FORMULATING FOR MILDNESS 61
Sulfate-free surfactants: what about sulfonates?
Dr Alexander T Wagner - Zschimmer & Schwarz, Germany
Sulfate-free or not sulfate-free? That is the question when customers are studying the INCI list of personal care products and find ingredients like surfactants based on sulfoacetates or sulfosuccinates. In fact, the discussion linked to that question is rather fruitless but one by one….
Conversations with customers show that the demand for sulfate-free cleansing products is growing more and more. Surfactant producers follow this trend and increasingly aim to develop sulfate-free surfactants. Additionally, already existing suitable products are claimed as sulfate- free. Of course there is no question that sulfur-free surfactants like for instance the mild and natural based Zschimmer & Schwarz Protelan- and Sebumol-Types (hydrophilic groups: amino acids or their derivatives and polyglycerol)1 free.
are sulfate-
In discussions, the clarity in answering the question “sulfate-free or not sulfate- free?” disappears often in that moment when sulfur-containing surfactants like sulfonates come into play. Therefore, in the following we will focus on sulfoacetates and sulfosuccinates as prominent representatives of sulfonate surfactants used in personal care applications. The perspectives that we take at first are driven by the chemistry followed by the application.
Chemical perspective Sulfate-free surfactants mean “free from alkyl sulfates and alkyl ether sulfates“ (sulfate surfactants). Both are organic sulfates (chemical bond: C-O-S, Fig 1) produced usually by the reaction of a fatty alcohol and a fatty alcohol ethoxylate respectively with sulfur trioxide. Sulfonate surfactants (chemical bond: C-S, Fig 2) like sulfoacetates and (ether) sulfosuccinates on the other hand are chemically synthesised with the help of hydrogen sulfite (Fig 3). Via a nucleophilic substitution reaction (sulfoacetates) or a Michael-Type addition (sulfosuccinates) a C-S bond is made. In reactions with hydrogen sulfite,
inorganic sulfates (Fig 4) are produced as by-products. That is the reason for the correct statement that sulfonate surfactant solutions always contain sulfates. These inorganic sulfates are nothing special. In the personal care area, magnesium sulfate is often added to stabilise water-in-oil emulsions (WO emulsions). Furthermore they are ingredients of seawater and essential for the human metabolism. In contrast to inorganic sulfates, alkyl sulfates and alkyl ether sulfates are surfactants with completely different properties Put briefly: From a chemical point of view surfactants based on sulfoacetates and sulfosuccinates are sulfate-free surfactants
O O S O O-
in the sense that they do not contain organic sulfates.
Mildness of surfactants
As the mildness of surfactants is becoming a more and more important aspect for customers, corresponding test results for sulfate surfactants let them fall behind other surfactants: alkyl sulfates and – to a lesser degree also alkyl ether sulfates – are more able to cause skin irritation than other anionic surfactants. For more information, see the corresponding studies cited in the literature, e. g. 2, 3.
This means, the true background behind the question “sulfate-free or not sulfate- free?” is to get information about the mildness of surfactants. There are two measures usually taken to
O O O O n Figure 1: Alkyl sulfate (top) and alkyl ether sulfate (n≥1, bottom): C-O-S bonds (red frame). April 2018 S O-
reduce the irritation potential caused by sulfate surfactants. The first one is to blend sulfate surfactants with milder surfactants. This was already the idea standing behind the now classical concept to use mixtures of ether sulfates with cocamido propyl betaine in shower and bath formulations. Further steps towards more mildness can be taken when for instance surfactants based on amino acids are added to this combination. The second measure – getting more and more popular – is to replace sulfate surfactants completely by mild sulfate-free surfactants. As proven later sulfonate surfactants based on sulfoacetates and sulfosuccinates are very mild. With this in mind the question may arise: Are all sulfonate surfactants mild? Put in chemical terms: Is the C-S bond of the sulfonate surfactants instead of the C-O-S bond of sulfates the secret of the success? Unfortunately, this is thought too simple. Counter examples are the sulfonate surfactants linear alkyl benzene sulfonates (LAS) and secondary alkane sulfonates (SAS) which cannot be classified as mild surfactants.2
Therefore,
they have few – if any – applications in personal care products. Besides charge properties, factors like the spatial structure of the whole surfactant and its biodegradability will probably be critical
PERSONAL CARE EUROPE
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