Covalent Modification of the Wool Fiber Surface: Removal of the Outer Lipid Layer

Abstract

This investigation provided a comparative assessment of strategies for the removal of 18-methyleicosanoic acid (18-MEA), and other surface bound lipids enveloping the wool fiber surface, by chemical and physical cleavage of the thioester bond. The removal of this lipid layer reveals an underlying proteinaceous layer, exposing functional chemical groups available for covalent attachment of new molecular or nanoparticulate entities by chosen treatments. Lipid removal treatments employing methanolic potassium hydroxide, t -butoxide in t-butanol, and aqueous hydroxylamine and a physical atmospheric pressure glow discharge (APGD) plasma treatment were compared. Treated wool fabrics were subsequently characterized by analysis of the exposed groups on the fiber surface by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), with the bulk surface properties of the fabrics assessed employing wettability testing techniques. An evaluation of chemical and plasma methods for removal of the surface bound lipid layer resulted in the selection of aqueous hydroxylamine/ non-ionic surfactant as the optimal treatment method for subsequent covalent attachment of novel entities by surface treatments. Optimized aqueous hydroxylamine treatment was found to remove up to 77% of the surface bound 18-MEA, providing a marked increase in surface wettability, without significantly affecting the handle of the treated fabric. Surface characterization demonstrated that the hydroxylamine treatment produces an increase in surface friction, with uniform and controlled removal of the surface lipid layer, and minimal surface oxidation of the surface thiols. Minimizing surface oxidation of thiols was a critical target of surface lipid removal in this study, as it maximizes the potential for subsequent surface modification via covalent attachment. The use of aqueous conditions, short reaction times, and moderate temperatures with hydroxylamine treatment are advantageous in comparison with treatments employing non-aqueous solvents such as methanol and anhydrous t-butanol. The economic and environmental advantages of an aqueous, effective, and non-damaging approach to surface lipid removal highlight this approach as a potential avenue for future textile application of novel wool surface chemistries.