Effects of Acid\nDissociation and Ionic Solutions on\nthe Aggregation of 2‑Pyrone-4,6-dicarboxylic Acid

The conversion of lignin can produce biomass-derived\naromatic compounds\nsuch as 2-pyrone-4,6-dicarboxylic acid (PDC), which is a potential\nsustainable precursor of bioplastics. PDC is a pseudoaromatic dicarboxylic\nacid that can aggregate in aqueous solution. Aggregation depends upon\nPDC–PDC, PDC-water, and PDC-ion interactions that are representative\nof interactions in similar charged, aromatic compounds. These interactions\nboth dictate PDC aggregation and the likelihood that PDC aggregates\nexhibit parallel stacking configurations that may promote PDC crystallization,\nwhich can be leveraged to separate PDC from solution. However, the\ninterplay of interactions that drive aggregation and structure formation,\nand how these depend upon the charge of PDC and ionic species present\nin solution, remains unclear. In this work, we investigate PDC aggregation\nin diverse ionic solutions using all-atom molecular dynamics simulations\nand molecular clustering analysis. We consider ion-induced dipole\ninteractions by using a modified Lennard-Jones nonbonded model for\ndivalent ions in solutions. From molecular clustering analysis, we\nderive characteristic parameters to quantify aggregate sizes and parallel\nstacking configurations. We show that acid dissociation facilitates\nPDC aggregation in ionic solutions via ion-mediated interactions,\nand different ionic solutions influence both the likelihood of aggregation\nand the formation of parallel aggregates. In particular, we find that\nparallel stacking is primarily found in solutions with monovalent\nions, whereas divalent ions promote larger, but less structured, aggregates.\nThese results provide molecular-scale insight into the effects of\nspecific ions on the aggregation of like-charged PDC molecules to\ninform understanding of related separation processes.