EFFECT OF ALKALINE MODIFICATION ON VISCOMETRIC AND SURFACE-ACTIVE PROPERTIES OF SOLUBLE LIGNIN

摘要

The effect of alkaline modification of Kraft lignin (KL) on its viscosity and surface tension at the water-air interface is studied.It was found out that the aqueous solution properties of KL change non-linearly with increasing the modification time,which may be conditioned by the oscillating variation in KL functionality during alkaline modification.Such properties are mainly defined by the hydrophilic-hydrophobic balance of the lignin macromolecules,which depends on the modification time,as well as on the content of high-and low molecular fractions.With increasing ionic strength and with decreasing pH,both reduced viscosity and surface tension at the air-water interface of the dilute aqueous solutions of the initial and modified samples of KL drop.It can be seen that,with decreasing the pH values from 8.0 to 7.0,spasmodic changes in aqueous solution properties of the lignin samples,which make up practically 50% of their full change value in the pH range 10-6,take place.It has been suggested that loss of charge by the phenolic hydroxyl groups in this pH range results in a co-operative H-bonding of aromatic nuclei in a KL macromolecules,leading to the macromolecules transition from the expanded conformation to a compact one,similarly with the case of "coil-globule" transition.