New Insight into the Surface Structure of Lignin Nanoparticles Revealed by 1H Liquid-State NMR Spectroscopy

H-1 liquid-state nuclear magnetic resonance (NMR) spectroscopy was applied for the first time to lignin nanoparticles (LNPs) in an aqueous suspension to study the surface composition of LNPs and to acquire a better understanding of the mechanism of their formation. A series of LNPs were prepared from spruce and eucalyptus kraft lignin fractions with narrow molecular weight distributions and functionalities. An NMR pulse program combining presaturation with excitation sculpting increased the signal resolution, making it possible to observe a superficial layer of LNPs directly in the aqueous suspension, as prepared. The particle size, charge, and surface morphology were characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS). According to liquid-state NMR, methoxy groups from syringyl and guaiacyl units of lignin are the main groups present on the surface of LNPs. The presence of aliphatic moieties, mainly from side chains of lignin molecules, has also been confirmed. Taking into consideration the chemical composition of the lignin fractions, the structure of lignin NPs as shown by NMR spectroscopy and their size and surface charge, a pattern of lignin self-assembly into LNPs has been suggested.