Lignin Nanoparticle Morphology Depends on Polymer Properties and Solvent Composition: an Experimental and Computational Study

Kraft lignin is an underutilized sideproduct from the pulp and paper industry that can be used as a feedstock for various bio-based products. In particular, microstructured lignin particles are used as functionalized materials in applications such as the controlled release of active substances and the absorption of contaminants and as polymer additives. Gaps in our understanding of lignin molecular interactions and morphology in various solvents limit our ability to tailor processes to produce particles with desired properties. We empirically demonstrate that the dispersity and aqueous phase polarizability influence lignin polymer chain conformation and thus the resulting nanoparticle morphology. Our complementary experimental and computational studies investigate lignin dynamics and indicate that a high molar mass polymer with a low polydispersity index, which contains numerous aromatic structures, will aggregate via hydrophobic interactions and form compact globules in an aqueous environment. Even in the presence of a sacrificial negatively charged surfactant sodium dodecyl sulfate, electronic interactions of aromatic units predominate, resulting in a compact globule after the surfactant is removed. Additionally, the number of inter-lignin contacts, solvent-polymer hydrogen bonds, and radius of gyration for a high molar mass, low polydispersity index lignin polymer remained almost constant in water and in the mixed (water/ethanol) solvent system containing the surfactant. We also report dendritic lignin nanoparticles obtained experimentally from low molar mass, high polydispersity index lignin, in a water/ethanol mixed aqueous phase. Molecular simulation results suggest that the intermolecular forces driving this morphology are stable inter-lignin contacts over time and enhanced lignin-solvent hydrogen bonding in the mixed solvent. Our results demonstrate the controlled behavior of lignin nanoparticle morphologies in aqueous solvents. This work contributes toward establishing property-performance correlations for lignin and advances knowledge for the synthesis of shape-specific lignin nanoparticles.