The Study of Water Sorption with Hydrolysis Lignin by Solid-State NMR Spectroscopy

Hydrolysis lignin is formed as a by-product of cellulose production and has limited industrial application. The ability of hydrolysis lignin to absorb and retain some water is an important aspect for the study of its properties and modification methods. The processes of water sorption by hydrolysis lignin were studied with solid-state NMR spectroscopy. The samples were humidified in desiccators containing different saturated salts solutions with different relative air humidity above them. The sorption capacity of the samples was determined by water sorbed from the air, and it was found that lignin absorbs the amount of water equal to 40% of sample weight at a maximum relative humidity of the air. The cross-polarization (CP) and magic angle spinning (MAS) methods were used to register solid-state NMR spectra. Using the H-1-NMR spectra, it was found that the hydrolysis lignin is hydrated in the whole volume, and the water penetrates into the deep layers of polymer, however, the distribution of water at the likely sorption sites is uneven. It was obtained with the use of C-13-NMR spectroscopy that hydrolysis lignin hydrates in both hydrophilic and hydrophobic regions of the macromolecule, and the bulk of sorbed water (similar to 64%) concentrates around the hydroxyl and methoxyl groups of lignin and polysaccharide residues.