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

Tao Zou,Maria Sammalkorpi,Mohammad Khavani,Aleksi Zitting,Paavo Penttilä,Nonappa Nonappa,Juan José Valle-Delgado,Maisa Vuorte,Mikhail Balakshin,Dorothee Silbernagl,Anna Maria Elert,Monika Österberg

doi:10.1021/acs.jpcb.1c05319

Abstract

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.

Highlights

Spherical lignin nanoparticles (LNPs, called colloidal lignin particles (CLPs) or lignin nanospheres) fabricated from isolated lignins have emerged in recent years due to their advantageous features such as large surface area per unit mass, tunable surface charge, well-defined spherical shape, and colloidal stability in aqueous media in addition to the inherent properties of lignin like antioxidant and UV-screening properties.[1,2] These features have triggered numerous studies in exploiting advanced applications of LNPs
LNPs were prepared from softwood Kraft lignin (SKL) using the binary solvent of aqueous acetone or aqueous THF
We investigated the solvent effects on the intrinsic properties of spherical LNPs using the binary solvents of aqueous acetone and aqueous THF

Summary

■ INTRODUCTION

Spherical lignin nanoparticles (LNPs, called colloidal lignin particles (CLPs) or lignin nanospheres) fabricated from isolated lignins have emerged in recent years due to their advantageous features such as large surface area per unit mass, tunable surface charge, well-defined spherical shape, and colloidal stability in aqueous media in addition to the inherent properties of lignin like antioxidant and UV-screening properties.[1,2] These features have triggered numerous studies in exploiting advanced applications of LNPs. Similar mean diameters of 44 ± 16 nm for LNPsacetone and 61 ± 21 nm for LNPsTHF were obtained by small-angle X-ray scattering (SAXS), using a model of homogeneous spheres with log-normal size distribution to analyze the data measured from LNP dispersions (Figure 1g,h). The interaction distances between the C atoms of the organic solvents and the benzene or methoxy groups of the lignin models have the following order: DMSO < acetone < THF ∼ DXN (data, e for model L1 and in Figure S6 for models L2 and L3) This order correlates well with the particle size of LNPs. Combining the solvent−lignin hydrophobic interaction and the solvent−lignin H-bonds results, we suggest that the stronger the solvent− lignin interactions, the smaller the LNP size. The E values are comparable to that of PS NPs (1−3 GPa),[91] suggesting that LNPs could potentially replace PS NPs in many applications

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES