Abstract
The effect of solvent polarity on the quality of self-assembled n-octadecanethiol (C18SH) on Cu surfaces was systematically analyzed using first-principles calculations. The results indicate that the adsorption energy for C18SH on a Cu surface is −3.37 eV, which is higher than the adsorption energies of the solvent molecules. The higher adsorption energy of dissociated C18SH makes the monolayer self-assembly easier on a Cu (111) surface through competitive adsorption. Furthermore, the adsorption energy per unit area for C18SH decreases from −3.24 eV·Å−2 to −3.37 eV·Å−2 in solvents with an increased dielectric constant of 1 to 78.54. Detailed energy analysis reveals that the electrostatic energy gradually increases, while the kinetic energy decreases with increasing dielectric constant. The increased electrostatic energies are mainly attributable to the disappearance of electrostatic interactions on the sulfur end of C18SH. The decreased kinetic energy is mainly due to the generated push force in the polar solvent, which limits the mobility of C18SH. A molecular dynamics simulation also confirms that the -CH3 site has a great interaction with CH3(CH2)4CH3 molecules and a weak interaction with CH3CH2OH molecules. The different types of interactions help to explain why the surface coverage of C18SH on Cu in a high-polarity ethanol solution is significantly larger than that in a low-polarity n-hexane solution at the stabilized stage.
Highlights
Copper (Cu) and its alloys have been widely used in many industrial sectors, including electronic, chemical, and ocean engineering [1]
Based on the previous experimental findings, solvent polarity plays an important role in the adsorption of C18 SH SAMs on Cu surfaces
The results have revealed the molecular mechanisms behind the effect of solvent polarity
Summary
Copper (Cu) and its alloys have been widely used in many industrial sectors, including electronic, chemical, and ocean engineering [1]. Despite its many outstanding properties, Cu is very chemically active, and is prone to corrosion. Various ways to protect metals from corrosion have been developed based on different principles. Self-Assembled Monolayers (SAMs) are one of the most economic, highly efficient, and simple ways to protect metals and alloys from corrosion and oxidization [5,6,7]. Experimental research has been carried out to identify the protection mechanism of SAMs on Cu surfaces. It was generally believed that the densely packed monolayers were formed through chemisorption onto the surface of Cu [8]. Since the properties of the solvent affect the assembly of SAMs, the qualities of the SAMs formed in different solvents are expected to be different. Many researchers have studied the thiol-SAMs formed on gold surfaces in different solvents. Bain et al evaluated the effect of various solvents
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