Control over the size and shape heterogeneity of lignin particles (LP) has challenged their conversion and applications, the latter of which include optical devices, coatings, biochemical sensors, among many others. It is often of interest to fractionate particle into populations with a low size dispersity and controlled assembly. For this purpose, it is desirable to have a theoretical understanding of related fractionation and aggregation processes. <br/>Two main methods used for lignin fractionations include selective precipitation and ultra-filtration. However, both operations are energy-intensive and time-consuming. Hence, a particle size fractionation method by two-stage centrifugation is proposed to process highly polydispersed lignin colloids into monodispersed particles (PDI &lt;0.06, dynamic light scattering) in a broad range of average sizes, from 58 to 857 nm. The proposed methodology provides a systematic linear fractionation of LP based on the effective centrifuge capacity and capacity factor. Depending on the initial particle size distribution, one or multiple linear fractionation ranges could be developed, which guides the LP size fractionation on a potentially industrial scale. The correlation of LP size with functionality was further validated by <sup>31</sup>P nuclear magnetic resonance and gel permeation chromatography.<br/>The aggregation behavior of monodispersed LP (157 nm average size) was induced by pH and salt adjustment. A two-stage pattern was observed: fast diffusion-limited and slow reaction-limited stages. A population balance model was applied to quantitively describe the aggregation with parameters that fit the collision efficiency. The mass fractal index was measured by static light scattering. The collision efficiency was found to increase with aggregation rate and exponentially decreases with cluster size. The salt and pH critical aggregation points correspond to 24 mM and pH=2, respectively. In sum, we unravel the LP aggregation mechanism induced by pH adjustment or salt addition to construct a road map for LP particle assembly in aqueous systems, assisting in the development of materials for advanced applications.

Monodispersed lignin particles and self-assembly into superstructures

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