Introduction to Sodium Lauroyl Methyl Isethionate (SLMI)
Sodium lauroyl methyl isethionate (SLMI, CAS 928663-45-0) is a mild anionic surfactant of the acyl isethionate family, made from lauric acid and isethionic acid. It is the premium, “gentle” surfactant behind a large share of modern facial cleansers, body washes, shampoos, baby care products and syndet bars. It combines rich, creamy foam with one of the lowest skin-irritation profiles of any widely used anionic surfactant.
SLMI belongs to the same family as sodium cocoyl isethionate (SCI), the classic mild surfactant of beauty bars, but with a purer lauroyl (C12) chain, whiter colour and lower odour. It is a central ingredient in the “sulfate-free” and “gentle cleansing” positioning that has reshaped the personal-care market.
This article covers the identity, properties, mildness mechanism, production, specification, applications and sourcing of SLMI. PolyBlueChem can supply SLMI (CAS 928663-45-0); see the product page for the current specification.
Chemical Identity and Structure
SLMI is an acyl isethionate: the sodium salt of the ester formed between a fatty acid (lauroyl, C12) and an isethionate head group. The molecule therefore has a hydrophobic lauroyl tail and a hydrophilic isethionate (sulfonate) head, connected through an ester linkage. That ester linkage and the isethionate head are both important: the head gives mildness, and the ester linkage gives biodegradability.
| Property | Value |
|---|---|
| Common name | Sodium lauroyl methyl isethionate (SLMI) |
| Family | Acyl isethionate (anionic surfactant) |
| CAS number | 928663-45-0 |
| Molecular formula | C15H30O5S.Na (sodium salt) |
| Molecular weight | Approx. 344.4 g/mol |
| Appearance | White chips or powder |
| Hydrophobe | Lauroyl (C12) chain from lauric acid |
| Hydrophile | Sodium isethionate (2-hydroxyethanesulfonate-derived) head |
Physical and Chemical Properties
SLMI is supplied as a white solid (chips or powder) with a high active content, and it is dosed on an active-matter basis. The table below gives a representative specification.
| Parameter | Typical specification |
|---|---|
| Appearance | White chips / powder |
| Active matter | 79.0% min (basis MW 344) |
| Free fatty acid | 7.0% max (basis MW 200) |
| pH (10% in demineralised water, 35 deg C) | 5.5 – 7.0 |
| Colour (5% in isopropanol/water) | 25 max |
| Water content | 1.0% max |
| Ionic character | Anionic (sulfonate) |
Two features stand out. First, SLMI is a solid with a high active content, which makes it convenient for syndet bars and for concentrated formulations. Second, its native pH (around 5.5–7.0) sits in the mildly acidic to neutral range that suits skin, so less adjustment is needed than with alkaline soaps.
What Makes SLMI Mild
The mildness of acyl isethionates is not a marketing claim; it follows from their chemistry.
- Weak protein binding. The isethionate head group binds only weakly to skin keratin proteins. Strong surfactant binding is a major cause of irritation, because it denatures and disrupts skin proteins; weak binding means SLMI cleans without stripping the skin’s lipids and proteins.
- A favourable head-group chemistry. The sulfonate head sits between the harsher alkyl sulfates (SLS) and the very mild amino-acid surfactants in irritation potential, but with far better foam than most amino-acid surfactants.
- An ester linkage for biodegradability. The ester bond is cleaved by environmental micro-organisms, so SLMI is readily biodegradable.
- Good performance in hard water. As an isethionate it is tolerant of water hardness and does not form lime-soap scum, so it rinses cleanly on skin.
The practical consequence is a surfactant that lathers richly and cleans effectively, yet can be used in products for daily use, sensitive skin and babies — a balance that is difficult to achieve with the older sulfate surfactants.
The Acyl Isethionate Family and Its Rivals
SLMI competes and cooperates with a range of mild-to-moderate surfactants. The table positions it against the main alternatives.
| Surfactant | Type | Foam | Mildness | Typical use |
|---|---|---|---|---|
| SLMI | Acyl isethionate (C12) | Rich, creamy | Very high | Facial wash, premium cleansers, baby |
| SCI (sodium cocoyl isethionate) | Acyl isethionate (C12–C18) | Creamy | Very high | Beauty bars, syndet, cleansers |
| Sodium lauroyl sarcosinate | Amino-acid (sarcosinate) | Moderate | High | Liquid cleansers, toothpaste |
| SLES (sodium laureth sulfate) | Alkyl ether sulfate | High, bubbly | Moderate | Mass-market shampoos |
| SLS (sodium lauryl sulfate) | Alkyl sulfate | High | Low–moderate | Strong cleansing, toothpaste |
The key market fact is that SLMI delivers the mildness of an isethionate with a richer foam and cleaner colour than the mixed-chain SCI, and much greater mildness than the sulfates — which is why it anchors premium “gentle” and “sulfate-free” lines.
