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

Abstract

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.

Strategies for structuring diverse emulsion systems by using wood lignocellulose-derived stabilizers

“…68 Another approach is to introduce polar groups via carboxymethylation to make lignins water-soluble, adjust the balance of affinity between the oil and water phases, and optimize their surface activity. 19,20 The solution-related properties of carboxymethylated lignins are dependent upon DS and pH. In one study, the droplet size of carboxymethyl lignin-stabilized kerosene o/w emulsion remained relatively constant for one month.…”

“…In one study, the droplet size of carboxymethyl lignin-stabilized kerosene o/w emulsion remained relatively constant for one month. 19 The complexity of lignins' structure and the numerous possibilities for further derivatization makes lignins a challenging, but versatile, raw material for producing effective emulsion stabilizers. 4.1.3.…”

“…For example, optimized aqueous extraction of hemicelluloses yields native functional emulsifiers; [12][13][14][15] cellulose may be disintegrated into nanoparticles 16,17 or modified to become a water-soluble derivative; 18 and lignin may be transformed into amphiphilic molecules via grafting with other building blocks. 19,20 Subsequently, emulsion stability is achieved through interfacial activity of the amphiphilic surfactants, Pickering stabilization with insoluble particles, or an increase in the viscosity of the continuous phase. This review discusses these conversion methods and the functionality they achieve, emphasizing the design of new safe green materials.…”

Grafting strategies for hydroxy groups of lignin for producing materials

“…The carboxymethylation reaction has been carried out by mixing lignin with NaOH followed by sodium chloroacetate or monochloroacetic acid in the time and temperature range of 1-6 h and 30-90°C, respectively. 40,[115][116][117][118][119] Alternative pathways were considered for purifying the products, such as acidification, 118 membrane dialysis, 40 and washing with ethanol. 116,119,120 Carboxymethylated lignin has been proposed as an effective dispersant for oil-water emulsions, 109 crude bitumen emulsions, 121 and clay, 40 cement, 116 and graphite suspensions.…”

“…116,119,120 Carboxymethylated lignin has been proposed as an effective dispersant for oil-water emulsions, 109 crude bitumen emulsions, 121 and clay, 40 cement, 116 and graphite suspensions. 122 Carboxymethylated lignin was also used as a stabilizer in kerosene-in-water emulsions 117 and as a foaming agent. 110 The composite of carboxymethylated lignin-tetra ethoxysilane was tested as a packaging and antimicrobial formula as well as in wound dressings.…”