Dr Paul Cornwell, Director at TRI
Dr Philippa Cranwell, Technical Content Creator at TRI
While there are always trends within cosmetics that come and go, one trend that has consistently been on the increase in recent years is for ‘sulfate-free’ shampoos, with consumers increasingly curious about the benefits of going sulfate-free. Rightly or wrongly, sulfates receive a lot of bad press, with numerous websites suggesting that sulfates strip lipids from hair, cause excessive dryness and brittleness and are irritating to scalp. These outcomes are clearly undesirable for consumers, but is there any truth in these claims? Are sulfate-free products any better for your hair and the planet? This extended article will look at what sulfates are and how they work, the benefits and concerns surrounding their use, and consider the alternatives when developing a sulfate-free product.
The unstoppable rise of ‘sulfate-free’ shampoos
A recent report suggests that the value of the sulfate-free market by 2030 will be USD 5.61 billion, with a CAGR between 2022 to 2030 of 3.3%. With social media influencers, websites and marketers claiming that sulfate-free products are more gentle, reduce irritation and are more environmentally friendly, this trend isn’t going anyway any time soon. But is there any truth behind the claims?
When discussing sulfate-free products, the term ‘sulfate’ often relates to the surfactants present; molecules that reduce the surface tension of a liquid and are key for cleansing and removal of dirt. More specifically, the sulfate-based surfactants in question are usually sodium laureth sulfate (SLES) or sodium lauryl sulfate (SLS).
Behind water, surfactants are usually the second most common ingredient in shampoos by weight, forming the core of the formulation’s active ingredients. There are numerous surfactants available, but sulfates have become a stalwart within cosmetic formulations due to their efficiency as cleansers, their ability to tune the properties of the bulk material, low price-point, ready transportation and easy incorporation into a range of items.
A shampoo usually contains both a primary and secondary surfactant. The primary surfactant is present at the highest concentration in the formulation and is fundamental to the formulation’s properties. Shampoos normally contain an anionic surfactant (with a negatively charged head group) as the primary surfactant, Figure 1. A secondary surfactant is often used to boost the properties of the primary surfactant, or add further characteristics, for example mildness or improved foaming.
Figure 1: The four groups of surfactants. The designation is based upon the charge upon the headgroup.
Alkyl and alkyl ether sulfates are the most widely used primary surfactants in hair-care items. In an alkyl sulfate, the sulfate group is bonded directly to the parent alcohol residue, whereas in an alkyl ether sulfate, there is an ethylene oxide spacer(s) between the parent alcohol and the sulfate group, Figure 2a. The most common parent alcohol is lauryl alcohol, which is usually derived from palm oil. As the sulfate group is anionic a counter-ion is required, which is most often a sodium ion or an ammonium group. Commonly used alkyl sulfates include sodium lauryl sulfate (SLS) and ammonium lauryl sulfate (ALS), with alkyl ether sulfates including sodium laureth sulfate (SLES) and ammonium laureth sulfate (ALES).
Alkyl sulfates are synthesized through sulfonylation of the parent alcohol with sulfur trioxide, followed by basic work-up to deprotonate the sulfonic acid and install the counterion; sodium hydroxide giving the sodium salt and ammonium hydroxide the ammonium salt, Figure 2b. Alkyl ether sulfates are prepared in a similar manner, but prior to sulfonylation the ether group is added by reaction of the parent alcohol with epoxirane (ethylene oxide). Under the reaction conditions between 1 and 3 ethylene oxide units are usually added, giving an intermediate alcohol that can then undergo sulfonylation and deprotonation to install the counter-ion.
Figure 2: (a) The structures of four commonly used surfactants in shampoo products; (b) synthesis of ALS/SLS and SLES/ALES.
With the structures of key sulfate-containing surfactants disclosed, we can now turn our attention to the pros and cons of using sulfate-based surfactants.
Four key benefits of sulfate-based surfactants
Sulfate-based surfactants provide advantages over alternatives in four main areas: cleansing, foaming, rheological modification and deposition of polymers.
The main reason for addition of surfactants to a formulation is to improve its cleansing properties. There are four main ways in which surfactants are proposed to remove sebum: roll-up, spontaneous emulsification, penetration and solubilization. Good evidence is provided for all four methods, but it is unclear which is more important and in reality, it is probably a combination of the four mechanisms that leads to cleaning. In essence, when surfactants encounter sebum oils on the hair surface they enclose them in the micelles, where the hydrophilic heads face into the aqueous medium and the lipophilic tails solubilize the oil molecules, meaning that they can be easily washed away, Figure 3.
Figure 3: The roll-up mechanism of oily soil removal. Surfactants agglomerate around grease (b), then the surface tension at the dirt/water interface is reduced, encouraging oil droplet formation (c), and finally micelles are formed where the dirt/grease molecules are encased within a ‘surfactant shell’.
Sulfates have been shown to be extremely effective in removing all types of dirt, including oils, particulates and deposited materials at low concentrations.
Although all cosmetic formulators know that foaming is not linked to cleansing power, the formation of foam is often seen by consumers as a key indicator that a product is working well. This means that foam is a good thing! Sulfates are known to make large volumes of uniform foam very quickly in both hard and soft water therefore they are likely to score highly for consumer satisfaction.
The rheological behavior of a product is extremely important for manufacture, packing, filling, storage and consumer experience. Viscosity is an important parameter to control, particularly in relation to customer perception: Shampoo that is too thin, or non-viscous, may feel cheap, whereas shampoo that is thick, or extremely viscous, may not flow or spread on the hair well. The viscosity of alkyl sulfates and alkyl ether sulfates is easily adjusted through the use of sodium chloride (NaCl). Addition of just 6% w/w NaCl can lead to nearly a 10,000-fold increase in viscosity cheaply, with the relationship between viscosity and w/w% NaCl being well-defined.
