Well-Defined Lignin Model Films from Colloidal Lignin Particles

The transformation of a molecularly complex and irregularly shaped lignin into a nanoscale spherical architecture is anticipated to play a pivotal role in the promotion of lignin valorization. From the standpoint of using colloidal lignin particles (CLPs) as building blocks for a diverse range of applications, it has become essential to study their interactions with soluble compounds of varied origin. However, the lack of model films with well-defined surface properties similar to those of CLPs has hindered fundamental studies using surface-sensitive techniques. Here, we report well-defined and stable thin films prepared from CLPs and demonstrate their suitability for investigation of surface phenomena. Direct adsorption on substrates coated with a cationic anchoring polymer resulted in uniform distribution of CLPs as shown with atomic force microscopy (AFM). Quartz crystal microbalance with dissipation monitoring (QCM-D) experiments revealed higher adsorbed mass of cationic lignin onto the CLP-coated substrate in comparison to the film prepared from dissolved lignin, suggesting preferential adsorption via the carboxylic acid enriched surfaces of CLPs. QCM-D further enabled detection of small changes such as particle swelling or partial dissolution not detectable via bulk methods such as light scattering. The CLP thin films remained stable until pH 8 and displayed only a low degree of swelling. Increasing the pH to 10 led to some instability, but their spherical geometry remained intact until complete dissolution was observed at pH 12. Particles prepared from aqueous acetone or aqueous tetrahydrofuran solution followed similar trends regarding adsorption, pH stability, and wetting, although the particle size affected the magnitude of adsorption. Overall, our results present a practical way to prepare well-defined CLP thin films that will be useful not only for fundamental studies but also as a platform for testing stability and interactions of lignin nanoparticles with materials of technical and biomedical relevance.