Comparison of bottom-up and top-down precipitation strategies for lignin nanoparticle obtention from organosolv and ionosolv Eucalyptus globulus liquors.

This work offers a side-by-side overview of the behaviour of two liquors obtained via two fractionation processes, ionosolv and organosolv, from Eucalyptus globulus wood, and how the precipitation strategy that follows may affect the final yield, morphology, and particle size of every kind of lignin nanoparticle. For lignin nanoparticle precipitation, two bottom-up techniques and two top-down approaches were employed to determine which combination of fractionation process and synthesis treatment would provide the nanoparticles with the best characteristics. The results demonstrated the importance of the fractionation process in the final lignin nanoparticle yield, as ionosolv fractionation gave enhanced yields of more than 60% lignin in the form of nanoparticles. However, sphericity, particle sizes, and non-agglomerated structures were easily obtained from organosolv liquors, in which precipitation was carried out progressively in the absence of sonication. The use of ultrasound mostly resulted in the breakage of particles into smaller and irregular pieces. However, in the case of ionosolv liquors, homogeneous spherical nanoparticles were fused, forming agglomerates of smaller particles through the top-down strategy of complete addition of the antisolvent followed by sonication. The highest precipitation yield of nanoparticles was obtained from ionosolv liquors in which the full amount of antisolvent was added in one step to precipitate lignin, and then sonication was applied. In contrast, the lignin nanoparticles (LNPs) precipitation strategy that resulted in more spherical LNPs was the bottom-up strategy of precipitation by progressive antisolvent addition, resulting in visually observed non-aggregated spherical particles with a particle size distribution of 200 nm < d_p < 500 nm, molecular weight of M_w = 14 000 g mol^-1, and thermal degradation property of T_10% = 310 °C.