Analysis of the hydration water on the surface of human hair using a combination of infrared absorption vibrational spectroscopy and multivariate curve resolution

Modern society's keen regard for aesthetics made hair products an integral part of a multi‐billion‐dollar cosmetic industry. Hair care products (e.g., shampoos and conditioners) and chemical treatments (e.g., bleaching and permanent waving) result in various effects on the morphological attributes of hair. Generally, water adsorbed on the hair surface is known to significantly dictate the hair's mechanical characteristics (smoothness and friction), and hair's macroscopic wettability has been commonly used to indicate its surface properties. However, an approach to selectively characterize the hydration water in the hair surface is required to accurately understand the intermolecular events between the hair and its vicinal water. In this paper, we successfully obtained the infrared (IR) absorption spectra of the hydration water of human hair. We employed the multivariate curve resolution‐alternating least square (MCR‐ALS) method to separate the hydration and bulk water spectra from the whole spectra. Comparing the IR spectra of the hydration water of chemically untreated and bleached hair samples, we conclude that water molecules form strong hydrogen bonds with the bleached hair surface due to the destruction of the hair's hydrophobic outer layer and the consequent formation of hydrophilic residues.