Oil‐in‐Water Emulsions Stabilized by Carboxymethylated Lignins: Properties and Energy Prospects

We take advantage of the amphiphilic properties of technical lignin macromolecules and their inherent high calorific values to formulate oil-in-water (O/W) fuel emulsions with high internal-phase ratios. For the oil phase, we used a combustible hydrocarbon (kerosene) with a measured equivalent alkane carbon number of 12. To adjust the balance of affinity with the oil and water phases and their surface activity, pine kraft lignins were carboxymethylated to different degrees, as quantified by (13) C NMR spectroscopy, potentiometric titrations, and zeta potential measurements. Carboxymethylated lignins (CMLs) with a degree of substitution of 30 % displayed a critical aggregation concentration of 3 %. The salinity and pH of the aqueous phase were chosen as formulation variables and adjusted within the Winsor framework. The O/W emulsions were produced by following standard protocols. The drop-size distributions of emulsions with varying pH, degree of substitution, and composition (water-to-oil ratio, WOR) were determined, and the long-term stabilities and rheological behavior of these emulsions were analyzed. Most of the obtained O/W fuel emulsions showed shear-thinning behavior with a drop size of approximately 2.5 μm and were stable for over 30 days. The combustion of the lignins and their respective emulsions was performed, and their higher heating values (HHVs) were quantified. The HHVs of CML and a high-internal-phase (WOR=30:70) O/W emulsion were 20 and 30 MJ kg(-1) , respectively. Overall, we propose the stabilization of O/W fuel emulsions by lignin as an important avenue in the utilization of this abundant biomacromolecule.