Smithers Rapra Hosts Third Set of Twin Conferences on the Future of Pigments & Colour Cosmetics in Miami

Abstract

Lignin is a biopolymer in plant cell walls that has intrinsic interfacial functions and is produced in vast quantities but has found limited application as a surfactant. Here we demonstrate that low-density functionalization of anionic lignin derivatives with a corona of poly(ethylene glycol) (PEG) significantly augments their native interfacial activities, in contrast to previous studies of PEGylated lignins in which extensive functionalization was performed based on hydrophilic-lipophilic balance (HLB) predictions of solubility. Commercially available kraft lignin and lignosulfonate having high concentrations of carboxylate and sulfonate groups, respectively, were PEGylated with monoreactive grafts having molecular weight 750, 2000, and 5000 g/mol at constant grafting density and PEG fractions as low as 20% for the 750 g/mol species whereas previous studies focused on 50% PEGylation of greater. Both classes of PEGylated lignin demonstrated modest reductions in air-water surface tension, which decreased monotonically with PEGylated lignin concentration to values as low as 50 dyn/cm but did not demonstrate a sharp critical micelle concentration (CMC). However, the air-water surface tension was found to be relatively insensitive to both the chemistry of the lignin core and the graft length. In contrast, the cyclohexane-water interfacial tension was strongly dependent on lignin core, graft length, and concentration, with PEGylated kraft lignin driving the interfacial tension value to 3 dyn/cm at a concentration of 0.1 mg/mL that corresponded with an apparent critical micelle concentration while the PEGylated lignosulfonates had a weaker effect, with uniformly higher values of interfacial tension measured and no CMC observed. Finally, cyclohexane-water emulsions were prepared, and the volume fraction of the emulsion phase was much greater for PEGylated kraft lignin than for PEGylated lignosulfonate, suggesting formulations based on kraft lignin are significantly more effective at stabilizing oil-water interfaces. This disparity suggests that specific interactions between the lignin core and the non-aqueous phase may augment interfacial activities.