Sulfates stay dominant because nothing else delivers the same performance as cheaply.
• The co-surfactant is usually an amphoteric such as CAPB, at about 1-5%. It forms mixed micelles with the anionic, lowers their charge density (reducing irritation), helps thicken the product and boosts foam.
• Adding ethylene-oxide units (turning lauryl sulfate into laureth sulfate) makes the anionic milder but harder to thicken.

Why blends are milder

Several mechanisms seem to contribute, and the theory has shifted over two decades.

• Micelle size. Pure SDS (the lab name for SLS) micelles are small enough to fit the pores of the skin's outer layer, and mixing in a non-ionic made bigger micelles and reduced SDS penetration in vitro (Moore 2003). Sodium cocoyl isethionate forms micelles too large for those pores, which the authors proposed explains its mildness (Ghosh 2007).
• Free surfactant and barrier damage. A study of 16 surfactant systems on human cadaver skin found penetration tracked the critical micelle concentration and zeta potential (R2 = 0.76) but not micelle diameter (R2 = 0.12) (Morris 2019). In SDS plus lauramidopropyl betaine mixtures, SDS penetration fell steadily as the betaine fraction rose (R2 = 0.95 against predicted free surfactant) (Morris 2021).
• Real-world mismatch. A 2026 study found cell assays ranked one betaine blend as milder, while 24-hour human patch tests showed the opposite (Chen 2026). Lab rankings are a guide, not a guarantee.

In human patch tests on 15 volunteers, sodium lauroyl glutamate was much less irritating than SLS, and mixing it into SLS reduced SLS irritation (Lee 1994). In 105 patients patch-tested with eight cleansing surfactants, none caused irritation at the tested dilution, and the best tolerated included sodium cocoyl glutamate (Corazza 2010).

How mildness is measured

Test	What it measures	Notes
Zein test	How much corn protein a surfactant solution dissolves; more predicts more irritation	Cheap screen. Magnesium salts artificially lower scores (Goffin 1995). Small, fast micelles gave higher scores and lower skin-model viability (Lechuga 2026).
Corneosurfametry	Staining of skin-surface strips after brief contact	Agreed with established tests and was not distorted by magnesium (Goffin 1995).
Patch tests and water loss	Redness and water loss after covered application	The human reference standard, but covering the skin exaggerates exposure compared with a one-minute shampoo (Chen 2026).
Reconstructed skin and cell assays	Cell survival, inflammatory signals	Can disagree with human results (Chen 2026).
Hair protein loss	Protein washed out of hair	Damage correlates with surfactant charge density (Pires-Oliveira 2014).

Foam and lather

Foam forms when surfactant stabilizes air bubbles and collapses as liquid drains and bubble walls rupture. Foaming capability does not affect cleaning, but consumers read more lather as better performance, so foam boosters exist to satisfy that expectation (Thompson 2023; D'Souza 2015). A low-foam sulfate-free shampoo can clean adequately, and a shampoo that barely lathers on very oily hair may simply be overwhelmed by sebum.

What surfactants do to hair itself

• Surface oils come off in one wash. Differences between surfactants reflected how much internal lipid they also eluted, roughly half of the ether-extractable lipid in total (Shaw 1979).
• Hair soaked in surfactant solutions at 38 C loses proteins, cuticle fragments and pigment granules, in an amount that scales with the surfactant's charge density (in vitro) (Pires-Oliveira 2014).
• Lipid loss can be reduced by formulation. Industry work shows polymer treatments can partly prevent surfactant-driven lipid loss in vitro (Song 2019). In a Procter & Gamble-funded study, four weeks of daily washing with a mild formula did not reduce internal hair lipids (Punyani 2021).
• The hair surface's water-repellent fatty acid layer (18-MEA) is gradually lost through combined daily stress. Repeated cycles of shampooing, blow-drying and sunlight caused marked loss in a Henkel study, though it did not isolate shampoo alone (Wiesche 2011).

Conditioning inside the shampoo

Modern shampoos also deposit things. Cationic polymers form coacervates with the anionic surfactant as the shampoo is diluted during rinsing, and these carry silicones or other conditioners onto hair. This is how "2-in-1" shampoos work (Thompson 2023). Procter & Gamble has also shown, in the lab, that a gel network of fatty alcohols in a shampoo can deliver them into hair (Marsh 2017).

What it means by hair profile

Profile	What matters most in the blend
Oily scalp, fine straight hair	Enough cleaning power to remove sebum. A sulfate or olefin sulfonate primary is reasonable; skip heavy conditioning agents.
Dry, curly or coily hair (3A-4C)	Gentler primaries (isethionates, amino-acid surfactants, glucosides) or lower surfactant levels. Lather is not a quality signal.
Sensitive or itchy scalp	Avoid high-SLS formulas; if itch persists, check for contact allergens. See itchy scalp.
Colour-treated, bleached, high porosity	Damaged hair loses more protein, so milder blends are logical, though hair-specific trials are scarce.
Heavy product, oil or silicone users	A stronger anionic cleanser now and then removes buildup that mild blends leave behind.

Myths and facts

Claim	Verdict
More lather means cleaner hair.	False. Foam does not affect cleaning.
Sulfate-free means gentle.	Oversimplified. Sulfates mixed with amphoterics are much milder than SLS alone, and some sulfate-free primaries (olefin sulfonates) are strong cleansers.
Non-ionic surfactants strip the most oil.	False. They are the weakest cleansers.
Natural surfactants clean better.	Not supported. Saponin-based natural cleansers lather but clean poorly, according to a dermatology review (D'Souza 2015).
Surfactants strip all your hair's lipids every wash.	Partly. Surface sebum yes; internal lipids only partly.

Bottom line

Read the first surfactant on the ingredient list: that is the main cleanser, and the next one or two are the mildness and foam helpers. Match cleaning strength to your scalp, not your ends. If a gentle shampoo does not feel like it is cleaning, two short washes work better than one long one. If your scalp reacts to many shampoos, a contact allergen may be the cause rather than "harshness", and patch testing can find it.