Lignosulfonate, also referred to as sulfonated lignin, is a product of the wood pulping process. Initially considered as a waste material, refined lignosulfonate nowadays constitutes a viable alternative to synthetic chemicals in many applications.Among others, it is used in dispersant formulations in concrete, for rheology control of drilling fluids and coal–water slurries, and as a germicide, a flotation agent, and an emulsion stabilizer. In part because of its origin, lignosulfonate is a renewable chemical with low toxicity and good biodegradability.
With respect to the chemical composition and structure, lignosulfonate is similar to its precursor lignin. From a simplified viewpoint, lignin is a biopolymer that consists of monomers based on p-hydroxyphenyl, guaiacyl, and syringyl, which are randomly connected via ether or carbon–carbon bonds. During sulfite pulping of lignocellulose biomass, the lignin polymer is degraded into smaller fragments and sulfonate groups are added, which account for the good solubility of lignosulfonates in water. The chemical composition depends largely on the substrate, where softwood lignins and hardwood lignins can be distinguished. Softwood lignin is reported to consist almost entirely of guaiacyclpropane units, whereas hardwood lignin contains both guaiacycl- and syringylpropane units. The molecular weight distribution of hardwood lignin was found to be lower than that for softwood lignin. Other factors that can influence the composition and characteristics of lignosulfonates are the reaction conditions during the sulfonation reaction, fractionation, and purification procedures, as well as chemical modifications.
The behavior and characteristics of lignosulfonates in an aqueous solution depend strongly on factors such as salinity, pH, and lignosulfonate composition. Solubilized lignosulfonate molecules were reported to exhibit an ellipsoidal shape and self-associate on the flat edges into planar agglomerates. Results from fluorescence spectrometry have suggested that lignosulfonate agglomeration can start at concentrations of 0.05–0.24 g/L. Qian et al. further reported that increasing the temperature above ambient conditions can enhance hydrophobic interactions, which can cause lignosulfonate aggregation at elevated temperatures. Besides, the presence of electrolytes can induce lignosulfonate precipitation, during which flocculates are formed, which have dimensions much larger than lignosulfonate aggregates. This destabilization was discovered to follow the Hofmeister series with the exception of a few ions. An increase in pH was reported to lead to size expansion by structural unfolding of both dissolved and aggregated lignosulfonates because of ionization of weakly hydrophilic groups. The carboxylic groups were stated to ionize at about pH 3–4, whereas the phenolic groups may ionize at around pH 9–10. Because lignosulfonate composition is precursor-dependent, hardwood and softwood lignosulfonates exhibit slight differences in solubility. Softwood lignosulfonate was found to have a Hansen solubility parameter closer to water, as compared to hardwood lignosulfonate.
Adsorption of lignosulfonates on solid surfaces has been stated to follow the Langmuir isotherm. The authors further found that straight-chain alcohols can enhance this adsorption. In a different approach, the adsorption characteristics were investigated by building up multilayers of lignosulfonate and the cationic polymer, and it was concluded that hydrophobic interactions and cation−π interactions were dominant rather than electrostatic interactions. Evidence for adsorption of lignosulfonates on the interface of binary oil–water mixtures is given by measurements of interfacial tension (IFT) or compression isotherms.
The stabilization and destabilization of emulsions is a contemporary research topic with importance to, for example, food science or fuel production. Lignosulfonate is a known stabilizer for oil-in-water emulsions. The stabilization mechanism is most likely a combination of electrostatic repulsion, stearic hindrance, particle stabilization, and the formation of a semirigid interface layer. Mixing of lignosulfonate and an anionic surfactant can yield improved surface activity, but mixing with a cationic surfactant showed less potential.
Lignosulfonate characterization generally measures properties such as elemental composition, the presence of functional groups, molecular weight distribution, and hydrophobicity. The molecular weight is traditionally measured by size exclusion chromatography (SEC). An improvement to SEC was done by coupling with multiangle light scattering as the detection method. Two-dimensional nuclear magnetic resonance spectroscopy has been extended to study the structural characterization of lignin and its derivatives. Hydrophobic interaction chromatography (HIC) is a technique that uses lignosulfonate adsorption on a stationary phase and subsequent desorption with solvents of different polarities for fractionation. As the elution time progresses, the eluent ratio of alcohol to water is increased stepwise, each producing a new eluent peak that can be used to quantify the hydrophobicity of the sample.
Recent developments have enabled better understanding of lignosulfonate properties and behavior in aqueous solutions. However, a lack of systematic studies was stated, which could establish a connection between these properties and practical applications. In addition, industrial efforts have yielded more specialized lignosulfonate products, which are reflected, for example, by a diversification of lignosulfonate hydrophobicity. In this article, we therefore adapted and compared methods to evaluate lignosulfonates, where the focus is on salt tolerance and emulsion stability. In addition, the effect of lignosulfonate on emulsion characteristics and IFT were studied. The goal was to establish templates for testing procedures and comparison, which would benefit lignosulfonate utilization in technical applications.