Solubility of Lignin and Acetylated Lignin in Organic Solvents

The solubility of four lignin samples and their acetylated forms was determined in a series of organic solvents to investigate the relationship between solubility and the solubility parameter. The solubility parameter of lignin samples and acetylated lignin was calculated based on the number of atoms or groups on lignin units. Lignin samples were obtained by isolating lignin from lignocellulosic bioethanol residues (Lignin 1 [L1]), isolating lignin from kraft hardwood black liquor (Lignin 2 [L2]), commercial kraft softwood lignin (Lignin 3 [L3]), and commercial soda non-wood lignin (Lignin 4 [4]). The solubility of lignin in organic solvents was not predictable due to poor correlation between the solubility of lignin and its solubility parameter. However, the solubility of lignin in an organic solvent depended on the molecular weight and the aliphatic hydroxyl number of the lignin. L2, with a lower molecular weight than other lignin samples, had the highest solubility in organic solvents, and L3, with highest aliphatic hydroxyl number, had the lowest solubility in organic solvents. All acetylated lignins were soluble in most of the organic solvents. Furthermore, the molecular weights of the soluble parts of all four lignins in ethyl acetate were found to be lower than the original lignins.