For many years, sodium lauryl sulfate (SLS) and sodium laureth sulfate (SLES) have been the mainstay surfactant base in personal care cleansing formulations. Increasingly, however, there is a move to replace these and other sulfate-containing surfactants with sulfate-free alternatives. This trend appears to have started 8-10 years ago in North America when consumers sought sulfate-free formulations, perceiving the benefits to include mild, gentle, non-stripping, more naturally-derived products. The number of sulfate-free cleansing products (hair and body) launched has grown significantly since 2007.

Many sulfate-free surfactants exist but can be difficult to formulate with and tend to suffer drawbacks such as poor foaming, unwanted by-products, non-natural hydrophobe sources, ethylene oxide (EO) moieties present, pH instability and undesirable irritancy profiles. Table 1 summarizes the classes of sulfate-free surfactants and their properties.

While there are viable sulfate-free alternatives to SLS/SLES, performance in the finished product is often compromised; many of these options are only suitable as secondary or even co-surfactants. For example, betaines are excellent secondary surfactants most often used in combination with SLES; they help to enhance the performance and foam aesthetics and reduce the irritancy of the primary sulfate-based component. They are, however, not ideal as primary surfactants since, if used alone, the foam quality and stability are poor and they are unable to build viscosity without external thickening agents. Furthermore, many other sulfate-free surfactant options which meet all other criteria for primary surfactants often fail on viscosity, are high in solids, and cannot be thickened to achieve a consistency which is appealing to the end user.

In addition, these sulfate-free surfactants are typically more expensive than traditional sulfate-containing ingredients. This cost is compounded further given that in order to achieve adequate performance in finished products, high levels of the sulfate-free surfactants may have to be used, meaning that total solids in the finished formulation can be high and ultimately the cost is passed on to the consumer.

However, it is possible to formulate successful sulfate-free formulations that will produce desirable products and minimize cost in the bottle.

Sodium cocoyl isethionate (SCI) is widely recognized as a versatile mild primary surfactant providing excellent lather in both liquid and solid formulations. However, it has a limitation in aesthetic appeal since it creates opaque products (unless used at very low levels, below that expected of a primary surfactant). Following extensive research, it was found that a minor modification of the structure of SCI affords a molecule with all the desired properties of a sulfate-free primary surfactant.  This novel ingredient is known by the INCI name sodium lauroyl methyl isethionate (SLMI). This molecule has excellent water solubility, setting it apart from SCI. This means that transparent formulations are easily created, helping meet today’s trends for clear products.

SLMI has demonstrated versatility as a primary and secondary surfactant in liquid cleansing applications thanks to improved hydrolytic stability and foaming performance. This mild surfactant is 1,4-dioxane free and nitrosamine-free, while being readily biodegradable and sourced from approximately 80% natural/renewable feedstocks.

Several parameters have been measured in order to substantiate performance claims for SLMI and to provide information to help formulators optimize product performance. 

Viscosity Building
Product viscosity is important since it controls the way a product looks and feels in-use and has an impact on consumer perception of the efficacy and quality of an end product.

It is also vital to achieve good viscosity with yield for creating novel visual effects such as suspended air bubbles or exfoliating beads. The simplest and cheapest way to thicken a surfactant solution is by using electrolytes such as sodium chloride.

However, many of the sulfate-free surfactants currently available cannot be thickened with electrolytes. SLMI, on the other hand, in combination with cocamidopropyl betaine or other amphoterics, shows good thickening ability with electrolytes.

Viscosity can be further increased by the addition of co-surfactants such as cocoyl taurates, oleoyl taurates, sulfosuccinates and/or alkyl polyglucosides.

Foaming
The appearance, density and longevity of foam are perhaps the most important parameters in judging a good surfactant. A dense, creamy foam that persists in the presence of hard water or high soil level is the ultimate goal. Many sulfate-free surfactants are poor foamers, but SLMI creates a dense, creamy, long-lasting lather. In foam testing, SLMI was benchmarked against SLES.

Close up inspection of the bubbles under a microscope revealed a much smaller bubble size than those of SLES which are more tightly packed in the same sized area as SLES. Bubbles were measured and the size distribution compared. SLMI contains more small bubbles that support the observations made just by looking at the foam with the naked eye.

Foam height tests using the Ross Miles methodology demonstrated that initial foam height using SLMI at 1% active in distilled water was, in fact, higher than SLES at the same concentration.

The results after 10 minutes still showed SLMI to have the greater volume of foam with minimal loss in volume over that time.  The same trend in results was found when using hard water (100ppm Ca2+, 200ppm Mg2+). 

Blends of SLES/CAPB and SLMI/CAPB were also tested for foam quality and, again, SLMI containing blends gave a smaller bubble size distribution than SLES blends.

Other benefits of SLMI are as follows:

• Cumulative irritancy testing of 0.5% active solutions of SLMI on the skin has shown that it is an extremely mild surfactant. In a similar test, it was also shown that SLMI can have a mollifying effect in combination with other surfactants, reducing the inherent irritancy of individual surfactants.
• It is readily biodegradable according to the industry standard OECD 301B test protocol.
• It shows good hydrolytic stability over extended time periods with no loss in activity over 28 days. Activity was also maintained across a broad pH range (4.5-9.5) over 30 days.

Conclusion
Sodium lauroyl methyl isethionate has been developed as a sulfate-free primary surfactant for use in personal care cleansing applications.

This novel new approach to formulating sulfate-free systems has proven to be a simple cost effective and efficient way to achieve high performing, stable formulations.

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Samantha Brown, Tony Gough and Virginia Kropac
Innospec Performance Chemicals
500 Hinkle Lane
Salisbury, NC  28144
704-633-8028
www.innospecinc.com
[email protected]