Aggregation of sodium lignosulfonate above a critical temperature

Abstract

Single molecules of sodium lignosulfonate (NaLS) were prepared and their aggregation behavior was studied as a function of temperature. The molecular dispersity of the NaLS solutions were confirmed by dynamic light scattering (DLS), and the aggregation behavior was studied by DLS and static light scattering (SLS), fluorescence spectrometry (FS), and atomic force microscopy (AFM). It was demonstrated that NaLS molecules started to aggregate above a critical temperature and the aggregation behavior was related to their spherical microgel conformation and hydrophobic interaction. During the DLS measurement, when NaLS was dissolved in 1.2 mol·l -1 NaCl aqueous solution and then filtered with 0.45 μm syringe filter, the slowmode diffusion (smD) corresponded to the polyelectrolyte effect and the original aggregates disappeared and the fast-mode diffusion (fmD) corresponded to the monomolecular NaLS (not yet aggregated) in the solution. When the temperature of the NaLS solution was raised to 38°C, the disappeared smD in DLS analysis appeared again, indicating that NaLS molecules started to form new aggregates. Due to the aggregation at 38°C, molecular weight (M w ) obtained from the SLS had a sharp increase, intensity ratio (I 1 /I 3 ) of pyrene in FS suddenly decreased, and the adsorption of NaLS on a solid substrate and the corresponding roughness of the surface increased significantly.

Light scattering characterization of lignosulfonate structure in saline solutions

“…Because of their spherical structure, LSs tend to lose their charge at about 40°C in 0.1 mol l -1 NaCl (Kontturi et al 1992). Our experiments also proved that LSs have a minimum Zeta potential at about 38°C (Qian et al 2013b). Vainio et al (2008) suggested an oblate spheroid shape of LSs in solutions.…”

“…If aqueous solutions of sodium lignosulfonate (NaLS) are investigated by dynamic light scattering (LS dyn ), a fast mode and a slow mode dispersion can be differentiated (Qian et al 2013a). The former is attributed to the presence of single NaLS molecules, and the latter to NaLS aggregates, which are large temporal molecular associations with Coulomb interactions as the active force (Qian et al 2013b). Actually, LSs are never completely dissolved as molecular disperse solutions in water.…”

A Critical Review of the Physicochemical Properties of Lignosulfonates: Chemical Structure and Behavior in Aqueous Solution, at Surfaces and Interfaces

“…It should be noted that the geometrical representation for individual lignosulfonate molecules used by Vainio et al [85], i.e., flat cuboid-like particles, is not consistent with the spheroidal conformation reported by other authors [4,21,115]. Lignosulfonate aggregation can be induced by increasing lignosulfonate or salt concentration, by adding alcohol, by reducing pH, and by increasing the temperature [82,85,123]. A common denominator among most of these actions is that electrostatic repulsion is reduced, which may further enhance hydrophobic interactions.…”

“…Lignosulfonate aggregation was also reported to be caused by a temperature increase to 38 °C and above [123]. This was accompanied by a reduction of the zeta potential [123], which would be coherent with a lower degree of dissociation [52]. As discussed previously, increasing the temperature will reduce the polyelectrolyte expansion of lignosulfonates, as a higher number of conformations become thermodynamically possible [116,118].…”

“…Lignosulfonate aggregation was also reported to be caused by a temperature increase to 38 °C and above [123]. This was accompanied by a reduction of the zeta potential [123], which would be coherent with a lower degree of dissociation [52].…”