Green Nanocoatings Prepared by Crosslinking Self‐Assembled Fatty Acids on Metals

Abstract

Fatty acids (FA) are carboxylic acids with long aliphatic chains of carbon atoms. They are important components of living cells in humans, animals, plants, and microorganisms. Their biodegradation to products with low toxicity is relatively easy, [1,2] which makes them a good choice for various applications. FAs are known to have the ability to self-assemble on a variety of substrates, including metals, by forming molecular layers on the surfaces, usually referred to as self-assembled monolayers or multilayers. [1,[3][4][5][6][7] Figure 1a is a simplified schematic of the assembly of FA on copper, where binding to the substrate by chemisorption is a mode to completely cover the surface. The driving force for the spontaneous adsorption of carboxylic acids on metals is the formation of a surface salt between the carboxylate anion and a metal cation on the surface. [7,8] The FAs are covalently bonded to the substrate surface and densely packed due to van der Waals interactions between neighboring FAs, which restricts their movement. [7,9] A major drawback of SAMs is their chemical instability and relatively easy removal from the surface. [10,11] Although the FA molecules cover the surface quite well and the film acts as a barrier, it is still possible for foreign molecules to get between the molecules onto the metal surface over time because there are only van der Waals interactions between the FAs, as shown in Figure 1a. Therefore, various post-treatments, such as heating, were applied to increase the effectiveness of the SAMs. Converting the van der Waals interactions between adjacent FAs into covalent cross-links could eliminate this problem and consequently improve the film's protective properties by preventing foreign molecules' access to the substrate.Radiation crosslinking refers to the process of using UV light, gamma rays, or electron beams to induce the rapid formation of chemical covalent bonds between two monomers or polymer chains. When UV light is used to induce crosslinking of SAMs, [12][13][14][15] the energy is typically too low to successfully complete the process, especially when irradiation occurs in the presence of oxygen. In such cases, the energy transfer to the film is too low and it is more likely that the molecular films will degrade rather than crosslink. On the other hand, high-energy gamma rays or accelerated electrons arrive at the surface of an object at extremely high velocity and cause rapid and homogeneous crosslinking without the use of solvents, initiators, and/or catalysts. [16] Another advantage of gamma rays and electrons is their deep penetration, which also ensures crosslinking of the inner surfaces. [17] It has Self-assembled monolayers (SAMs) are an important element of modern nanotechnology and surface functionalization. However, their application is still limited because they are easily removed from the surface of the object in corrosive environments. Crosslinking would make SAMs more resistant to the corrosive environment they are exposed to. In this work, how to strongl...