pH-Dependent interaction mechanism of lignin nanofilms

Lignin has been spotlighted as an abundant renewable bioresource for use in material technologies and applications such as biofuels, binders, composites, and nanomaterials for drug delivery. However, owing to its complex and irregular structure, it is difficult to investigate its fundamental interaction mechanism, which is necessary to promote its use. In this study, a surface forces apparatus (SFA) was used to investigate the pH-dependent molecular interactions between a lignin nanofilm and five functionalized self-assembled monolayers (SAMs). The lignin nanofilm adhered most strongly to the amine-functionalized SAM, indicating that the molecular interactions with lignin were mainly electrostatic and cation-π interactions. The force-distance profile between lignin and a methyl-functionalized SAM revealed pH-dependent interactions similar to those between two lignin nanofilms. This finding indicates that the dominant cohesion mechanism is hydrophobic interactions. A quartz crystal microbalance with dissipation was used to investigate the adsorption of free lignin molecules on functionalized SAMs. Lignin molecules, which were free in solution, were most effectively adsorbed to the phenyl-functionalized SAM. To investigate whether the nanoscopic interaction forces could be extended to macroscopic properties, the compressive strength of activated carbon-lignin composites prepared at different pH values was evaluated. As the pH increased, the compressive strength decreased owing to the reduced hydrophobic interactions between the activated carbon and lignin, consistent with the SFA results. These quantitative results regarding lignin interactions can advance the potential use of lignin as an eco-friendly biomaterial.