In-Depth Study of the Interaction Mechanism between the Lignin Nanofilms: Toward a Renewable and Organic Solvent-Free Binder

Abstract

Lignin is an abundant biorenewable resource with an annual production of 50 million metric tons. Despite the abundance and high potential for applications, only ∼2% of the produced lignin was used for industrial applications. One of the main reasons for the low applicability is the lack of fundamental studies. In particular, the molecular binding mechanism of lignin is a key for the development and design of lignin into higher-value products. In this study, the interaction forces between homogeneous lignin nanofilms as thin as a phenylpropane unit monolayer (∼11 Å) are directly measured using a surface forces apparatus (SFA) at various concentrations of intervening electrolyte solution. The measured adhesion force decreases with increasing electrolyte concentration, the inverse of what would be expected according to the electric double layer theory. These findings, along with detailed analyses using Derjaguin–Landau–Verwey–Overbeek (DLVO) and hydrophobic theories, strongly indicate that hydrophobic interaction accounts for a large proportion of the interaction forces. Additional measurements between methyl-terminated self-assembled monolayer and lignin film confirm that hydrophobic interactions dominated the overall interaction potential of lignin films. Furthermore, lignin-supplemented activated carbon composites show enhanced compressive strength, which indicates the potential use of lignin as an ecofriendly reinforcing binder.