62 FORMULATING FOR MILDNESS
parameters for the mildness of surfactants: The ‘architecture’ of the surfactant molecule is expected to have an impact on its penetration potential into the skin and other cells. The biodegradability gives information about the ‘lifetime’ of a surfactant. In contrast to sulfate surfactants, LAS and SAS, the hydrophobic and hydrophilic parts of surfactants based on sulfoacetates and sulfosuccinates are linked via carboxylic acid ester bonds. Substances with these chemical bonds are abundant in the human metabolism (e.g. triglycerides) and – as a result – can be cleaved easily by nature´s enzymes (lipases). Under healthy conditions these enzymes are almost everywhere in the human organism. The ‘lifetime’ of ester-linked sulfonate surfactants in contact with human cells therefore should be too short to cause (serious) irritation. The metabolites are no longer surfactants and have a natural origin (exception: fatty alcohol ethoxylates as cleavage products of ether sulfosuccinates). To summarise, the term “sulfate-free“ surfactant is synonymous for “mild“ surfactant. The mildness of sulfoacetate and sulfosuccinate surfactants cannot be traced back to the nature of the linkage between sulfur and carbon atoms (C-S instead of a C- O-S bond). Possible reasons for their mildness are the spatial and natural related surfactant structure leading to low skin penetration and easy biodegradability.
Why sulfonates as replacement for sulfate surfactants? As mentioned before surfactants, e.g. based on amino acids like cocoyl glutamates, are also sulfate-free.
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O S O Figure 2: Alkyl (ether) sulfosuccinate (top) and alkyl sulfoacetate (bottom): C-S bonds (red frame). O HO S O Figure 3: Hydrogen sulfite.
Nevertheless, sulfonate surfactants are often used to replace sulfate surfactants and the question arises; why? One reason is that in a physico-chemical sense sulfonates share the positive properties of alkyl sulfates. When about 85 years ago the first synthetic laundry detergents based on alkyl sulfates entered the market and competed with classical soaps two ‘new’ detergent
O O S O Sulfoacetate O O n Sulfosuccinate: Structure 1 O O O n Sulfosuccinate: Structure 2
Figure 5: Different location of the sulfo-group: top and middle: Sulfo-groups next to ester groups, bottom: ”Methyl-bridge” between sulfo- and ester groups.
PERSONAL CARE EUROPE O O
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S O O- O O- O- -O S O Figure 4: Inorganic sulfate.
properties were emphasised. At first, alkyl sulfates are much more ‘robust’ to water hardness: The ‘loss’ of surfactants by precipitation is drastically reduced. Secondly, whereas washing with soaps inherently means alkaline conditions, using alkyl sulfates does not change the pH of a solution: Washing in the neutral pH range is possible.
When the alkaline pH value of a soap solution is lowered by the addition of an acid more and more of the carboxylate- groups are protonated. The resulting fatty acids have only limited water solubility. In contrast, alkyl sulfates are charged over the whole pH-range relevant for cosmetic application. This means that sulfate surfactants are water-soluble irrespective of the pH-value. Both aspects (robustness to water hardness, pH-independent water solubility) also favour sulfonate surfactants. Another important point for using sulfonate surfactants like sulfoacetates and sulfosuccinates instead of other sulfate-free surfactants is the price. Both sulfonate surfactants enable the formulator to develop products compatible with mass market requirements.
Sulfoacetates and sulfosuccinates both
are esters. As known, esters are susceptible to hydrolysis even in weakly acidic formulations of about pH 5 which is commonly used in cosmetic formulations. Ester based surfactants are stable under these conditions when a sulfo-group is in direct neighbourhood to the ester group.4
April 2018 O-
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