FORMULATING FOR MILDNESS 81
OSO3
-
Na+
Figure 3: Sodium fatty alcohol sulfate.
compensated by organic acids like citric acid, more and more water-insoluble fatty acids will be created and at a certain point no longer be solubilised. So, the seemingly simple trick to make soap solutions pH- neutral or even pH-skin neutral by adding citric acid fails. To summarise: At a neutral pH the surfactant properties of soaps are lost to a great extent, as in this pH-range a high percentage of soap molecules becomes water-insoluble fatty acids. Later in this article, we will introduce an elegant way to produce carboxylate surfactants creating clear solutions in a neutral or even slightly acidic pH-range. But first, we are following the historical development, beginning in the 1920s: The first high-performing synthetic detergents were developed to overcome the drawbacks of using soaps. As all of these drawbacks can directly be traced back to the carboxylate group of soaps, the approach was to replace it by other hydrophilic groups.
Sulfate surfactants: as co- surfactants for ‘combo bars’ The “invention” mentioned in the opening patent citation of this article1
was to mix
soaps with sodium fatty alcohol sulfates (Fig 3), which were developed some years before. Fatty alcohol sulfates have as good foaming and cleansing properties as soaps. On top, the sulfo-group of fatty alcohol sulfates offers some advantages in comparison to carboxylate groups. Sulfate surfactants are much more robust to water hardness than soaps. Additionally, the sulfate group is not
a O O O b O SO3 O - Na+ Figure 4: Sulfonate surfactants: Disodium alkyl sulfo succinate (a) and sodium acyl isethionate (b).
protonated in the whole pH range relevant for cosmetic applications. Consequently, the addition of a fatty alcohol sulfate will not change the pH-value of aqueous solutions. So, in suitable blends of soaps and sulfate surfactants the properties cited in the introduction can indeed be achieved:1
High-
performing solid cleansing bars can be produced with a nearly neutral to weakly alkaline pH when dissolved in water. This is the reason why wash pieces based on a combination of soaps and fatty alcohol sulfates are still popular. Mixtures between soaps and other surfactants (mostly based on alkyl sulfates or alkyl ether sulfates) are generally called “combo bars”. However, to succeed in developing a pH skin-neutral solid cleansing bar (pH in aqueous solutions: 5.0 - 5.5), soaps have to be replaced completely. These bars are called “syndets” and are exclusively based on synthetic detergents.
Sulfonate surfactants: as soap replacement for ‘syndet bars’ Syndet bars on the basis of sulfate surfactants alone leave an unpleasant skin feel as they are more irritant than soaps (‘harsh’ surfactants).6
There are different
ways to create mild syndet bars. For example, alkyl sulfates can be replaced by the milder alkyl ether sulfates. Another solution is to combine alkyl sulfates with very mild surfactants to reduce the irritation potential. In some syndet bars sulfate surfactants are even completely replaced by milder ones (‘sulfate-free’ syndet bars). As mild additives or replacements frequently sulfonate surfactants are used. They share all the positive properties of sulfate surfactants (robustness to water hardness, pH-independent charge, good foaming and cleansing properties). They are mass-market compatible and, additionally, some of them provide very good skin compatibility.7
In solid cleansing bars, often
used sulfonate surfactants are disodium lauryl sulfo succinates and sodium cocoyl isethionates (Fig 4).6
The hydrophobic part Figure 5: Transparent solid cleansing bars. September 2019
of both sulfonate surfactants is - as usual for cosmetic applications - made from fatty acids. Coconut fatty acid and lauric acid are obtained by cleaving the corresponding natural oils. So, the production process is the same as for soaps. The lauryl alcohol which is needed to produce lauryl sulfo succinates is made by a hydrogenation step. The final product (Fig 4a) carries a carboxylate group. In contrast to soaps, the protonation degree of this group does not play a role for its water solubility as there is an additional sulfo-group within the molecule. In the second example, the cocoyl
PERSONAL CARE ASIA PACIFIC SO3 - O- 2 Na+
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