Wetting and adhesion evaluation of cosmetic ingredients and products: correlation of in vitro – in vivo contact angle measurements

Abstract

Objective

The aim of this work was to use the contact angle measurement in order to predict the behavior of ingredients and finished cosmetic products on skin to improve skin feel and product texture.

Method

Different classes of cosmetic ingredients and formulations were evaluated. The contact angle measurements were carried out by the sessile drop methodusing an apparatus, designed and set up in laboratory. Glass, Teflon and human skin were the reference substrates. In a preliminary phase TEWL parameter, sebum content and hydration of human skin were measured in order to set up method.

Results

Data demonstrated that glass substrate may be used as replacement of the skin:critical surface tension of skin and glass were about of 27 and 31 dyne/cm, respectively. Non ionic surfactant with increasing HLB were evaluated: a correlation between contact angle measured and HLB was not observed because of different and complex molecular structure. In detail, Ethylexylhydroxystearate (θ_glass = 17,1°) showed lower contact angle value with respect to Polysorbate 20 (θ_glass = 28.1°). Sodium Laureth Sulfate and Steralkonium Chloride were also evaluated: anionic molecule showed more affinity for glass with respect to Teflon (θ_glass = 21.7° and θ_Teflon = 52.3°). Lipids and silicones showed different affinity for substrate according to hydrophilic groups and hydrocarbon chain: contact angles of silicones remained unchanged independently from substrate. Finished cosmetic products (O/W, W/O emulsions, cleansing oil, dry skin oil) showed different profiles according to surfactant and its affinity for continuous phase of the formulation. Comparing the values of the contact angle on skin of non-ionic surfactants, as Ethylexylhydroxystearate and Polysorbate 20, they showed values lower (near to zero) than ones of Sodium Laureth Sulfate and Steralkonium Chloride (21.7° and 66.8°, respectively). Finally, finished cosmetic productstested on human skin, showed different profile: corresponded contact angle values were less than 20°. The product tended to be quickly adsorbed on human skin.

Conclusion

Systematic study carried out by evaluating the wettability of single cosmetic ingredients on different substrates allowed to find correlations between the use of certain ingredients and the final performance of a cosmetic product.

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