SCI (Sodium Cocoyl Isethionate)

Overview

Sodium Cocoyl Isethionate — SCI for short — is a mild anionic surfactant made by reacting coconut fatty acids with isethionic acid. It comes as a soft, white-cream powder, prill, or noodle and is the single most popular surfactant for premium syndet bars (synthetic detergent bars) and sulfate-free shampoo bars.

The reason everyone loves SCI is the foam quality. Where SCS gives you fluffy, bubbly lather, SCI gives you a dense, creamy, almost lotion-like cushion — the kind of foam you find in a luxury cleanser. It cleanses without that squeaky, stripped feeling, which is why it is the default choice for baby washes, facial cleansers, and shampoos for sensitive scalps.

It is more expensive than SCS — typically $12-15 per kilo — but a little goes a long way and the upgrade in feel is immediately obvious. Most commercial ‘sulfate-free’ bars on the shelf are SCI-based, sometimes blended with a little SCS or SLSA for extra foam.

What it does in a formula

Primary role: mild anionic cleanser. SCI cleanses thoroughly but does not strip the skin’s natural lipids the way harsher surfactants do. It is well-tolerated even at high percentages, which is unusual for an anionic.

Secondary roles: foam builder (creamy not fluffy), texture giver in solid bars (it has a wonderful smooth, glide-y feel), and a buffer against the harshness of other surfactants when blended. In a bar formula, SCI is often the structural backbone that holds everything else together.

How to use

SCI comes as a solid, usually around 80-85% active (most cosmetic-grade powders sit at ~80-84% SCI salt content). The rest is stearic acid plus residual sodium isethionate from manufacturing. Some suppliers sell a 95%-active stearic-acid-free version — check your spec sheet, because that 10-15% difference matters for active surfactant matter (ASM) math. Two pH grades are common in DIY supply: a pH 4-6 grade and a pH 5-7 grade — the lower-pH grade is the easier one to work with in liquid acidic cleansers.

It is famously stubborn to dissolve. Heat the water phase to 75-80 C and add the SCI slowly while stirring. Many formulators add a drop of glycerin or a small amount of a co-surfactant to help it melt down faster. For solid bars, the technique is different: SCI is melted dry in a double boiler with stearic acid, cetyl alcohol, and a little water until it forms a smooth dough, then poured or pressed into molds.

For solid syndet bars: 30-60% SCI as the primary surfactant. For liquid sulfate-free shampoos: 5-15% of the total formula, which translates to roughly 4-12.5% ASM at the 80%-active grade. CIR has approved use up to 50% in rinse-off products and 17% in leave-on, so there is plenty of safety headroom.

Wear a mask when handling the powder.

Best for / Worst for

Best for: syndet bars, baby shampoos, sulfate-free shampoos, facial cleansers, shampoos for color-treated or curly hair, sensitive-scalp cleansers. Anywhere foam quality and mildness matter more than foam volume.

Worst for: deep-cleansing clarifying shampoos (not strong enough), bubble baths (foam is too dense, not bubbly enough), formulas where cost is the priority over feel.

Common pitfalls

The biggest pitfall is the dissolving time. SCI is famous for being slow — the powder will sit on top of hot water staring at you for a long time before it breaks down. Patience and heat. Some people add the SCI before the water heats up and let it soften gradually as the temperature climbs.

Second pitfall is the dust. SCI dust is fine, easily airborne, and irritating. Mask up.

Third: confusing the active percentage. 85%-active SCI and 95%-active SCI behave differently in a formula — the 85% version contains stearic acid that adds slip and structure, the 95% version is cleaner but harder to bar-form. Know what you bought.

Fourth: pH drift and hydrolysis. SCI is happiest in liquid formulas above pH 6 — below that, it slowly hydrolyzes and the formula can destabilize over weeks. The “pH 4-6” cosmetic-grade is engineered to be more acid-tolerant, but for any regular SCI, keep liquid cleansers at pH 6-8 and adjust at the very end with citric or lactic acid only if needed. In solid bars this is much less of an issue.

Substitutes

• SCS (Sodium Coco-Sulfate) — much cheaper, stronger cleansing, fluffier foam, more drying. Swap this in only if cost is the priority and you can compensate with more conditioning agents.
• SLSA (Sodium Lauryl Sulfoacetate) — similar mildness, much fluffier foam, slightly easier to dissolve. Use for bubble bars or kids’ products.
• Sodium Cocoyl Glutamate / Sodium Lauroyl Glutamate — even gentler, amino-acid based, lower foam, more expensive ($18-22/kg). Best swap for very sensitive skin.