Lignins emulsifying properties according to pH to control their behavior at oil-water interface

This study focuses on the mechanisms governing the ability for fractionated lignins to stabilize emulsion depending on the pH. It is hypothesized that adsorption mechanisms of lignins at the oil/water interface sharply impact their ability to stabilize emulsions. Stabilization mechanisms may vary because lignins could exist either in the form of particles and/or in the form of surfactants depending on pH treatment. The interfacial layer formed by lignins was examined by both interfacial tension and interfacial viscoelasticity as a function of pH history. Emulsifying capacity of corresponding lignins dispersions was investigated and correlated with both the lignins adsorption performance at the oil/water interface and the interfacial rheological properties. The coexistence of two distinct mechanisms responsible for O/W stabilization as a function of pH was identified. First, for lignins in the form of surfactants, emulsion is formed but a marked coalescence occurs rapidly. Secondly, when lignin mainly exists in the form of particles, rapid phase separation occurs due to poor interfacial properties. Finally, when both lignin particles and lignin surfactants coexist, a synergy between Pickering and micellization mechanisms leads to stable emulsion. This study proves a synergistic effect between lignin particles and lignin surfactants resulting in optimal interfacial stabilization.