56 FORMULATING FOR MILDNESS
and sulfate-free formulations. There is the perception that formulators often compromise the performance of a product to achieve sulfate-free status The overall message to formulators, both
novice and experienced, who are more used to developing the traditional sulfate-based products; sulfate-free formulating can seem initially a daunting and complex process due to the variety of raw materials available. A general rule of thumb is that binary anionic and amphoteric surfactant combinations are not always effective when optimising the viscosity of sulfate-free compositions and, therefore, there tends to be a requirement to combine three or more surfactant types.
Importance of formulation pH on selection of mild surfactants Consideration of the pH of the finished formulation cannot be underestimated when aiming for a sulfate-free product. Unlike alkyl ether sulfates, that demonstrate high water solubility (low Krafft temperature) and good hydrolytic stability across the personal care formulation pH range,1
the optimum and
effective pH window for the mild sulfate-free surfactants is much tighter. In the case of ester surfactants, that can be
hydrolysed at a low acidic pH, this will lead to losses in both performance and clarity. Those surfactants with an easily ionisable carboxylic acid head group will demonstrate a change in viscosity build as changes in pH will dramatically alter the packing parameter and the size of micelles formed.2
Middle East & Africa ■ Latin America ■ Asia Pacific ■
16.000 12.000 8.000 4.000 0
North America ■ Europe ■
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 Figure 1: Mintel data showing number of ‘sulfate-free’ product launches per year
in the case where the formulation is thickened by use of an electrolyte, additional salt will be required to achieve the desired viscosity. It is also advised that the surfactant ratio
of amphoteric to primary anionics, need to be ‘tuned’ specifically for the fragrance. For example, as shown in Figure 4, By altering the ratio of SLMI to SMCT in formulations the level of NaCl needed to build viscosity can be easily modified to accommodate wide range of fragrances and viscosity
The pH sensitivity
of Sodium Lauroyl Glutamate and Sodium Lauroyl Sarcosinate are seen in Figures 2 and 3, the effect of pH continue to impact the viscosity even when present as a co-surfactant.
Optimising electrolyte thickening The inclusion of fragrances is another factor that will have a significant effect on the finished formulation. The typical inclusion range of fragrances is between 0.2 and 0.8%, depending on levels of allergens and the application type of the formulation, be that either leave on or rinse off. It is rather an obvious statement that
6%act SLMI, 2%act SLGT & 3%act CAPB at pH 5.2 6%act SLMI, 2%act SLGT & 3%act CAPB at pH 5.8 6%act SLMI, 2%act SLGT & 3%act CAPB at pH 6.4
8000 7000 6000 5000 4000 3000 2000 1000 0
Relative mildness of ‘sulfate-free’ surfactants compared to SLS and SLES Mildness to skin and, in the case of shampoos, eyes is a key characteristic that surfactants need to possess when used in finished formulations. Surprisingly there is no singular scientific definition or measure of the mildness of surfactants and blends thereof, instead this parameter is usually defined by the absence of a series of negatives. In general, mildness is associated with the absence of any irritation to the skin, commonly
6%act SLMI 3%act CAPB at pH 6.4 6%act SLMI 3%act CAPB at pH 5.8 6%act SLMI 3%act CAPB at pH 5.2
manifested as redness, elevated temperature, swelling and a general perception of pain. From a medical standpoint skin irritation can be classified as irritant contact dermatitis, defined as a biological response to abrasion to the skin or irritation due to exposure to a chemical, such as a surfactant.3 The skin can also demonstrate irritation in the absence of skin sensitisation, known as allergic contact dermatitis. This type of dermatitis is commonly associated as resulting from the skins contact with other factors such as certain metals in jewellery, fragrances and preservatives.3
The latter two components
require the modern-day formulator to exercise caution in their selection of ingredients for finished formulations. Outside of direct irritation or sensitisation
of the skin, mildness can also be defined as the lack of skin barrier disruption and the absence or lack of induced skin dryness. These effects lead to the predisposition of the skin to irritation arising from the surfactants themselves or to other chemicals. Surfactants possess the ability to affect
6%act SMCT, 4%act CAPB pH 4.5 6%act SMCT, 4%act CAPB pH 5.0
6%act SMCT, 4%act CAPB pH 5.5 6%act SMCT, 4%act CAPB pH 6.0
14000 12000 10000 8000 6000 4000 2000 0
0 0.5 1
1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 Salt / % (w/w)
Figure 2: Viscosity response of Sodium Lauryl Methyl Isethionate and Cocamidopropyl betaine with Sodium Lauryl Glycinate effect of pH
PERSONAL CARE November 2022
4.0%act SMCT, 4%act CAPB 2% SLSar pH 4.5 4.0%act SMCT, 4%act CAPB 2% SLSar pH 5.0
4.0%act SMCT, 4%act CAPB 2% SLSar pH 5.5 4.0%act SMCT, 4%act CAPB 2% SLSar pH 6.0
0
2
4 Salt / % (w/w)
Figure 3: Viscosity response of Sodium Methyl Cocoyl Taurate and Cocamidopropyl betaine with Sodium Lauryl Sarcosinate effect of pH
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6
8
Viscoisty 20˚C/cps
Viscoisty / (Cp) 20˚C
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