Introduction
As formulators seek to respond to market needs with easy to use and affordable technologies, one very common substation we’ve seen is the replacement of Sodium Laureth Sulfate (SLES) with Sodium C14-16 Olefin Sulfonate (AOS). In this blog post we’ll examine the potential benefits and drawbacks of such a replacement strategy.
Technical Differences
While SLES and AOS follow similar reaction schemes, there are important differences in feedstocks and end products. For SLES, Lauryl Alcohol is ethoxylated, typically with anywhere from 1-4 moles of ethylene oxide. Biobased carbon content cany vary from 0 to 100%, depending on supply chain. For AOS, narrow range hydrocarbons with a terminal (alpha) double bond follow a similar reaction process. Biobased carbon content is 0%. Both reaction schemes will produce sodium sulfate as a byproduct. Depending on the exact process, Sodium Chloride may also be produced as a byproduct. SLES will also produce 1,4-dioxane as a byproduct, which may be substantially removed by post-reaction treatment.
Performance Drivers and Stability
The INCI designation “Sodium Laureth Sulfate” can encompass a wide range of performance attributes. The purity of Lauryl Alcohol and degree of ethoxylation strongly influence performance characteristics like foam, viscosity building, and irritation potential. Sodium C14-16 Olefin Sulfonate can also exhibit a range of performance attributes, driven primarily by the precise alkyl distribution. Most grades of SLES will not exhibit a Krafft temperature above freezing, while most grades of AOS will exhibit a Krafft temperature, typically around 10°C. AOS also exhibits superior hydrolytic stability in low-pH formulations due to the inherent stability of the sulfonate functional group. In contrast, SLES shows poor stability below approximately pH 5, where sulfate ester linkages become increasingly susceptible to hydrolysis. As a result, AOS is generally preferred for highly acidic formulations.
Foam Profile & Viscosity Response
The foam profile of AOS is comparable to higher-ethoxylated SLES grades such as SLES-3, but lower than low-ethoxylated grades like SLES-1. With optimized formulation strategies, AOS systems can achieve near parity with SLES-2. In terms of salt response, SLES-based systems typically achieve peak viscosity with lower salt levels compared to AOS-based systems, which may require higher levels of secondary or tertiary surfactants.
Irritation Potential
While the stated goal of replacing SLES with AOS is for improved mildness due to the “harshness” of sulfates, AOS has been demonstrated in formulated systems to be close to SLES with the lowest degree of ethoxylation. More frequently used higher ethoxylates of SLES like two and three mole
versions have much lower irritation potential, even in formulated systems. Increasing the use of secondary amphoteric surfactants and/or introducing other zero irritation surfactants in AOS-based formulations can allow formulators to achieve much closer parity in irritation or even improvements on the most commonly used grades of SLES.
Cost, Regulatory & Market Drivers
AOS is experiencing rapid global adoption primarily due to its cost advantages. These are driven by historically low crude oil prices, high lauryl alcohol costs, fewer processing steps, and the absence of dioxane removal requirements. SLES production requires additional processing and specialized equipment to reduce 1,4-dioxane levels.
Regulatory & Marketing Advantages
Regulatory pressure, particularly in the United States, has significantly impacted SLES usage due to strict limitations on 1,4-dioxane content. Achieving ultra-low dioxane levels is increasingly challenging. AOS, which does not generate dioxane, offers a clear regulatory advantage. From a marketing standpoint, AOS enables ‘SLS and SLES-Free’ and ‘PEG-Free’ claims, aligning strongly with current consumer and brand owner preferences.
Sustainability & Future Outlook
Although not yet widely commercialized, suppliers are actively developing AOS derived from biobased olefins. This development could enable AOS to transition from 0% to 100% biobased content, significantly strengthening its sustainability profile. This evolution is expected to further establish AOS as the surfactant of choice in the coming decades.
Conclusion
While SLES remains a well-established surfactant, AOS is an increasingly popular option with superior short-term advantages in cost efficiency, regulatory compliance, formulation ease, and consumer acceptance. Long-term continued development in biobased olefin technologies will improve its environmental footprint. Some reformulation work to maximize benefits of AOS is expected and needed to ensure all stated goals are met.