The use of conditioning cationic polymers and silicones to offer hair conditioning benefits such as wet detangling, ease of wet combing and dry smoothness has become increasingly popular as consumers demand more from their haircare products. Surfactants play a key role in delivering these benefits as they are deeply involved in the mechanisms through which cationic polymers deposit on the hair surface. Sulfate-based surfactants are particularly good due to the interaction between the cationic conditioners and anionic head groups on the surfactant. Replacement of sulphates with alternative surfactants can markedly affect polymer deposition, and 2-in-1 shampoo benefits.
Concerns around sulfate-based surfactants
Until this point, only the positive aspects of sulfate-based surfactant systems have been discussed. We’ll now consider the drawbacks.
One of the usual arguments against use of sulfates in a formulation is that they are not mild, and cause skin irritation or negative effects such as stinging, burning, itching and tightness. There are two accepted ways through which surfactants can cause irritation: through interaction with skin proteins or through lipid removal. In vitro zein tests, used for interrogating surfactant–protein interactions, have shown that surfactant irritancy runs: SLS and soap > cationics and SLES > APGs and betaines, with SLS and soap being the most irritating. However, mildness to proteins and mildness to skin lipids do not always correlate, and Ananthapadmanabhan et al. have shown that in some cases a blend that is mild to skin proteins can significantly increase lipid removal, Figure 4.
Another study by Takagi et al. showed that blending SLES with other surfactants (in this case sodium laureth carboxylate and lauryl glucoside) led to a formulation that was milder than some sulfate-free alternatives. As with all things, a balance is necessary and if sulfates are blended correctly, they can be mild to skin.
Take home: sulfate-free does not always guarantee mildness!
During the installation of ethylene oxide groups to prepare SLES or ALES, 1,4-dioxane can be made as a by-product. Although 1,4-dioxane is categorized as ‘Category 2 CMR’ in Europe and ‘likely to cause cancer in humans’ in the US, the amounts of dioxane present in any sulfate-based surfactant are likely to be very small if sourced from a reputable supplier. By way of comparison, the EU proposes a maximum exposure to 1,4-dioxane of 10 ppm in a finished product. Good quality 70% SLES contains <14 ppm 1,4-dioxane, with dilution to 10% SLES in a finished product giving an exposure of <2 ppm, much lower than the EU maximum exposure.
Take home: it is likely that there are some traces of 1,4-dioxane present in SLES or ALES, but provided the dioxane levels are controlled and the supplier knows the levels of contamination, the risk is minimal.
“Naturalness” and environmental impact
The definition of the term ‘natural’ within cosmetic items is a thorny issue, as there is no agreed legal definition although certifications standards exist, for example ISO, BCI and the Natural Original Index. Alongside the ‘naturalness’ of an ingredient of product, environmental impact is increasingly coming under scrutiny. A common argument levelled at sulfates is that they are bad for the environment and non-sustainable, but for SLS and SLES, the parent lauryl alcohol can be derived from 100% sustainable palm oil and installation of the ether groups to give SLES can be achieved using natural sugars, therefore sulfates can be derived from natural feedstocks. In addition, both SLES and SLS, as well as other associated long-chain sulfates, readily biodegrade in the environment without requirement for specialist conditions.
Take home: sulfate-based surfactants can be derived from renewable resources and are biodegradable.
What are the alternatives?
The key question when developing new products is whether to ‘play safe’ with commonly used sulfate-based surfactants or to innovate with a new, less well-understood blend. There is a great deal to consider including cleaning power, foamability, rheology control, mildness, polymer deposition, cost, environmental sustainability, pH stability and processing. In all cases, there is no magic bullet, and a cost/benefit analysis is key. However, there are two areas in which sulfate-free formulations regularly struggle: cost and performance.
In terms of cost, a sulfate-free formulation is often more expensive, which is likely due to reduced economies of scale for suppliers of alternative surfactants. In addition to this, the amount of replacement surfactant required will likely be greater, and blends of replacement surfactants may be complex; reading the back of a sulfate-free shampoo bottle regularly reveals a mixture of three, four or five different surfactants, whereas a sulfate-containing product usually only uses two. In addition, formulation viscosity is unlikely to be controlled through addition of NaCl, therefore additional, more expensive, thickeners are needed that will add to cost and may slow product development. Finally, in terms of performance there may be reduced deposition of active ingredients, for example cationic polymers, unwanted phase behavior and lower chemical stability of the sulfate-free surfactants themselves.
However, if you are still thinking of using an alternative surfactant, or are just curious, a few are outlined, Figure 5.
Figure 5: Examples of alternative surfactants to SLES or ALES (or similar).
This article has outlined the pros and cons of using sulfate-based surfactants within shampoo formulations. The use of sulfate-based surfactants is well-established, and sulfates are low cost and provide an excellent performance profile. While there are concerns over irritation, toxicity, natural-ness and environmental impact, these are often overstated and subject to over-simplification. In addition, the behavior of sulfates within a range of formulations is known, therefore the risk for development of a new product when using a sulfate-based surfactant is lower.
However, as more manufacturers move towards the use of alternative, sulfate-free surfactants, the knowledge base around them will grow and their behavior will be more understood. While some of the reasons for generating sulfate-free cosmetic items may be based on shaky science, the knowledge and innovation gleaned through expanding their use could be very useful, bringing opportunities and benefits that could be applied beyond the cosmetics industry.
For more information about how TRI could help with your formulations (sulfate-free or otherwise!) and claims, contact us.
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