Industrial Production
SLMI is made by transesterification (also called interesterification): a fatty acid ester, typically methyl laurate, is reacted with sodium isethionate (or the corresponding methyl isethionate precursor) at elevated temperature, with the by-product alcohol (methanol) removed to drive the reaction. The route gives a cleaner, whiter product than the older direct acylation of isethionic acid with fatty acid chloride, and it avoids chloride by-products.
| Stage | Purpose | Notes |
|---|---|---|
| 1. Feedstocks | Provide the fatty chain and the isethionate head | High-purity methyl laurate (from coconut/palm kernel fatty acids) and sodium isethionate |
| 2. Transesterification | Form the acyl isethionate | Elevated temperature (roughly 150–200 deg C) under vacuum, with a catalyst; methanol removed to drive equilibrium |
| 3. Finishing | Meet the specification | Neutralisation, bleaching and flaking to white chips; dried to ≤ 1% water |
| 4. Quality control | Confirm performance | Active matter (~79%), free fatty acid (≤ 7%), colour, pH and water |
Process control centres on the reaction conversion (which sets the residual free fatty acid), the colour, and the consistency of the active matter, since formulators dose SLMI on an active basis and need it to be reproducible.
Specifications and Quality Control
The specification controls what matters to a formulator: active content, free fatty acid, colour, pH and water.
| Parameter | Why it matters |
|---|---|
| Active matter (79% min) | Defines the surfactant content and the dosing basis |
| Free fatty acid (7% max) | Unreacted fat; excess can affect foam, feel and stability |
| pH (5.5–7.0) | Compatibility with skin-friendly, mildly acidic formulas |
| Colour (25 max) | Whiteness of the final product, especially syndet bars |
| Water (1% max) | Stability, dosing and shelf life of a solid surfactant |
For customers making clinical mildness claims, additional irritation test data (for example HET-CAM or patch testing) may be requested from the supplier.
Grades and Forms
- Standard lauroyl methyl isethionate chips (79% active). The main grade; white chips for syndet and cleanser manufacture.
- Powder / milled forms. For easier dissolution in liquid processes.
- Solutions and blends. SLMI is supplied in liquid blends for cold-process formulation.
- Cosmetic-grade. The normal commercial grade; confirm the documentation for your market.
Applications Overview
| Application | Role of SLMI |
|---|---|
| Facial cleansers | Primary mild surfactant; non-stripping foaming wash |
| Body washes and shower gels | Creamy foam with low tightness after rinse |
| Shampoos and baby washes | Low eye/skin irritation; gentle daily cleansing |
| Syndet (soap-free) bars | Main surfactant for mild, hard-water-tolerant bars |
| Foaming hand washes and shave foams | Stable, dense lather |
| Premium “sulfate-free” ranges | Anchor surfactant delivering the gentle positioning |
SLMI in Facial Cleansers and Body Wash
In facial cleansers and body washes, SLMI is chosen for the balance of rich foam, thorough cleansing and a soft, non-stripping after-feel. It is usually the primary surfactant, blended with an amphoteric (commonly cocamidopropyl betaine) and a non-ionic (for example an alkyl polyglucoside) to tune foam, mildness and viscosity. Because SLMI is a solid at high active content, it is typically dissolved into the aqueous phase with mild heating or supplied pre-blended for cold processing. The result is a cleanser that removes sebum and sunscreen thoroughly yet leaves skin comfortable — the profile consumers describe as “gentle”.
SLMI in Baby and Sensitive-Skin Products
The combination of weak protein binding, a near-neutral native pH and good hard-water performance makes SLMI a natural fit for baby washes, shampoos and sensitive-skin cleansers. In these products the low irritation profile is the principal selling point, and SLMI is often used at higher levels than it would be in a mass-market product. As always, claims such as “tear-free” or “for sensitive skin” must be supported by appropriate testing and comply with the target market’s cosmetics rules.
SLMI in Bar Soap and Syndet Bars
SLMI is a key surfactant in syndet (synthetic detergent, soap-free) bars, where it replaces part or all of the soap. Syndet bars built on acyl isethionates give a creamy lather, are tolerant of hard water, do not leave the “squeaky” soap residue, and are much milder than traditional soap, which is alkaline and raises skin pH. SLMI’s white colour and low odour are advantages in a bar, where colour and smell matter to the consumer.
Formulation Guidance
- Dose on active matter. Because SLMI is ~79% active, calculate the use level on an active basis (typical use levels are 5–20% in rinse-off cleansers).
- Co-surfactants. Pair with amphoteric (betaines) and non-ionic (alkyl glucosides, alkanolamides) surfactants to balance foam, mildness and viscosity.
