Differential scanning calorimetric studies on the melting behavior of water in stratum corneum

The melting behavior of water in human stratum corneum (s. corneum) has been studied by using differential scanning calorimetry (DSC) in the temperature range from -40 degrees to 20 degrees C. The DSC thermogram was analyzed in terms of the amount of bound water and the melting temperature of water in s. corneum. Extraction of the s. corneum with the mixed solvent of chloroform: methanol (2:1, v/v) or 0.5% sodium dodecyl sulfate aqueous solution decreased the bound water content, whereas extraction with water did not change the bound water content. The melting temperature of water in the s. corneum was lowered as the water contents decreased. Extraction of the water-soluble components from the s. corneum increased the melting temperature of water when the water contents were constant. The results suggest that 20-30% of water in the s. corneum is bound water interacting strongly with the protein or lipids in the s. corneum, and the excess of water over the bound water content is unbound water solubilizing the water-soluble components such as amino acids and urea in the s. corneum. The thermodynamic theory for freezing-point depression is favorably applied to the melting temperature change of the unbound water, which implies that the water-soluble components are present as an aqueous solution in the s. corneum. Measurements of the melting-point depression of water in s. corneum provide us the quantitative information on the amount of water-soluble components in the s. corneum. This technique is a sensitive and useful tool to evaluate the hydration behavior of s. corneum.