Experimental and Simulation Study of the Solvent Effects on the Intrinsic Properties of Spherical Lignin Nanoparticles

This work made use of Aalto University Bioeconomy Facilities and OtaNano─Nanomicroscopy Center Facilities. Bin Zhao is thanked for conducting the N2 adsorption–desorption experiments. T.Z. acknowledges funding from the Novo Nordisk Foundation (SUSCELL project, Reference Number: NNF17OC0027658), M.V. acknowledges funding from Fortum and Neste Foundation (grant no. 20200079), P.P. acknowledges funding from the Academy of Finland (grant no. 315768), and M.S. acknowledges funding from the Academy of Finland (grant no. 309324). Computational resources by CSC IT Centre for Science, Finland, and RAMI─RawMatTERS Finland Infrastructure are gratefully acknowledged. The authors are also grateful for the support by the FinnCERES Materials Bioeconomy Ecosystem and Photonics Research and Innovation (PREIN) flagship.Spherical lignin nanoparticles (LNPs) fabricated via nanoprecipitation of dissolved lignin are among the most attractive biomass-derived nanomaterials. Despite various studies exploring the methods to improve the uniformity of LNPs or seeking more application opportunities for LNPs, little attention has been given to the fundamental aspects of the solvent effects on the intrinsic properties of LNPs. In this study, we employed a variety of experimental techniques and molecular dynamics (MD) simulations to investigate the solvent effects on the intrinsic properties of LNPs. The LNPs were prepared from softwood Kraft lignin (SKL) using the binary solvents of aqueous acetone or aqueous tetrahydrofuran (THF) via nanoprecipitation. The internal morphology, porosity, and mechanical properties of the LNPs were analyzed with electron tomography (ET), small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), and intermodulation AFM (ImAFM). We found that aqueous acetone resulted in smaller LNPs with higher uniformity compared to aqueous THF, mainly ascribing to stronger solvent–lignin interactions as suggested by MD simulation results and confirmed with aqueous 1,4-dioxane (DXN) and aqueous dimethyl sulfoxide (DMSO). More importantly, we report that both LNPs were compact particles with relatively homogeneous density distribution and very low porosity in the internal structure. The stiffness of the particles was independent of the size, and the Young’s modulus was in the range of 0.3–4 GPa. Overall, the fundamental understandings of LNPs gained in this study are essential for the design of LNPs with optimal performance in applications.Peer reviewe