Pickering/Non-Pickering Emulsions of Nanostructured Sulfonated Lignin Derivatives

Sulfoethylated lignin (SEKL) polymeric surfactant and sulfoethylated lignin nanoparticles (N-SEKL) with a size of 750±50 nm are produced by using a facile green process involving a solvent-free reaction and acidification-based fractionation. SEKL forms a liquid-like conventional emulsion with low viscosity that has temporary stability (5 h) at pH 7. However, N-SEKL forms a gel-like, motionless, and ultra-stable Pickering emulsion through a network of interactions between N-SEKL particles, which creates steric hindrance among the oil droplets at pH 3. The deposition of SEKL and N-SEKL on the oil surface is monitored by a using a quartz crystal microbalance. Experimentally, the formation of emulsions at pH 7 is found to be reversible owing to the low adsorption energy ΔE of SEKL on the oil droplet (ΔE≈15 k_B T), which is determined with the help of three-phase contact-angle measurements. However, the high desorption energy (ΔE≈6.0×10⁵  k_B T) of N-SEKL makes it irreversibly adsorb on the oil droplets. SEKL is too hydrophilic to attach to the oil interface (ΔE≈0) and thus does not facilitate emulsion formation at pH 11. Therefore, it is feasible to apply SEKL for the formulation of Pickering or non-Pickering emulsions in the form of nanoparticles or polymeric surfactants, depending on the targeted application.