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.

To overcome hydrophobicity and low charge density of kraft lignin (KL), softwood KL was reacted with tannic acid (TA), a green reagent, under alkaline conditions. The mechanism of this reaction was identified to proceed through three steps: (i) oxidation of TA, (ii) transesterification of ester groups of TA (in hydroquinone form) by lignin, and (iii) phenolic ring opening of TA. The enhancement in the carboxylate group was observed, resulting in KL–TA with a high charge density and water solubility in a wide pH range. The modified lignin was utilized as an emulsifier to stabilize oil (hexadecane) in water (O/W) emulations. Vertical scan analysis revealed the greater stability of the system against phase separation at low pH values. The stability of the emulsion containing KL–TA could be tuned by pH, while the emulsion containing lignosulfonate (LS) was not pH-responsive. The LS-containing emulsion had lower stability than the KL–TA-containing one. The rheological studies of KL–TA emulsions showed that all samples displayed shear-thinning characteristics.

Lignin has gained intensive interest as an excellent raw material for the generation of advanced green products.

It is hypothesized that polymeric lignin surfactants have different affinities for stabilizing oil–water emulsions and that the emulsifying performance of these surfactants is highly affected by their adsorption performance at the oil–water interface. To validate this hypothesis, the adsorption performance of sulfethylated lignin (SEKL) surfactant at different oil–water interfaces was examined by assessing the contact angle, dynamic interfacial tension, and surface loading (Γ). Moreover, the interfacial adsorption kinetics of SEKL was comprehensively assessed in different oil–water systems to reveal the mechanisms of the SEKL adsorption at the interface. Also, the impacts of SEKL concentration and ionic strength on the performance of SEKL as an effective emulsifier for the emulsions were assessed. Furthermore, the droplet size and instability index of the emulsions were systematically correlated with the adsorption performance of SEKL at the interface of oil and water. For the first time, by implementing a modified Ward Toradai diffusion model, two distinct early stages of the adsorption of SEKL at the oil interface were identified. Interestingly, the second stage was the determining stage of adsorption with the diffusion-controlled mechanism when polymers reconfigured at the oil–water interface. Salt screening facilitated the clustering of SEKL upon charge repulsion elimination, which removed the energy barrier in the first stage of adsorption (ΔE p →0 = 0), but it introduced a steric barrier upon the reconfiguration of polymers at the oil interfaces in the second stage of adsorption. In addition to the kinetics of adsorption, satisfactory correlations were observed between surface pressure (Δγ = γ∞ – γ0), surface loading (Γ) of polymers, and contact angle at oil interfaces on one hand and the oil droplet size and emulsion stability on the other hand.