Formulation base
Sodium Cocoyl Isethionate
INCI: Sodium Cocoyl Isethionate · also SCI, coco isethionate, the syndet surfactant
A coconut-derived cleansing agent, the one that makes syndet bars behave like soap without behaving like soap. It builds a dense, creamy foam at a pH close to skin's own.
Why it is here
Cleans without the stripped feeling that starts the whole apology cycle. Its micelles are too big to get into the skin and take lipids out with them, which is a better mildness argument than a leaf on the label.
Surfactants clean by forming micelles that carry oil and dirt into the rinse water. The trouble starts when those micelles are small enough to slip into the water-filled pores of the stratum corneum and take skin proteins and lipids with them. SCI micelles are measurably larger than those pores, which is the current best explanation for why it reads as mild.
The receipt
SCI micelles measured 33.5 plus or minus 1 angstrom, larger than the 29 plus or minus 5 angstrom average radius of stratum corneum aqueous pores, so they were sterically hindered from penetrating; SCI skin penetration was dose independent.
Why is sodium cocoyl isethionate (SCI) mild to the skin barrier? An in vitro investigation based on the relative sizes of the SCI micelles and the skin aqueous pores. Journal of Cosmetic Science, 2007. in vitro. PMID 17598025
In practice
- Typical use
- 5 to 30 percent in syndet bars; roughly 3 to 15 percent active in liquid and cream cleansers. There is no single studied level, because it is a base, not an active.
- Pairs with
- Glycerin, Jojoba Beads
- Keep apart from
- High-pH soap bases: the mildness argument for SCI depends on staying near skin pH, and pH 10 alone increases stratum corneum swelling even without surfactant (PMID 14728695).
- Cautions
- None notable for most skin. It is a cleansing surfactant, so contact time is short by design. As with any wash-off product, leaving it on longer does not make it work better.
Questions people ask
Is sodium cocoyl isethionate actually gentler than sulfates?
The in vitro work says yes, and gives a physical reason rather than a marketing one. SCI micelles are about 33.5 angstroms across, wider than the roughly 29 angstrom pores in the stratum corneum, so they cannot get in and disturb skin proteins and lipids the way smaller SDS micelles can. Mild is a measurement here, not a mood.
Is it a sulfate?
No. It is an isethionate, made from coconut fatty acids and isethionic acid. The confusion is understandable, because it appears in products marketed as sulfate free and it foams the way people expect a sulfate to foam. Chemically it is a different head group, and the head group is exactly what determines how a surfactant interacts with skin protein.
Why does a cleanser with SCI still feel foamy when it is supposed to be mild?
Foam and harshness are not the same variable. Lather comes from how a surfactant behaves at the air and water interface. Irritation comes from how it behaves at the skin and water interface. SCI happens to be good at the first and unremarkable at the second, which is the whole reason formulators reach for it.