Sodium lignosulfonate(SL)

recovered from spent pulping liquor

has been used as a polyanion to build up multilayer films through an electrostatic sequential adsorption process by using poly(diallyldimethylammonium chloride)(PDAC) as the polycation

and the effect of pH value on the electrostatic layer-by-layer self-assembly has also been studied in this work.The electrostatic sequential adsorption process was monitored by UV-Vis spectrosxopy

and the morphology of the SL/PDAC multilayers was observed by AFM.Results show that during the sequential adsorption process

the UV-Vis absorbance of the SL/PDAC multilayers increases linearly

indicating that the thickness of multilayes increases in a layer-by-layer manner.The solution pH has a significant influence on the thickness and morphology of the SL/PDAC multilayers.In the studied pH value range

the lower is the solution pH

the higher is the UV-Vis absorbance and the larger is the roughness of the multilayer films.This phenomenon is caused by the specific molecular structure of lignosulfonate

and its conformation characteristics in solutions with different pH values.The sodium lignosulfonate exhibits different ionization degrees in the solutions with different pH values

because it includes not only sulfuric groups

but also carboxylic and phenolic groups.Although sulfuric groups can ionize at any pH value

but the ionization degrees of carboxylic and phenolic groups in the solutions increase with increasing pH values.The higher absorbance and the larger roughness of the layer-by-layer films can be attributed to the low ionization degree and the shrinkage conformation of sodium lignosulfonate in the solution with the low pH value.This work is helpful for understanding the adsorption characteristics of SL at a molecular level

and further expanding functionalities of SL through manipulation of the adsorption capacity by changing solution pH values.