An empirical assessment of the physicochemical properties of lignin solutions in aqueous sodium hydroxide – corroboration and demystification of some widely accepted statements

Abstract

Lignin is one of the main byproducts of the pulp, paper, and cellulosic ethanol industries. For the past 35 years, it has received increased interest in applications other than its use as an energy source. Although much of this research requires the use of lignin solubilized in solvents such as alkalis, little is known about the impact of the main process conditions – initial lignin mass, alkali concentration, and temperature and time of dissolution – on key solution properties – density, mass fraction of lignin, and pH. A central composite design, with these process conditions as input variables and these key solution properties as output variables, was made by varying the temperature from 30 to 80 °C, the time from 1 to 3 h, the concentration from 0.1 to 0.5 M, and, instead of directly working with lignin mass, a ratio of added lignin to alkali concentration of 30 to 60 g L mol⁻¹. The hypothesis made by Sarkanen et al. (Macromolecules, 1984, 17(12), 2588–2597) that lignin may aggregate under strong alkaline media and Lindströmn's (Colloid Polym. Sci., 1979, 257, 277–285) hypothesis that there are thermally induced processes that also cause aggregation – and further agglomeration – were attested and updated to indicate a joint action of both factors. Surprisingly, mass fraction displayed a maximum value using fixed ratio conditions instead of a saturation point. That shows lignin solubilization depends on more factors than simply the ratio of hydroxide anions vs. phenolic-OH groups and the pH. pH evolution was governed by slow aggregation and agglomeration reactions and conformational changes sensitive to time and temperature. The resulting polynomial models achieved adjusted R² > 0.996 for all responses, and ten validation experiments exhibited maximum relative errors ≤1.6%. These results furnish quantitative guidelines for tailoring lignin solution properties and suggest further studies into rheology, extended factor ranges, alternative lignin sources, and developing theoretical – and possibly more universal – models to predict lignin solution properties.