Temporal and spatial visualization of water uptake into human hair studied by infrared microscopy

Real-time examination of water behavior in cuticle, cortex, and medulla of untreated and bleached hair was performed by Fourier-transform infrared (FT-IR) microscopy. Penetration of heavy water from the cuticle to the medulla was visualized and quantified by monitoring the O-D stretching band located at 2500 cm^-1. The medulla of untreated hair had a significantly higher peak O-D intensity from 20 min after the heavy water penetration to 20 min after drying than that of bleached hair. Furthermore, heavy water evaporated 1 h slower in untreated hair than in bleached hair in any part. These differences may be due to the increase of S=O stretching of sulfonic acid caused to breaking of the disulfide and thioester bonds by bleach treatment. The method proposed in this study revealed potential application in visualizing and quantifying real-time water behavior in hair.