- Viscosity building. SLMI can be built into salt/surfactant systems for gel viscosity; it is compatible with the mildly acidic pH (5.5–6.5) preferred for skin care.
- Avoid harsh conditions. The ester linkage is hydrolytically sensitive, so avoid prolonged strong alkali or excessive heat, which can hydrolyse SLMI to free fatty acid.
- Dissolution. Dissolve the chips with gentle heating and agitation, or use a pre-blended liquid grade for cold processing.
- Storage. Keep sealed, cool and dry; SLMI is hygroscopic and should be protected from moisture and heat.
Safety, Health and Regulatory
- Low irritation. SLMI has a very favourable skin and eye irritation profile, supporting its use in baby and sensitive-skin products; final claims must follow regional cosmetics regulations.
- Handling. As a solid surfactant it can generate dust; handle with gloves and appropriate dust protection and follow the SDS.
- Biodegradability. The ester linkage makes SLMI readily biodegradable; check local requirements for surfactant biodegradability.
- Regulatory. SLMI is used as a cosmetic surfactant in major markets; confirm the grade and documentation for your intended use and market.
Market and Supply Overview
- Demand driver. The global shift toward mild, sulfate-free and gentle cleansers, especially in facial and baby care.
- Production base. A limited number of specialty surfactant producers, including well-known names in acyl isethionates.
- Price drivers. The cost of lauric acid/methyl laurate and isethionate feedstock, process energy and the required colour/purity.
- Quality differentiators. Active matter, free fatty acid, colour and odour.
- Packaging. 25 kg multi-layer bags or cartons with a PE liner are typical; keep sealed.
Procurement and Sourcing Guide
- Active matter. Confirm the active content (79% min) and dose on an active basis.
- Free fatty acid and colour. Check both; they affect foam, feel and the whiteness of bars.
- Odour. Low odour matters for fragrance-free and premium products.
- Grade and documentation. Confirm the grade, and require the SDS, per-batch COA and any irritation data for claims.
- Form. Chips, powder or pre-blended liquid, to suit your process.
- Storage. Cool, dry and sealed; confirm shelf life (often around 24 months).
PolyBlueChem can supply sodium lauroyl methyl isethionate (CAS 928663-45-0). To request current specifications, a COA or a quotation, contact the PolyBlueChem team.
How Acyl Isethionates Clean: Surface Activity and Foam
To see why SLMI works, it helps to look at what a surfactant does. Surfactant molecules have a water-loving (hydrophilic) head and an oil-loving (lipophilic) tail. In water they arrange themselves at interfaces: at the surface they lower surface tension and create foam, and on skin or soil they lift oils and hold them in the wash water so they can be rinsed away.
| Function | How SLMI delivers it |
|---|---|
| Wetting | The lauroyl tail lowers surface tension, letting water spread and wet skin |
| Soil removal | The tail solubilises sebum and oils into micelles |
| Foam | SLMI forms dense, creamy lather, which consumers associate with effective cleansing |
| Rinsing | Hard-water tolerance means no lime-soap scum, so it rinses clean |
| Skin feel | Weak keratin binding leaves the skin soft rather than tight |
The way acyl isethionates form foam is also part of their appeal. Their lather is finer and creamier than the big, bubbly foam of the sulfates, which suits premium personal-care products where the feel of the product is part of the experience.
SLMI vs Amino-Acid and Other Mild Surfactants
Formulators choosing a mild surfactant weigh SLMI against the amino-acid surfactants (glutamates, sarcosinates, taurates, alaninates) and against the isethionate SCI. The table summarises the practical comparison.
| Surfactant class | Examples | Foam | Mildness | Skin feel |
|---|---|---|---|---|
| Acyl isethionate | SLMI, SCI | Rich, creamy | Very high | Soft, cushiony |
| Glutamate | Sodium cocoyl glutamate | Fine, creamy | Very high | Soft, silky |
| Sarcosinate | Sodium lauroyl sarcosinate | Moderate | High | Silky |
| Taurate | Sodium methyl cocoyl taurate | Rich | High | Soft |
| Alkyl ether sulfate | SLES | High, bubbly | Moderate | Clean, sometimes tight |
In practice, mild “sulfate-free” systems are often built from a blend: SLMI for creamy foam and cleansing power, with an amino-acid surfactant to fine-tune skin feel and pH behaviour. The blend gives a better balance than any single mild surfactant.
Colour, Odour and Skin Feel: What Buyers Notice
For a solid surfactant used in bars and premium cleansers, three sensory attributes matter as much as the certificate of analysis:
- Colour. SLMI’s white chips let formulators make white or lightly coloured bars and creams without having to mask a yellow or beige base. Colour is controlled by the specification (25 max) and by the bleaching step in production.
- Odour. Acyl isethionates can carry a faint fatty odour from residual free fatty acid. Low-odour, low-free-fatty-acid SLMI is preferred for fragrance-free and premium lines, where any base odour is a defect.
