Background: Natures and amounts of water retained at the surface of stratum corneum (SC) of human skins, affecting skin health and penetration of chemical components, remain unclear.
Methods: A keratin film, a main component of human SC surface, was measured by IR micro-spectroscopy combined with a quartz crystal microbalance (QCM) and a relative humidity (RH) control system.
Results: Water contents increased with RH up to about 19 wt% and were correlated linearly with the OH + NH band areas in IR spectra of the keratin film. The OH + NH band areas for the triple helix collagen film are about twice as large as those for the keratin film (double helix). The free water component increases with RH by keeping the bound water component minor for the keratin film. About twice of water retention capacity of the collagen film can be due to increasing adsorption of free water, interacting possibly with hydrophobic aliphatic CH surfaces.
Conclusion: The present results suggest relatively low water contents less than about 19 wt% of outermost SC layers of human skin composed mostly of keratin exposed to ambient RH conditions. The triple helix collagen can be used as an effective moisturizing agent.
Keywords: bound water; collagen; free water; hydrogen bonding; infrared spectroscopy; keratin; quartz crystal microbalance; relative humidity; water molecules.
© 2018 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.