FORMULATING FOR MILDNESS 93 between the molecules.4 For acyl
sarcosinates, however, this is not possible (Fig 3). In the following some of the consequences are described. Amino acid surfactants are produced via a two-step synthesis route starting from fatty acid. This process leads to a salt content of 5% and more in customary 30% amino acid surfactant solutions. Additionally, these solutions contain unreacted free amino acid. For some consumer products, however, a high purity grade is required. The reasons, for instance, can be reduced stabilities of ‘salty’ products or unwanted colour reactions of the free amino acid with other ingredients (e.g. essential oils). Some customers therefore prefer a limited content of salt and free amino acid of maximum 0.5% or even lower. For the purification process the melting point of the protonated amino acid surfactant plays a major role. For protonated lauroyl sarcosinate (lauroyl sarcosine) the melting point is about 50°C whereas for lauroyl glycine and lauroyl glutamic acid it is about 120°C and 100°C.5 Protonated amino acid surfactants which are liquid at a temperature below 80°C – 90°C enable an easy purification step. So, this is only possible for lauroyl sarcosinate. As a consequence, commercial aqueous lauroyl sarcosinate solutions often have a low salt concentration. In contrast to this, there are only very few suppliers for highly pure cocoyl glycinate and cocoyl glutamate solutions on the world-wide market. To desalt them a more sophisticated procedure is necessary. Another significant difference between
the lauroyl derivatives of sarcosine and glycine is their solubility. Additional intermolecular hydrogen bonds stabilise the solid acyl glycinate form which reduces the water solubility. A suitable measure to prove this is the Krafft-temperature. It informs about the minimum temperature above which surfactants are water-soluble. For sodium lauroyl glycinate it is above room temperature even in weakly alkaline solutions.6
In contrast, sodium lauroyl
sarcosinate is easy to handle in aqueous solutions. At pH 7.5 the deprotonated anionic form is predominant 2,7
and the
Krafft-temperature is below room temperature. Sodium lauroyl sarcosinate is soluble even in cold water and in slightly acidic solutions. In a solution of the soap sodium laurate, on the other hand, at pH 7.5 already 50% of the soap is turned into water-insoluble lauric acid. This confirms the statement that sodium lauroyl sarcosinate is the “better soap“.2 The different behaviour of acyl
sarcosinates and acyl glycinates can also be seen in the viscosity of their 30% aqueous solutions. Whereas the viscosity of sodium cocoyl sarcosinate solutions is water-like, the
April 2020 Acyl glycinate O O
N H
O- O O
N H
O- O O N O-
Figure 3: Two amino acid surfactants: Hydrogen bonds (hashed line) can be created only in acyl glycinates.
Disordered micellar structures: Low viscous Ordered structures: Viscous N O- Acyl sarcosinate O O
Protonation of acyl sar- cosinate/acyl glycinate
Hydrophilic surfactant part Hydrophobic surfactant part
Figure 4: Radical change of micellar structures: Thickening by lowering the pH.
viscosity of sodium cocoyl glycinate solutions can be in the range of several thousand mPa∙s. Therefore, already in the production process problems can arise. Also, for some applications (highly) viscous cocoyl glycinate solutions are difficult to handle. Two very effective measures to reduce the viscosity of cocoyl glycinate solutions will be suggested when Zschimmer & Schwarz products are introduced.
The capability of acyl glycinate to create
intermolecular hydrogen bonds also has advantages. By these bonds, the stability of foam in comparison to acyl sarcosinate is enhanced.4
It was shown that via
intermolecular hydrogen bonds the foam lamella elasticity is increased.8
This means
that the bursting of foam bubbles is slowed down. In cosmetic applications consumers often perceive stable foam as a very creamy foam. The stabilisation of foam by intermolecular hydrogen bonds is a general phenomenon.9
In cosmetic products normally
mixtures of different surfactants are used. Acyl glycinates with their ability to create hydrogen bonds are a smart choice to create a creamy foam also in surfactant mixtures. A unique property of cocoyl glycinate is
Acyl glutamates: completely sustainable With a longer carbon chain and an additional carboxylate group the derivatives of glutamates (Figs 1a,2a) have some different properties than acyl glycinates and acyl sarcosinates. In the production process the additional carboxylate group leads to side-reactions which reduce the yield of the target product. To avoid this, surfactants based on acidic amino acids like acyl glutamates usually are synthesised in aqueous solutions to which a solvent is added. This was already recommended in the patent of the 1930s.1
When the solvent is volatile like acetone it is removed afterwards to avoid PERSONAL CARE EUROPE
its ultra-mildness, which is proven in clinical tests.10
In addition, the preferred pH-range to use acyl glycinates is the neutral to alkaline pH-range. These facts turn cocoyl glycinate into an ideal candidate for pH-neutral baby care applications as their skin has a pH of about 7. As shown later in a frame formulation, by adding suitable co- surfactants clear formulations can also be created in a pH-range of about 6.
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