Structuring natural deep eutectic solvents with epoxidised lignin-enriched residues: a green alternative to petroleum-based thickened formulations

Abstract

• Natural Deep Eutectic Solvents (NADESs) were used as material base to obtain novel structuring materials based on lignin. • A crosslinked microstructural network was promoted by NADESs-epoxy-modified lignin chemical interactions. • The NADESs chemical nature showed a noticeable effect on the rheological properties of final structuring formulations. • The significant hydroxyl value of NADESs enabled the tunability of the ensuing formulations’ macroscopic properties. Natural deep eutectic solvents (NADESs) differing in the number of hydroxyl groups in the hydrogen bond donor (HBD) compound were investigated for usage as a matrix base in reactions involving a lignin-enriched fraction. NADESs were prepared by mixing citric acid with different HBDs (glycerol, xylitol, and sucrose). In addition, waste lignocellulose was epoxidised for use as a thickening agent for NADESs by promoting chemical crosslinking between the HBD hydroxyl groups and the epoxy rings of the lignocellulosic material. Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and thermogravimetric analysis were used to verify the reaction between NADES and the epoxidised lignin-rich material. In addition, full rheological characterisation was performed to evaluate the effects of the viscosity of NADESs and lignin-enriched residue concentration. A heating process was applied to determine the influence of water on the viscoelastic properties of the final products. The chemical interaction between NADESs and epoxidised lignocellulose was successfully achieved, resulting in various rheological responses, from liquid-like to gel-like, depending on the HBD compound comprising NADESs and the lignin concentration. The higher the number of hydroxyl groups in the HBD, the higher was the viscosity of the lignin-structured NADESs. An analysis of the relative viscosity data reveals that the epoxidised ligninocellulose residue exerts a similar structuring role in the three different NADESs.