Some physical properties of acetosolv lignins from bagasse

Abstract

Acetosolv-solubilized lignins were characterized by their solubility in different organic solvents, Fourier transform infrared, 13 C-NMR, UV, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis. Solvents having a solubility parameter in the range of 10-12.7 and a hydrogenbonding parameter in the range of 3.6-5 were considered good solvents for acetosolv lignins. Fourier transform infrared spectra of the lignins were typical for lignins containing p-hydroxy phenylpropane (H), guaiacyl (G), and syringyl (S) units. The lignins contained more conjugated and fewer nonconjugated C¼ ¼O groups, and the guaiacyl groups were etherified and condensed.13 C-NMR confirmed partial acetylation of the lignins and the presence of b-O-4 and b-5 linkages. Acetosolv lignins also showed the typical UV spectrum of annual plants.The effects of the acetic acid concentration and pulping time on the molecular weights of the lignins were explained with the presieving and condensation concepts. The thermal behavior of the acetosolv lignins was also studied.

Solubility of Lignin and Acetylated Lignin in Organic Solvents

“…Due to the capability of hydrogen bonding, the hydrogen bonding parameter of organic solvents such as ethanol or methanol is much higher than other organic solvents. The hydrogen bonding parameter was used to evaluate the hydrogen bonding capacities of the organic solvents for the solubility of lignin because hydrogen bonding is a major molecular interaction between hydroxyl groups of lignins and the organic solvents with the ability to form hydrogen bonding (Pan and Sano 1999;Shukry et al 2008).…”

“…Pyridine is also a good lignin solvent, having an even smaller solubility parameter than lignin. This behavior can be explained by an acid-base interaction between pyridine and the phenolic groups in lignin, which results in the high solubility of lignin in pyridine (Shukry et al 2008). It is important to note that the chemical interactions such as the acid-base interaction are not considered in Hildebrand theory.…”

“…The low solubility of lignin in ethanol can be explained based on the Hansen theory, where the hydrogen bonding parameter (9.5 (cal/cm 3 ) 1/2 ) of ethanol is very high as compared with other organic solvents (Table 5). Shukry et al (2008) reported that ethanol was not efficient for dissolving acetosolv lignins due to a high value of hydrogen bonding parameter. Although the solubility parameter of lignin is closer to the solubility parameter of ethanol than that of methanol, it seems methanol is a better solvent for lignin.…”

“…Unknown molecular structure, broad molecular weight distribution, and variability in the physico-chemical properties of lignin make the lignin less applicable in many areas (Lora and Glasser 2002). The solubility of lignin in organic solvents is also not clear and depends on many variables such as chemical structure, molecular weight, and the presence of hydrophilic moieties in the lignin molecule (Shukry et al 2008). Acetylation is a technique that can improve the solubility of lignin in organic solvents (Olarte 2011), and it can be used as a pretreatment method when a soluble form of lignin is required in the manufacturing processes.…”

“…Lignin solubility in different organic solvents has been characterized previously (Ni and Hu 1995;Shukry et al 2008;Zhang and LeBoeuf 2009;Quesada-Medina et al 2010;Wang et al 2011;Cybulska et al 2012). In general, the dissolution of a polymer into a solvent involves chain disentanglement and solvent diffusion when the polymer makes contact with a thermodynamically compatible solvent (Miller-Chou and Koenig 2003).…”