Molecular modelling of lignin behavior in fiber walls

Lignin is a major component of wood cell walls, and as such, contributes to the properties of pulp fibers and fiber materials. Despite the natural abundance of lignins, their structure-function relationships remain rather poorly understood. This is both due to the chemical heterogeneity of lignins and the difficulty of isolating them without altering their structure. Recent advances in the chemical analysis of lignins have enabled a statistical description of their complex molecular structure [1]. This, in turn, has enabled the use of molecular modelling to study their fundamental properties and behaviors.

We report molecular simulations on the structure and physical properties of softwood lignin, including its role in the moisture behavior of wood cell wall structures and thermal response in the hot-pressing process for a fiber web. We have created a molecular model for native-like softwood lignin based on a statistical description of its chemical structure [1]. Using a molecular force field developed for lignins [2], we have carried out atomistic simulations to study the behavior of cellulose-lignin interfaces and neat lignin at different moisture contents and temperatures. We compare the predictions at ambient temperature with X-ray scattering experiments on native and delignified spruce samples to understand the role of lignin in the moisture response of wood microfibril structures [3]. We compare the predictions at elevated temperatures with hot-pressing experiments on fiber webs at varied temperatures, sample moisture contents and pressing times to determine the role of lignin softening and diffusion in the observed wet strengthening.

Our work is an early step toward more systematic use of molecular modelling to understand lignin structure-function relationships. We anticipate that combined chemical analysis and simulation will find increasing use in the development of lignin-based materials, including the thermal and wet processing of lignin-containing fiber materials.

[1] M. Balakshin, E. A. Capanema, X. Zhu, I. Sulaeva, A. Potthast, T. Rosenau and O. J. Rojas. Spruce milled wood lignin: linear, branched or cross-linked? Green Chem., 22:3985-4001, 2020.
[2] J. Vermaas, L. Petridis, J. Ralph, M. F. Crowley and G. T. Beckham. Systematic parameterization of lignin for the CHARMM force field. Green Chem., 21:109-122, 2019.
[3] A, Zitting, A. Paajanen, M. Altgen, L. Rautkari and P. A. Penttilä. Role of Lignin in Moisture Interactions of Cellulose Microfibril Structures in Wood. Small Struct., 5:2400167, 2024.