Effect of Nonionic Surfactants on Dispersion and Polar Interactions in the Adsorption of Cellulases onto Lignin.

Abstract

Residual lignin in pretreated biomass impedes enzymatic hydrolysis. Nonionic surfactants are known to enhance the enzymatic hydrolysis of lignocellulosic biomass but their mechanisms of action are incompletely understood. This study investigates the effect of a nonionic surfactant, Tween 80, on the adsorption of cellulases onto model lignin substrates. Lignin substrates were prepared by spin coating of flat substrates with three different types of lignin: organosolv lignin, kraft lignin, and milled wood lignin. The functional group distributions in the lignins were quantitatively analyzed by 31P NMR spectroscopy. The surface energies and surface roughnesses of the substrates were determined by contact angle measurements and atomic force microscopy, respectively. Tween 80 and cellulase adsorption onto the lignin substrates was analyzed with a quartz crystal microbalance with dissipation monitoring. Tween 80 adsorbed rapidly and primarily (≥85%) via dispersion interactions onto the lignin substrates and effected solubilization of lignin molecules, most notably with organosolv lignin, having the largest dispersive surface energy component and smallest molar mass. Cellulase adsorption onto the lignin substrates was mostly irreversible and had both a rapid and a gradual adsorption stage. Rapid cellulase adsorption was primarily (≥64%) mediated by dispersion interactions. The subsequent gradual mass increase is postulated to involve swelling of the lignin substrates. Adsorbed Tween 80 rendered lignin surfaces more hydrophilic by increasing their polar surface energy component and reduced both the extent of rapid cellulase adsorption as well as the rate of the subsequent gradual mass increase. The effect of Tween 80 on the rate and extent of the gradual mass increase depended strongly on the chemical properties of the lignin.