- Skin feel. Because SLMI binds weakly to keratin, the after-feel is soft and comfortable rather than tight — the sensation consumers describe as “gentle” and “moisturising”, even though SLMI adds no moisturiser of its own.
These attributes explain why SLMI commands a premium over commodity surfactants: it delivers a perceptible quality difference in the finished product, not just a technical specification.
The “Sulfate-Free” and Gentle-Cleansing Trend
SLMI sits at the centre of the major consumer trend in cleansing: the move away from sulfate surfactants and toward milder systems.
- Consumer perception. “Sulfate-free” and “gentle” have become powerful marketing claims, driven by consumer concern about irritation and by social-media discussion of “harsh” cleansers.
- Technical reality. Isethionates and amino-acid surfactants are genuinely milder and more skin-pH-friendly than the sulfates, so the trend is backed by real performance.
- Trade-offs. Mild surfactants are usually more expensive and can be harder to thicken and to build to high foam, so formulations are tailored to the product’s positioning.
- Market segmentation. SLMI dominates premium facial and baby care, while SLES remains the volume choice for mass-market products; many lines use blends to bridge the two.
For a buyer, the trend means steady demand growth for SLMI and other mild surfactants, and it makes consistent quality — colour, odour, active content — particularly valuable, because the finished product’s premium positioning depends on them.
Quality and Consistency for Contract Manufacturing
For contract manufacturers and brands, SLMI’s value depends on batch-to-batch consistency, because the finished product’s performance and appearance were validated against a specific grade.
- Active matter consistency. Because SLMI is dosed on an active basis, small variations in active content change the effective surfactant level; a consistent active matter figure keeps the formula on target without rework.
- Colour consistency. White chips that vary in colour force adjustment of pigments or cause visible variation in bars and creams.
- Free fatty acid and odour. Residual fat drives base odour and can affect foam; consistent low levels matter for fragrance-free products.
- Documentation. Per-batch COAs with measured values, plus irritation data where claims are made, support both quality control and the brand’s substantiation file.
The practical buying lesson is that, for SLMI, a supplier’s consistency over time is as important as the specification on paper. Buyers with a validated formulation should review batch history and agree tolerances with the supplier.
Frequently Asked Questions
What is SLMI and what is its CAS number?
SLMI is sodium lauroyl methyl isethionate, a mild anionic acyl isethionate surfactant; its CAS number is 928663-45-0. See the product page for specifications.
Is SLMI the same as SCI?
No. Both are acyl isethionates, but SCI uses a mixed cocoyl (C12–C18) chain while SLMI uses a pure lauroyl (C12) chain, giving whiter chips, lower odour and a cleaner profile.
Is SLMI sulfate-free?
Yes — it is not a sulfate; it is an isethionate, which is why it is used in “sulfate-free” gentle cleansers.
How is SLMI made?
By transesterification of methyl laurate with sodium isethionate, with methanol removed to drive the reaction, then finishing to white chips.
Is SLMI gentle enough for babies?
Its very low protein binding makes it one of the mildest anionic surfactants, widely used in baby washes; claims must still comply with local cosmetics rules.
Can SLMI replace SLES in a shampoo?
It upgrades mildness but gives a different foam character, so most formulators blend it with co-surfactants to keep foam and viscosity.
What is the active content of SLMI?
Typically 79% min on a MW-344 basis; dose on an active-matter basis.
How should SLMI be stored?
Sealed, cool and dry, protected from moisture and heat; typical shelf life is around 24 months.
Is SLMI a sulfate?
No. SLMI is an acyl isethionate, not a sulfate, which is why it appears in “sulfate-free” cleansers.
What is the difference between SLMI and sodium lauroyl sarcosinate?
Both are mild anionics. SLMI gives richer, creamier foam; sarcosinate gives a silky feel and is often used in toothpaste and liquid cleansers. They are frequently blended.
Can SLMI be used at the acidic pH of a facial cleanser?
Yes. SLMI’s native pH is mildly acidic (around 5.5-7.0), so it suits skin-friendly, slightly acidic formulas.
Does SLMI work in hard water?
Yes. As an isethionate it tolerates water hardness and does not form lime-soap scum, so it rinses cleanly even in hard water.
References and Disclaimer
References. Substance identification data for sodium lauroyl methyl isethionate (CAS 928663-45-0); standard chemistry and manufacture of acyl isethionate surfactants (transesterification of fatty esters with isethionates); manufacturer technical data for the chips grade. Specifications quoted are representative and should be confirmed against the current certificate of analysis.
Disclaimer. This article is provided for general technical and commercial information only and does not constitute a specification or a warranty of fitness for any particular use. Users are responsible for confirming regulatory status, safe handling and suitability of the material for their own application and market.