Lignosulfonate precipitation from solution can be caused by a number of changes that destabilize its solubility. The addition of another electrolyte can invoke precipitation by salting out . As has been reported, the salting out tendency for various ions is in line with both the Schulze‑Hardy rule and the Hofmeister series , with the exception of a few ions. It was further discussed that the observed effects could not be explained by the common ion effect or screening effects. Adding a solvent to an aqueous solution can cause precipitation of lignosulfonates as well , as there are many solvents that are miscible with water but pose as poor solvents for lignosulfonates . Precipitation of technical lignin by pH reduction is a common method, for example to separate the lignin from alkali black liquor during wood pulping . At pH 3 or lower, both the phenolic and carboxylic acid groups are mostly undissociated, which can lead to lignosulfonate precipitation if the ratio of sulfonic to carboxylic acid groups is too low.
Adsorption at Surfaces and Interfaces
Adsorption and desorption of lignosulfonates follow a similar behavior as that of other surfactants. Langmuir isotherm has been reported by several authors to describe the equilibrium adsorption of lignosulfonates on solids ]. At the water‑air surface or water‑oil interface, lignosulfonate adsorption is evident by a decrease in surface or interfacial tension . This decrease follows a linear‑logarithmic progression with increasing lignosulfonate concentration , but above a certain concentration the effect can decrease in slope, which has been related to the aggregation onset by some authors . Figure 4 exhibits two comparisons of surface tension plots for lignosulfonates with other surfactants and polymers.
Figure 4. Equilibrium surface tension (water-air) in dependence of surfactant or polymer concentration. Comparison of lignosulfonates (LS) with polyelectrolyte polymers (left) or with the surfactants dodecyl benzenesulfonate (DBS), nonylphenyl polyoxyethylene glycol (PONP, Ingepal CO-720) and polysorbate 20 (PS20, Tween 20) (right) .
Overall, lignosulfonate addition caused a larger reduction of surface tension than polyelectrolyte polymers, such as sodium polyacrylate or sodium polystyrenesulfonate . On the other hand, less reduction of surface or interfacial tension than regular surfactants was reported , such as sodium dodecyl sulfate or nonylphenyl polyoxyethylene glycol. Reports are contradictory on the effect of molecular weight. One study stated that lignosulfonates with a lower average molecular weight displayed a tendency to induce larger changes of interfacial tension , whereas another reported that with increasing molecular weight the effect on surface tension became stronger . The reduction of surface or interfacial tension can further be enhanced by increasing ionic strength or by reducing the pH .
Measurements of surface or interfacial tension are also instrumental for studying the kinetics of lignosulfonate adsorption. Several hours or more are usually needed to attain an equilibrium state . To explain this comparably long equilibration time, it was proposed that lignosulfonate molecules undergo diffusion exchange at the interface, and that individual molecules are subject to rearrangement with respect to the interface and to each other. Such conformational realignment has for example been described for petroleum asphaltenes at the water‑oil interface . Both lignosulfonates and asphaltenes are polybranched and exhibiting a tendency for self‑association. In addition, lignosulfonates and petroleum asphaltenes have in common that emulsions stabilized by these components require overnight storage before processing , as the emulsions would otherwise be less stable. These two species have hence been compared in terms of interfacial phenomena .