Layer-by-layer assembly of lignosulfonates for hydrophilic surface modification

Abstract

Lignosulfonates (LS) were used to modify the surface of a mica substrate using Cu²⁺ as the binding agent through layer-by-layer (LbL) self-assembly. The average thickness and roughness of the self-assembled multilayer of LS–Cu²⁺ complexes increased with the number of layers as revealed by atomic force microscopy. The hydrophilicity of the modified surface decreased with the increase in the number of layers. The contact angle was increased from 6.5° to 86° after the mica surface was coated with 18 layers of LS–Cu²⁺ complexes. This suggests that surface hydrophilicity can be modified in a controllable manner via LbL assembly of lignosulfonates.

Introduction

Lignosulfonates (LS), the most available commercial lignin, are produced as by-products of sulfite pulping (Nägele et al., 2002). LS macromolecule contains a hydrophobic backbone (C6–C3 structure monomers linked together by ether and carbon bonds) and hydrophilic branches (sulfonic, carboxyl, and phenolic hydroxyl groups), and thus possesses a certain degree of surface activity (Myrvold, 2008, Johansson and Svensson, 2001, Telysheva et al., 2001). It has negative charges and exhibits polyelectrolyte behavior in aqueous solution due to the ionization of functional groups (Telysheva et al., 2001, Fredheim and Christensen, 2003). Because of the amphiphilic and anionic properties, LS have been increasingly used to modify the surface wetting properties of solids, such as plastics, coal, sphalerite concentrate particles, etc. (Telysheva et al., 2008, Yang et al., 2007, Peng et al., 2009). It has been demonstrated that the hydrophobicity of a solid surface can be improved by adsorbing LS in an aqueous solution (Telysheva et al., 2008). This conventional adsorption method is not very efficient due to the limited amount of LS can be adsorbed at any one time. Furthermore, it negatively affects some other surface properties of the raw material when a large amount of LS is applied. For example, the addition of LS to bleached pulp may change the color and decrease the brightness of the final paper products.

Layer-by-layer (LbL) self-assembly of polyelectrolyte is a promising approach for selective surface modification (Hammond, 2000). By building up a multilayer ultrathin-film coating of macromolecules, the desired wetting properties of the modified surface can be achieved in a controlled manner (Chen and McCarthy, 1997). Recently, self-assembled multilayer films of LS and poly-(o-ethoxyaniline) (POEA) have been successfully produced using the LbL technique (Paterno and Mattoso, 2001, Paterno and Mattoso, 2002, Paterno et al., 2002). The strategy was the alternate adsorption of oppositely charged polyelectrolytes onto a substrate via electrostatic interactions (Paterno et al., 2002). Unfortunately, polycations, such as POEA, are relatively expensive, which limits the applications of electrostatic LbL assembly to high-end products. It has been reported that the metal–ligand interactions can also be used for LbL assembly (Hatzor et al., 2000, Doron-Mor et al., 2004). LS are capable of binding transition metal ions, such as Co²⁺, Cu²⁺, and Fe²⁺, etc., resulting in the formation of LS–metal complexes (Khvan and Abduazimov, 1990, Dalimova et al., 1998). Therefore, it is possible to construct LbL multilayers of LS using metal–ligand interactions. Because LS are inexpensive industrial by-products and available in large quantities (Nägele et al., 2002), LbL self-assembly of LS for surface modification has technical as well as economical significance.

The objective of this study was to demonstrate the feasibility of LbL assembly of LS for surface modification of mica using Cu²⁺ as the binding agent. Atomic force microscopy (AFM) was used to examine the morphology of LS–Cu²⁺ multilayers. The water contact angle was measured to describe the effect of LbL coating on the hydrophobicity of the modified surface. This may provide a new approach for controlling the surface morphology and hydrophobicity of mica properly using the LbL technique.