Alkali lignin stabilization of oil-in-water emulsions via simple dispersion and ozonation processes†

Abstract

An alkali lignin was subjected to successive processes enabling the stabilization of oil-in-water (O/W) emulsions. Firstly, by merely using a water/acetone mixture and evaporating acetone, more than 90% of this lignin could be dispersed in pure water, with no chemical modification involved. The obtained lignin dispersion was then subjected to an ozonation reaction, without requiring any additional steps, resulting in an increase of the carboxylic acid content (+193%), through effective cleavage of phenolic structures (−93%), and a decrease in the lignin molar masses, as evidenced by ³¹P quantitative NMR, 2D HSQC NMR, FT-IR spectroscopy and size-exclusion chromatography. These features allowed this alkali lignin to be easily re-dispersed in water. Aqueous dispersions of lignin before and after ozonation were characterized by an acidic pH and a bimodal nano-objects composition. These nano-objects exhibited a great affinity for interfaces, especially the ozonated lignin with an ability to reduce the surface tension of water down to 48 mN m⁻¹ and the interfacial tension of a water/sunflower oil system down to 8 mN m⁻¹. Both lignins were consequently involved in the preparation of O/W emulsions and were able to stabilize micro-sized oil droplets, through a supposed Pickering mechanism, preventing oil release under both accelerated and real-time conditions (30 days). These results demonstrated that stable emulsions could be obtained from simply processed lignins for potential applications in cosmetic formulations or for the entrapment of hydrophobic compounds in the agrochemical or pharmaceutical industries.