FORMULATING FOR MILDNESS 9%act SLMI 3%act CAPB
6%act SLMI 3%SMCT, 3%act CAPB 4.5%SLMI, 4.5%SMCT, 3%act CAPB
18000 16000 14000 12000 10000 8000 6000 4000 2000 0
3%act SLMI 6%act, 3%act CAPB 9%act SMCT 3%act CAPB
35 30 25 20 15 10 5 0
57 14 Day cumulative Skin Irritation 0.5% Active pH 6.0
0
1
2
3 NaCI / % (w/w) Figure 4: Viscosity response of SLMI and SMCT with CAPB effect of NaCl
the integrity of the lipid barrier in the stratum corneum and thus increase the skin permeability therein.4
Surfactant solutions,
when topically applied can also swell corneocyte proteins, resulting in the removal of natural moisturising factors. This can increase and enhance the penetration of irritant chemicals into the viable epidermis.5 The belief here is that the penetration
directly leads to skin dryness and increases the potential and likelihood of the development of skin irritation. Additionally, surfactants can demonstrate an effect on the biology of the stratum corneum. As an example, sodium lauryl ether sulfates are reported to inhibit the enzymes that control and regulate desquamation within the stratum corneum. This can lead to scaling, dryness of the skin and result in impairment of the skin barrier function.6
Mildness testing We have mentioned that specifically in the case of shampoos, mildness to the eyes is exceptionally important. The advent of the
Pureact WS Conc & CAPB
Figure 5: Relative irritation potential of anionic surfactants via human volunteer repeat insult patch testing
introduction of a ban on animal testing as part of the current cosmetics directive, its prohibition in other markets outside of the EU, and the general feeling of consternation by consumers, has meant alternative in vitro methods to support mildness and ‘no tears’ claims for personal care ingredients and finished products, particularly the red blood cell test.7 Investigation of the results and test data
from several studies has correlated the relationship between the red blood cell test data for a series of surfactant classes and in vitro eye irritation data.6
The observations
demonstrated a clear relationship between reported eye irritation data and the concentration of surfactants and finished products (in this case shampoos), required to result in breakage of cytoplasmic membranes (cell haemolysis) and denaturation of haemoglobin. Additionally, the data demonstrated a trend
that the order of irritancy followed this path: cationics (demonstrating severe eye irritation)
Conc & CAPB & Iselux Pureact WS
Pureact WS Conc & CAPB & Pureact I-78
> anionics and amphoterics (moderate) > non-ionics (moderate to non-irritant). The red blood cell test continues to be utilised as a tool to screen shampoo formulations for their mildness to mucosal membranes. One additional example uses red blood cell test data to demonstrate that increasing the proportion of an amphoteric surfactant, e.g. Cocamidopropyl betaine, as part of an anionic/amphoteric blend significantly reduces the potential for irritancy.8
Skin mildness testing The mildness of surfactants on skin has also been correlated with their tendency (or lack of) to interact and denature zein proteins.9 The study demonstrated that for a series of surfactants, skin patch test data correlate well very well with the results from zein dissolution tests.
Another benefit from this study is that the data has allowed for the development of a structure-activity relationship that can accurately predict the zein dissolution data for surfactant structures. Mildness of surfactants has also been linked to their effects on skin lipids.10
Results of the work reported
demonstrated that lipid solubilisation and disruption tests can produce data that proports to the inverse of the protein dissolution data. An example cited refers to
Pureact WS Conc & CAPB & Pureact LSR
Pureact WS Conc & CAPB & Pureact Gluco C
Pureact WS Conc & Empicol® CDL-60P
alkylpolyglucosides, well known for being mild and non-ionic in character, typically have low zein dissolution scores (therefore low protein disruption), also report high lipid bilayer disruption. This indicates that alkylpolglucosides can potentially be drying to the skin. Obviously, there will always be anomalous
Figure 6: Foam characteristics of co-surfactants blended with sodium methyl cocoyl taurate and cocamidopropylbetaine
www.personalcaremagazine.com
results, and the report surprisingly reveals that isethionates deliver a rather unique combination of both low zein dissolution scores and low lipid bilayer disruption effect. One final observation of note in that investigating the increasing the level of amphoterics (cocamidopropyl betaine) as a secondary surfactant alongside an anionic surfactant will reduce the zein dissolution score. However, at higher levels of cocamidopropyl betaine there is a noticeable increase in
4
5
6
PC November 2022 PERSONAL CARE
Viscoisty / (Cp20˚C)
Relative Irritation Score
Sodium Lauryl Sulfate Sodium Laureth Sulfate Cocamidopropyl Betaine
Sodium Lauroyl Methyl Isethionate Sodium Cocoyl Isethionate Sodium methly Cocoyl Taurate Sodium Lauroamphoacetate Decyl Glucoside
Sodium Lauryl Glycinate Deionised Water
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