Effect of molecular weight on the adsorption characteristics of lignosulfonates

Sequentially adsorbed multilayers composed of sodium lignosulfonate (SL) and poly(diallyldimethylammonium chloride) (PDAC) were built up by a layer-by-layer self-assembly technique to investigate the adsorption characteristics of SL microgels for sensitivity improvement. The effect of molecular weight on the adsorption characteristics was investigated by light scattering, UV-visible spectroscopy, ellipsometer, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements. Results show that the self-assembly of SL microgels and PDAC is not mainly driven by electrostatic interaction but by cation-π interaction and hydrophobic interaction. The Coulomb interaction in SL microgels causes a "slow mode" peak in dynamic light scattering measurement, but it can be completely suppressed when SL is dissolved in 1.2 mol/L NaCl solutions due to charge-screening. The SL in 1.2 mol/L NaCl solutions can be well adsorbed on a PDAC surface, indicating that the electrostatic interaction is not the main driving force. The presence of cation-π interaction between SL and PDAC is detected by a stoichiometric ratio of SL to PDAC in SL/PDAC self-assembled films. During SL/PDAC self-assembly, hydrophobic interaction plays an important role in determining the adsorption rate, film thickness, surface morphology, and surface properties. SL with a higher M(w) has a stronger hydrophobic ability in water, so it exhibits a slower adsorption rate, a larger film thickness, and a higher film roughness. SL with different M(w) has almost the same wettability because of a larger hydrophobic effect that increases contact angle and a higher film roughness that decreases contact angle. In this case, the opposite effects cancel each other to make no difference in wettability, but it still reflects the role of the hydrophobic effect in surface properties.