106 FORMULATING FOR MILDNESS
skin. Sometimes the residual surfactants on the skin after washing can penetrate the barrier structure and disrupt the lamellar layer of the barrier layer creating skin defects. It has been extensively documented that surfactant molecules can penetrate the skin but not only molecules in the form of monomers but the micelles also have been found to penetrate the skin and create defects leading to low hydration of skin by high Trans Epidermal Water Loss.2 Molecular size, size of the hydrophilic head group and length of the hydrophobic chain all have an effect on the mildness of the surfactants.3
Skin pH and surfactant binding The pH of skin is normally acidic in the range of about 6.0, while the internal body pH is near neutral around 7-8. If the pH difference between the skin pH and the cleansing product is high then the cleanser has a potential effect on skin irritation.4 Another reason for skin harshness is due to the binding of the surfactant with the SC protein of the skin. The behaviour of binding of anionic surfactants varies in different surfactants. Normally it is observed that the binding potential depends upon the head group of the surfactant, the binding tendency of surfactant that has a larger head group is lower compared to the smaller head group surfactant, for example sodium dodecyl sulfate (SDS) binding potential is higher than sodium cocoyl isethionate (SCI).5 Normal bar soaps have high pH in the range of 9.5-10.5 in comparison to the skin pH of about 6.0. It is not possible to reduce the pH of normal soap because of its high pKa value. This is the reason why normal soaps are harsher, have profound effect on sensitive skin and are highly sensitive to weather changes. In winter season use of normal soap can cause skin drying and damage the barrier structure leading to psoriasis. The harshness of the soap increases further with the soap compositions having high level of lauric component. Normally soaps can have fatty acid components from C8 to C18 where the irritancy potential is low for higher chain fatty acid soaps and for C12 which is sodium laurate it is the highest.
Table 2: Superfatted soap formulation Ingredients
%
Anhydrous Soap Free Fatty Acid Salt
Glycerin Water
Preservative Perfume
78.0 – 85.0 0.5 – 7.0 0.5 – 0.8 0.0 – 0.5
10.0 – 11.0 0.02 – 0.05 0.5 – 1.5
PERSONAL CARE EUROPE Air
Water Rod Micelles
Spherical Micelles
Hexagonal L.C. Figure 2: Liquid phases of surfactant.
Table 1: General formula for bar soap. Ingredients
%
Anhydrous Soap Salt
Glycerin Water
Preservative Perfume
85 – 86 0.5 – 0.8 0.0 – 0.5
12.0 – 13.5 0.02 – 0.05 0.5 – 1.5
Superfatted soap To reduce the irritancy potential in bar soaps sometimes formulators use 1.0-7.0% free fatty acid as a superfat. Superfatted soaps do not have a large effect on pH reduction. These soaps give creamy lather but the quantity is low. The solid phases in superfatted soaps are transformed to delta phase which helps in reducing the mush in the soap.
Combars
Addition of small amount of non-soapy surfactants in superfatted soaps gives combars. Formulators use various surfactants in combars but the basic criteria should be to use surfactants with a Krafft Point just above room temperature. If the Krafft Point is low then soap will be very soft in combination with superfatting and one can face a problem of processing in the plant. The surfactants with high Krafft Point are not very effective because they work as a structurant without improvement in lather quantity. These soaps have an additional advantage of lime soap dispersing and are
highly recommended for places with hard water. One of the most famous combars is Lever 2000 in North America. These soaps have been developed based on superior mildness and moisturisation.
Syndet bars Syndet bars are the mildest because in these bars the pH can be adjusted from 6.5 to 8.56
Lamellar L.C.
by using high levels of fatty acids, mainly a combination of stearic acid and palmitic acid. It does not mean that these bars are free of soaps. Some quantity of conventional soap noodle and insitu generated sodium stearate is added in the formulation for imparting plasticity for the processing. Without adding the soap the processing is difficult. Syndet bars were developed after the Second World War by the three multinational companies Unilever, Colgate Palmolive and Proctor and Gamble using different surfactants. These are shown in Figure 3. Out of these three, sodium cocoyl isethionate was found to be the best because of its appropriate Krafft Point and mildness. This is the reason why Dove is the best syndet bar manufactured by Unilever. Its mildness and moisturising properties are well established. These products are highly recommended for sensitive skin because of the balanced pH as well as very smooth skin-feel after washing. In tropical areas because of high humidity people do not prefer the after-wash feel but in cold weather it protects the skin and helps in the repairing of barrier layer structure.
April 2019
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