Effects of pH on aggregation behavior of sodium lignosulfonate (NaLS) in concentrated solutions

Concentrated sodium lignosulfonate (NaLS) solutions have wide industrial applications. Therefore, their aggregation behavior at different pH (2.89–11.81) was investigated by means of rheology, conductivity, acid–base titration, and zeta potential measurements. It was found that all the NaLS solutions were pseudoplastic fluids. In the pH range of 2.89–10.34, higher pH resulted in increased viscosity, pseudoplasticity, and thixotropy. The dynamic viscoelastic measurements characterized by frequency and temperature sweeps showed that the storage modulus, loss modulus, and complex viscosity increased, but the loss tangent and the crossover temperature where storage modulus equaled loss modulus decreased with increasing pH. The change in rheological properties as a function of pH may be related to the molecular expansion effect. The conductivity, acid–base titration, and zeta potential measurements revealed the formation of a larger and higher-strength network structure at pH 2.89–10.34 as a result of the ionization of sulfonic, carboxyl, and phenolic hydroxyl groups. As the pH exceeded 10.34, NaLS solutions exhibited decreased viscosity, pseudoplasticity, thixotropy, and elasticity, and the aggregation effect weakened, which may be caused by the electrostatic repulsive forces becoming too strong and disruption of hydrogen bonds.