Well-Defined Lignin Model Films from Colloidal Lignin Particles

Abstract

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.

Experimental and Simulation Study of the Solvent Effects on the Intrinsic Properties of Spherical Lignin Nanoparticles

“…This observation is consistent with our previous findings. 31 We emphasize that LNPs were already present in particle form and spherical shape in the dispersion state before drying, as confirmed using cryogenic transmission electron microscopy (cryo - TEM) ( Figure 1 b,e, inset). Quantitatively, the mean diameters of LNPs acetone and LNPs THF were 47 ± 13 and 66 ± 22 nm, respectively, calculated based on over 300 ambient-dried particles according to TEM images ( Figure 1 c,f; see more TEM images in Figure S3 ).…”

“…LNPs acetone and LNPs THF were prepared following the same procedure described earlier. 31 In brief, 2 g of dry SKL was dissolved in 200 g of aqueous acetone (75 wt %) or aqueous THF (75 wt %) under magnetic stirring for 3 h and the undissolved residues were removed by filtration with paper filters (Whatman, pore size 0.7 μm). Then, the solutions were poured immediately (within approximately 1 s) into vortex-stirring DI water (solution/water = 1:2.5, w/w) to form LNPs instantly.…”

“…In terms of solvent effects, it has been reported that the binary solvent of aqueous acetone results in smaller D h and PDI than aqueous THF. 31 However, the underlying reasons remain unclear. Similar phenomena have been reported earlier for NPs prepared from synthetic polymers ( e.g ., polylactides) using a pure organic solvent, that is, acetone results in smaller particles than THF.…”

Multifunctional lignin-based nanocomposites and nanohybrids

“…377 As LNPs are known to have more hydrophilic surfaces compared to bulk lignin, further studies on protein adsorption would benefit form the recently described model films from colloidal lignin particles. 104 There are many opportunities for fundamental studies to shed more light on lignin-protein interactions, which is becoming increasingly important as the biomedical applications of LNPs are rapidly developing. 48,378 4.4.2.…”

“… 103 Thereafter, the surface decoration of lignins with functional groups is related to the assembly strategy, as associated with the amphiphilic nature of lignins. For instance, with the hydrophilic group of lignins being oriented towards the outside of the surface for precipitation-driven strategies in polar solvents, 104 and with the hydrophilic groups of lignins being oriented towards the inside of the material for aerosol-formed particles, as a result of the air–water interface. 9 However, even in the case of the aerosol-formed particles an anionic zeta potential in the range of −30 mV is reported, which suggest charge migration upon exposure to solvent.…”