76 FORMULATING FOR MILDNESS
its smell or other adverse effects. In case of using the odourless propylene glycol the solvent usually remains in the product. Acyl glutamates have a hydrogen atom at
the nitrogen atom so they should be able to create intermolecular hydrogen bonds like acyl glycinates (Fig 1). But because of the ‘bulkiness’ of the glutamate group these hydrogen bonds play a minor role, if any. Regarding solubility properties in aqueous solutions, for instance, cocoyl glutamate behaves like cocoyl sarcosinate: It is easily soluble in cold water even in weakly acidic solutions, so it is also a ‘better soap’.2 In comparison to the sarcosine and
glycine derivatives, cocoyl glutamates have some further useful properties. For instance, they are excellent oil-in-water (OW) emulsifiers for oils of different polarity like vegetable oils and paraffins. Emulsions based on aqueous cocoyl glutamate solutions are cold-processable. Further ‘add-on’ properties of acyl glutamates are discussed in the next section. Acyl glutamates made from vegetable
oils are completely based on renewable raw materials. When propylene glycol made from glycerol is used in the production process, this claim is still valid. Nevertheless, some customers prefer propylene glycol-free cocoyl glutamate solutions. Surfactants in sulfate-free products for
the natural cosmetics market usually are combinations of cocoyl glutamate and sugar surfactants or their derivatives. As cocoyl glutamate is a main surfactant in these formulations the requirements in terms of purity usually are very high.
The principle ‘thickening without thickeners’ To create viscous aqueous cleansing formulations two different paths are taken. Alkyl (ether) sulfate solutions are thickened by adding salt. For other surfactant solutions the addition of thickeners is required which in some cases is unwanted. The usage of thickeners can be circumvented when lauroyl sarcosinate or cocoyl glycinate are combined with suitable other surfactants. Via pH- adjustment the viscosity of these surfactant solutions can easily be enhanced to 4.000 mPa∙s and more. In two formulations introduced in the next section both amino acid surfactants are combined with lauryl sulfoacetate and cocamidopropyl betaine (CAPB). Usually the surfactant concentration in aqueous cosmetic solutions is so high that they are associated in micelles. A small part is shown in Figure 4. The wedge-shape (‘triangle’-shape) for lauroyl sarcosinate (Fig 5) is the result of repulsive forces between the carboxylate group of the amino acid surfactant and other anionic head groups: the sulfonate
PERSONAL CARE ASIA PACIFIC Lauroyl Sarcosinate O- O R: C11 O
Hydrophilic part Hydrophobic part
R N
O O
R Lauroyl Sarcosine OH
N
Figure 5: Anionic and neutral form of an amino acid surfactant: Effective spatial arrangement within micelles. For the position of the “border” between hydrophobic and hydrophilic parts see literature.11
Acyl glutamate O- O-
O HN O
O
R
Figure 6: Schematic spatial arrangement of acyl glutamates within micelles.
group of the lauryl sulfoacetate and the carboxylic group of CAPB when it is deprotonated. The disordered micelle shape creates low-viscous solutions (Fig 4). Now advantage is taken of the fact that by lowering the pH-value below 7 the carboxylate group of amino acid surfactants is more and more discharged via protonation.2
Figuratively speaking, the
‘triangles’ are more and more turned to ‘rectangles’: ‘Ordered’ micelle structures can arise at a pH-value of about 5 – 6 (Figs 4,5). This can cause a drastic viscosity increase of the formulation. The negative charge of sulfonate groups does not change by lowering the pH3
whereas there
is probably also a contribution of CAPB’s carboxylic group to the viscosity increase. As a consequence of its betaine structure the pH-dependent behaviour of this carboxylic group (protonated or not protonated) is very special. It is discussed elsewhere.12,13
By the way, the basic thickening mechanism is the same as for
alkyl (ether) sulfates. The difference is the trigger (here: pH-value, there: salt). Some examples of thickening amino acid surfactant solutions via pH-adjustement are shown in the next section. The above-mentioned property
applies to lauroyl sarcosinate and cocoyl glycinate. For cocoyl glutamate the situation is completely different as the hydrophilic group here is inherently bulky (Figs 1,6). In alkaline solutions acyl glutamates are
double negatively charged. This combination (molecular structure of the hydrophilic group plus two charges) can be used for the opposite effect: To ‘break’ the viscosity of surfactant solutions (hydrotropic surfactant).14,15
As an
illustration of this effect see Figure 4 from right to left (reverse arrow). In the next section an example for the viscosity decreasing effect of cocoyl glutamates as well as an example for the viscosity increasing effect is introduced.
November 2020
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