Skin critical surface tension: A way to assess the skin wettability quantitatively.

Abstract

Surface energy phenomena result from molecular interaction between substances. On the skin, they are involved in sweat and sebum dispersion, and in foreign substances, in spreading and absorption. However, they are unknown due to the scarce number of studies. The aim of this work was to find the skin critical surface tension (CST), an index of wettability, through a reliable technique, and to study the influence of skin lipid and of some oil-in-water (O/W) and water-in-oil (W/O) preparations. The CST was calculated using the Zisman equation, from the contact angle at equilibrium, of droplets of liquids whose surface tension was known. Contact angles were computed from the base and the height of the droplets viewed from their side through an operating microscope provided with a slanted mirror. Both volar forearm and forehead were studied. On volar forearm, the skin CST was 27.5±2.4 dyne/cm (n=12) and normally distributed. Forehead CST was above 50.7 dyne/cm. On both areas, defatting with ether reduced CST down to 21.6±2.6 dyne/cm, and 29.3±1.7 dyne/cm, respectively (p<0.001). Washing with soap reduced forearm CST down to 23.7±1.0 dyne/cm (p<0.01), and occlusion down to 21.6±3.4 dyne/cm (p<0.001). O/W cream application strongly decreased both forearm and glass slide CST, whereas W/O cream application either raised CST or left it unchanged; in all cases, changes were less marked on skin than on glass. These data demonstrate that the skin surface is primarily hydrophobic and paradoxically becomes more wettable through its lipidic component, especially the sebum. Emulsions seemed to interact with the skin and change its CST as would do their continuous phase only.