Influence of pH on the Adsorption Characteristics of Sulfonated Alkali Lignin-based Polymers

The solution behaviors of sulfonated alkali lignin-based polymers (SBAL) at different pH values were investigated by means of dynamic light scattering and transmission electron microscopy (TEM). The results indicated that with the increase of pH,the hydrophilic functional groups of SBAL:sulfonic, carboxyl and phenolic hydroxyl groups were ionized gradually. With the increase of pH, the molecular configuration of SBAL changed from coil chain to extended chain. Moreover, the adsorption characteristics of SBAL on solid/liquid interface at different pH values were determined using a quartz crystal microbalance with dissipation monitoring (QCM-D) and electrostatic layer-by-layer self-assembly, as well as UV-Vis spectroscopy and atomic force microscopy (AFM). The results showed that with the pH increased from 3 to 12, the adsorption amount of SBAL on gold plate and quartz slide decreased at first and then increased, the lowest amount was at pH = 9. The adsorption configuration changed from rigid to soft, tight to loose and then tight again. The adsorption conformations of SBAL on gold plate and quartz slide with poly(diallyldimethylammonium chloride) (PDAC) showed mushroom-like structure, pancake-like structure and polymer brushes in the acidic, neutral and alkaline conditions, respectively. Cation-pi interaction was the main driving force between SBAL and PDAC in the acidic and neutral conditions and the adsorption strength was weak. However, electrostatic interaction played an important role between SBAL and PDAC in the alkaline condition and the adsorption strength was strong.