Carboxylation of lignin by oxidation with hydrogen peroxide and its use as emulsion stabilizer

Abstract

This article studied the carboxylation of technical lignin and subsequent use as emulsion stabilizer. Oxidation was conducted with hydrogen peroxide under alkaline conditions. As both titration and Fourier-transform infrared spectroscopy (FTIR) showed, phenolic units were converted to carboxyl groups by oxidation. The treatment was most effective for soda lignin from Arkansas/straw, but also had significant effect on the softwood kraft lignin and softwood soda lignin. An increase in molecular weight by size-exclusion chromatography was further noted, which was less pronounced for the Arkansas/straw lignin. It was argued that one contributing mechanism was the monolignol composition, as the lignin from annual plants also contained S-units in addition to the G-units that mostly made up the softwood lignin. Moreover, purification prior to oxidation, i.e., removal of inorganic components in the lignin, showed no significant effect on the carboxylation process. Emulsion stabilization was studied with respect to the pH using three oxidized kraft lignins. Here, lower pH yielded better emulsion stabilization, unless the lignin precipitated, which switched the stabilization mechanism from interfacial adsorption to particle stabilization. It was argued that the degree of ionization played a key role, as a lower degree of ionization corresponded with better emulsion stability at the same ionic strength. At last, measurements of interfacial tension and interfacial rheology found that oxidized lignin behaved similar to water-soluble lignosulfonates and created viscoelastic interface layers.