Abstract
Phosphorylated kraft lignin (PKL), a novel lignin polymer, has potential application in the preparation and modification of inorganic materials. In this article, the adsorption characteristics of PKL at the solid/liquid interface were investigated under different pH conditions. Quartz crystal microbalance combined with dissipation monitoring (QCM-D), UV spectroscopy, and atomic force microscopy (AFM) were applied for monitoring the adsorption. The adsorbed amount of PKL gradually decreased as a function of pH increment on the gold covered QCM-D crystals, which were pretreated with an adsorbed layer of poly(diallyldimethylammonium chloride) (PDAC). This was also the case for quartz slide substrates pretreated with PKL/PDAC multilayers by the method relying on the layer-by-layer (LBL) electrostatic self-assembly. The hydrophobicity of adsorbed PKL films increased with increasing pH from 3 to 12. In the course of this process, the adsorption morphology changed from a compact rigid state to a less compact soft state. Under acidic conditions, the adsorption is governed mainly by cation-π interaction between PKL and PDAC, and the adsorption configuration is mushroom-like. Under neutral and alkaline conditions, the cation-π interaction is less relevant owing to the longer distances between the anionic groups in PKL and the cation in PDAC. This causes stretching of the molecular chains in PKL, while the adsorption configuration changes to a planar pancake structure so that the electrostatic interaction dominates. The knowledge on adsorption characteristics of PKL at different pH values is helpful to prepare some lignin-based composite materials.