Inlfuence Of π-π Interaction And Hydrophobic Effect On The Aggregaiton Behavior Of Alkali Lignin

Industrial lignin has recently attracted worldwide attention because it can be used as asustainable alternative to nonrenewable materials. Alkali lignin (AL) is a by-product of thewaste liquor from alkaline pulping, and the modified AL has wide applications as a polyanionsurfactant. How to utilize AL with high efficiency is still a great challenge; a betterunderstanding of AL basic structures and properties is required to improve its value–addedapplication. At present, domestic and foreign researchers are mainly focused on the moleculardesign and chemical modification of alkali lignin, while the study on aggregation behavior ofalkali lignin, especially the aggregation driving force, is less of a focus. The dispersionstability, viscosity and adsorption of alkali lignin have a very important influence on itsmodification and application; moreover, all these are related to the aggregation behavior ofalkali lignin. Therefore, it is of significance in both theory and practice to study theaggregation behavior of alkali lignin.This work studied the lignin-iodine charge-transfer complexes formed from the alkalilignin and iodine, and the effects of iodine on the disaggregation behavior and the assemblycharacteristics of AL. Results show that AL form-aggregates (i.e J-aggregates) in THFdriven by the-interaction of the aromatic groups in AL, and the-aggregates undergodisaggregation in THF-I2media because of the formation of lignin-iodine charge-transfercomplexes. By using iodine as a probe to investigate the aggregation behaviors and assemblycharacteristics, it is estimated that about18mol%aromatic groups of AL form-aggregatesin AL molecular aggregates. When molecular iodine is introduced into the AL solutions,lignin-iodine complexes occur with charge-transfer transition from HOMO of the aromaticgroups of AL to the LUMO of iodine. The formation of lignin-iodine complexes reduces theaffinity of the aromatic groups approaching each other due to the electrostatic repulsion, andthen eliminates the-interaction of the aromatic groups. The disaggregation of the-aggregates brings a dissociation behavior of AL chains, and a pronounced molecularexpansion. This dissociation behavior and molecular expansion of AL in the dipping solutionsinduce a decrease in the adsorbed amount and an increase in the adsorption rate, when AL istransferred from the dipping solution to the self-assembled adsorbed films. Consequently, the adsorption behavior of AL can be controlled by adjusting the-aggregation.This work studied the influence of the hydrophobic effect on the aggregation of alkalilignin in solution by adding salt. Results showed that with addition of salt, the alkali lignintends to aggregate more. When the salt concentration is higher or the valence of the saltcations is higher, the degree of aggregation of alkali lignin increases, as demonstrated by ahigher hydraulic diameter of alkali lignin in solution. The aggregation behavior of the alkalilignin in solution will affect its adsorption behavior at the solid/liquid interface. The adsorbedamount and the surface roughness of the alkali lignin films increases with increasing saltconcentration and higher valence of the salt cations.