Extraction and Surfactant Properties of Glyoxylic Acid-Functionalized Lignin

Abstract

The amphiphilic chemical structure of native lignin, composed by a hydrophobic aromatic core and hydrophilic hydroxy groups, makes it a promising alternative for the development of bio-based surface-active compounds. However, the severe conditions traditionally needed during biomass fractionation make lignin prone to condensation and cause it to lose hydrophilic hydroxy groups in favour of the formation of C−C bonds, ultimately decreasing lignin's abilities to lower surface tension of water/oil mixtures. Therefore, it is often necessary to further functionalize lignin in additional synthetic steps in order to obtain a surfactant with suitable properties. In this work, multifunctional aldehyde-assisted fractionation with glyoxylic acid (GA) was used to prevent lignin condensation and simultaneously introduce a controlled amount of carboxylic acid on the lignin backbone for its further use as surfactant. After fully characterizing the extracted GA-lignin, its surface activity was measured in several water/oil systems at different pH values. Then, the stability of water/mineral oil emulsions was evaluated at different pH and over a course of 30 days by traditional photography and microscopy imaging. Further, the use of GA-lignin as a surfactant was investigated in the formulation of a cosmetic hand cream composed of industrially relevant ingredients. Contrary to industrial lignins such as Kraft lignin, GA-lignin did not alter the color or smell of the formulation. Finally, the surface activity of GA-lignin was compared with other lignin-based and fossil-based surfactants, showing that GA-lignin presented similar or better surface-active properties compared to some of the most commonly used surfactants. The overall results showed that GA-lignin, a biopolymer that can be made exclusively from renewable carbon, can successfully be extracted in one step from lignocellulosic biomass. This lignin can be used as an effective surfactant without further modification, and as such is a promising candidate for the development of new bio-based surface-active products.