Lignin self-assembly phenomena and valorization strategies for pulping, biorefining, and materials development: Part 2. Factors affecting the specificity of lignin self-assembly for industrial applications
Nelson Barrios, Ronald Márquez, Ramakrishna Trovagunta, Laura Tolosa, Antonio Suarez, Franklin Zambrano, Ronalds González, Lokendra Pal, Martin A. Hubbe
Abstract
This review considers a profoundly underutilized resource, technical lignin, and its potential for large scale upgrading for higher-valued industrial usage by means of self-assembly processes. Molecular interactions that can be used to guide lignin self-assembly are systematically explored, categorizing them into physicochemical interaction-driven assembly and external stimuli or template-driven assembly. Published findings are examined to reveal molecular mechanisms governing lignin aggregation into lignin nanoparticles (LNPs), films, and interfacial behavior in Pickering emulsions that have potential to be used industrially. Recent advancements in experimental techniques are explored to provide deeper insights into lignin's self-assembly processes. Hydrophobic effects, π-π stacking, hydrogen bonding, electrostatic layering, polyelectrolyte complex formation, chain entanglement, and covalent cross-linking are critically assessed as potential means to control the self-assembly of lignin and systems involving lignin. Additionally, external factors, such as chemical dehydration, solvent-mediated interactions, and external fields are examined related to their role in templating lignin assembly. Based on a comprehensive review of the literature, hydrophobic interactions are predominant in lignin aggregation, with hydrophobicity degrees varying significantly across lignin samples. Interfacial rheology studies demonstrate that lignosulfonate exhibits maximum storage moduli at oil-water interfaces, significantly enhancing emulsion stability. Additionally, modified lignins via esterification contribute larger lifetimes of water-in-oil emulsions stability under varying salinity and oil types. The integration of molecular modeling with experimental characterization techniques can further optimize lignin-based materials for multiple applications, such as drug delivery, catalysis, advanced pesticide delivery systems, bioplastics, 3D printing, and emulsification, among many others. Although there are existing technical and economic assessments (TEA) and life cycle assessments (LCA) involving lignin self-assembly that point to promising prospects, there is a need for more comprehensive TEA and LCA work to clear the way for the needed industrial innovations in this field.