Influence of Acetylation on the Microstructure of Alkali Lignin in Tetrahydrofuran

The effect of acetylation on the microstructure of alkali lignin (AL) in tetrahydrofuran (THF) was studied. The physicochemical properties of AL and acetylated alkali lignin (ACAL) were compared. Dynamic light scattering, gel penetration chromatography (GPC), transmission electron microscopy (TEM), intrinsic viscosity, fluorescence spectrophotometry, quartz crystal microbalance with dissipation (QCM-D) measurements, and layer-by-layer (LbL) self-assembly were used to investigate the aggregation and adsorption of AL and ACAL. Intermolecular and intramolecular associations in AL decreased following acetylation. AL in THF existed mainly as mono-molecules and intermolecular associations and had a more compact conformation. ACAL existed mainly as mono-molecules with a more stretched conformation. ACAL had a larger hydrodynamic radius than AL(of the same absolute molecular mass), because of its stretched conformation. QCM-D and LbL self-assembly measurements suggested that AL aggregates have a strong tendency to adsorb at the liquid-solid interface, but ACAL hardly adsorb at this interface because of its stronger interaction with THF. The compact conformation and strong adsorption property of AL resulted in inaccurate molecular weights of non-acetylated AL, as measured by GPC with a THF mobile phase. In order to get more reliable molecular weights from GPC measurements, acetylation for AL should be used in the sample pretreatment.