Introduction

An easy way to formulate gentle skin cleansers is to reduce the dose of surfactants, as is the case in micellar waters. However this poses the challenge of finding a good compromise with the cleansing properties, especially for makeup removal. Surfactant characteristics such as CMC (Critical Micellar Concentration), wetting properties, help in comparison but the measurement condition is far from the final formulation combining them with other ingredients. A method closer to the application condition is needed. This work aimed to develop a simple tool that provides a quick and reproducible answer to select effective cleansing ingredients for micellar water at an early stage of formulation development. Another interest of this in vitro tool is the ability to guide the development of new cleansing structures that can not be tested in vivo, pending comprehensive safety assessment and risk analysis processes. Well-known mild surfactant solutions and essential additives in micellar water compositions were tested at realistics use levels. The results were compared to negative controls and micellar water benchmarks to challenge the method and its ability to discriminate the cleansing performance.

Methods

Make-up removal performance evaluation is based on colorimetric measurement (L*,a*,b*) on a standard film of foundation before and after mechanical cleansing. The protocol has been adapted to be representative of the final micellar water routine. Two inverse emulsion foundations benchmarks claiming non-transferable efficacy (i.e. Water-in-Oil and Water-in-Silicone) were selected for their difficulty in being cleaned.
A foundation film at standard thickness was made on white carrier and then dried 12h to 24h at 40°C. Measurements of L* parameter (clarity) were firstly made, on the dry calibrated foundation film, using a chromameter (CR400 from Minolta company), in 5 different places of the film: these values represented the “make-up” values. A standard amount of ingredient solution (or benchmark formulas) was evenly dotted on a cotton pad attached to a cylindrical cleaning head. The cotton was applied to the film by 17 rotations for 30 seconds and L* measurements were again performed on the film in the same manner and represented the values “after make-up removal”. Cleansing performance was calculated by a ratio between the “makeup” and “ after makeup removal” values and expressed in % (average of 5 places on the film and 3 to 5 replicates).

Results

Among the parameters studied during the development of the method, pH of the surfactant solutions (4 to 11) showed no significant effect on performance and it was therefore decided to evaluate the solutions in their spontaneous state. On the contrary, the concentration of the materials proved to be critical. This highlighted the value of evaluating the performance of ingredients at low concentrations, representative of the use in micellar waters. A first evaluation of the performance of mild surfactants with different chemical structures at 10% Active Matter (AM) to compare the results with a previous publication, led to the selection of the most effective materials, in particular alkyl polyglucoside with short-chain structure and amino acid derivatives, while cocoyl sodium glutamate, lauroyl sodium sarcosinate, cocamidopropyl betaine and poloxamer 184 were discarded. A second, more realistic evaluation at 1% MA revealed significant differences between the surfactants tested and made it possible to compare their performances, alone and in combination. Essential additives, such as solubilizers and glycols, tested alone and in combination with surface-active solutions, had no impact on cleansing performance. Results obtained on micellar water benchmarks confirmed the consistency of the responses. Taken altogether, the trials demonstrated that the Water-in-Silicone foundation was the hardest to remove, irrespective of the cleanser's composition. It was also found that the method was not suitable for the evaluation of oils

Discussion and conclusion

The in vitro test based on colorimetric measurements after mechanical cleansing was revealed as a simple and reproducible tool to evaluate the effectiveness of the removal of makeup from surfactant solutions, at a concentration level similar to those used in micellar waters. It was also demonstrated that the method was suitable for studying the influence of ingredient combinations to optimize micellar water formulations. To date, only the clear solution form has been evaluated and an interesting prospect will be to question the suitability of the method for screening other water-based formulations: cream-gel and emulsions.