Solubilization of lignin in copolymer micelles in aqueous solution

Abstract

Abstract The solubilization of lignin in self-aggregating triblock amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) in aqueous solution has been studied utilizing liquid-state proton nuclear magnetic resonance spectroscopy ( 1 H NMR) and diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY NMR). Our hypothesis for copolymer micelle incorporation of lignaceous species includes a “hydrophilic-aromatic interaction model”, in which lignin derivatives with amphiphilic characteristics, known to be abundant in the effluent from pulp mill and pretreatment of various lignocellulosic materials, have an affinity for these micelles, and tend to form relatively organized structures within them. Here we show how the chemical/structural features of lignin units and small molecule lignin models determine their solubilization behavior in the micelles. It was found that a buildup of concentration series of lignin and lignin-based model compounds guaiacol, eugenol, phenol, guaiacylglycerol-beta-guaiacyl ether, ferulic acid in Pluronic ® F68-deuterium oxide solution results in significant upfield chemical shifts of PEO-(CH 2 -CH 2 ) and PPO-(CH 3 ) proton resonances, and at a critical guest concentration (CGC), dramatic upfield shifts due to gross structural transitions in the micelles. We present evidence that copolymer micelle-lignin interactions depend on both chemical functional group characteristics of solute (i.e., polarity, H-bonding ability) and π-π interactions between aromatic/conjugated groups. Our results demonstrate how the loci of incorporated solute in the block copolymer micelles are affected by these features.