Terahertz time-domain spectroscopy as a novel tool for crystallographic analysis in cellulose: insights into the complex lattice interaction with water molecules

This study explored a method for investigating crystalline cellulose and its interaction with water molecules using terahertz time-domain spectroscopy (THz-TDS). Cellulose-lignin composite samples with different cellulose contents were placed under varying humidity conditions to assess their spectral changes. The humidity-dependent terahertz response of cellulose-lignin composites with different cellulose contents was systematically investigated. The water content of all samples increased with increasing relative humidity, and time-domain nuclear magnetic resonance (TD-NMR) confirmed that adsorbed water existed in the form of bound water. THz-TDS revealed a strong sensitivity of the complex dielectric constant to humidity. Two characteristic dielectric loss peaks were observed near 2.13 THz and 3.04 THz. The peak center position of the 2.13 THz mode first decreased and then increased with humidity, while the 3.04 THz mode showed the opposite trend, indicating anisotropic water-cellulose interaction. Two-dimensional correlation spectroscopy (2D-COS) analysis demonstrated that the change in the centroid at these two positions was due to a change in the order of the response of the THz dielectric loss to humidity at different frequencies. These results demonstrate that THz-TDS is a sensitive and non-destructive method for detecting dynamic changes induced by hydration in crystalline cellulose.