Pickering/Non‐Pickering Emulsions of Nanostructured Sulfonated Lignin Derivatives

Abstract

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 kBT), which is determined with the help of three‐phase contact‐angle measurements. However, the high desorption energy (ΔE≈6.0×105 kBT) 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.

Recent Developments in the Formulation and Use of Polymers and Particles of Plant‐based Origin for Emulsion Stabilizations

“…The stabilized emulsion with carboxyalkylated and sulfonated lignin derivatives showed pH‐responsive performance. This behavior is believed to be a result of surface charge elimination via the protonation of functional groups [167,168,171] . Acylated lignin derivatives synthesized from the modification of lignin with succinic anhydride (SA) and dodecylsuccinic anhydride (DSA) also showed more surface tension depletion compared to the unmodified lignin, while the longer‐chain derivatives showed better emulsifying properties [170] …”

“…The advantage of LNPs compared to polymeric lignin surfactants was evaluated in our latest study [171] . Sulfo‐functionalized lignin particles (N‐SEKL) formed a gel‐like and ultra‐stable Pickering emulsion due to considerable viscosity enhancement of the emulsion (24 Pa s) compared with the unstable liquid‐like emulsions (0.24 Pa s) using polymeric surfactants [171] …”

“…Quartz crystal microbalance with dissipation monitoring (QCM−D) was used by Wei et al [136] . and Ghavidel and Fatehi [171] as a reliable technique to obtain information regarding the stabilization mechanism of emulsion systems at the nanoscale. QCM−D combined with other techniques [e. g., scanning electron microscopy (SEM) and rheological studies] can be implemented to investigate the adsorption kinetics and adsorbed mass/thickness at the oil/water interface as well as the structural properties of the interfacial films.…”

“…QCM−D combined with other techniques [e. g., scanning electron microscopy (SEM) and rheological studies] can be implemented to investigate the adsorption kinetics and adsorbed mass/thickness at the oil/water interface as well as the structural properties of the interfacial films. Ghavidel and Fatehi differentiated between the stabilization mechanism of non‐Pickering and Pickering systems using the adsorbed mass via QCM−D and the visualization of the adsorbed layer at the coated‐oil interface via SEM imaging [171] . In both studies, the intermolecular hydrophobic and hydrogen‐bonding interactions contributed to the formation of microstructures at the oil/water interface [136,171] .…”

“…Ghavidel and Fatehi differentiated between the stabilization mechanism of non‐Pickering and Pickering systems using the adsorbed mass via QCM−D and the visualization of the adsorbed layer at the coated‐oil interface via SEM imaging [171] . In both studies, the intermolecular hydrophobic and hydrogen‐bonding interactions contributed to the formation of microstructures at the oil/water interface [136,171] . For both OSA‐CFG (octenyl succinic anhydride‐modified corn fibre gum) and N‐SEKL systems, the diffusion of polymers to the oil/water interface occurred in the emulsion interface.…”