Kraft Lignin–Tannic Acid as a Green Stabilizer for Oil/Water Emulsion

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.