Solvent effect on the production of spherical lignin nanoparticles

Abstract

Controlling the self-assembly behaviors of soluble lignin oligomers is key to construct uniformly spherical lignin nanoparticles (LNPs) and benefit to their specific applications. Herein, several H₂O-organic solvent systems were selected for solvolysis of the lignin in wood chips to obtain the lignin oligomers with different structural features. Afterwards, the reaction liquors containing soluble lignin oligomers were employed to produce LNPs via continuous adding H₂O. The self-assembly behaviors of the lignin oligomers were strongly influenced by the structural features of the lignin oligomers and the existing organic solvents. From solvothermal extraction in H₂O-polar aprotic solvents (e.g. tetrahydrofuran, acetone, and γ-valerolactone), the lignin oligomers with widely-ranged molecular weight could self-assemble into regular-shaped and spherical LNPs, making use of every single space. Besides, the conjugated aliphatic side chains with benzene rings and the abundant negative charged groups of the lignin oligomers favored their self-assembly into LNPs with core–shell structures, and the resultant high electric surface charge prevented further aggregation of LNPs into irregular-shaped clusters. Conversely, the existing polar protic solvents (e.g. ethanol and ethylene glycol) and polysaccharides-derived chemicals, dispersed the lignin oligomers and restrained their π–π stacking induced self-assembly via generating undesirable hydrogen bonds. Therefore, ascertaining the ripple effects of the organic solvents on the whole fabrication process of LNPs benefits to control the morphology of LNPs for various applications.