Theoretical Insights into the Solvent Polarity Effect on the Quality of Self-Assembled N-Octadecanethiol Monolayers on Cu (111) Surfaces

The effect of solvent polarity on the quality of self-assembled n-octadecanethiol (C(18)SH) on Cu surfaces was systematically analyzed using first-principles calculations. The results indicate that the adsorption energy for C(18)SH 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 C(18)SH makes the monolayer self-assembly easier on a Cu (111) surface through competitive adsorption. Furthermore, the adsorption energy per unit area for C(18)SH 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 C(18)SH. The decreased kinetic energy is mainly due to the generated push force in the polar solvent, which limits the mobility of C(18)SH. A molecular dynamics simulation also confirms that the -CH(3) site has a great interaction with CH(3)(CH(2))(4)CH(3) molecules and a weak interaction with CH(3)CH(2)OH molecules. The different types of interactions help to explain why the surface coverage of C(18)SH on Cu in a high-polarity ethanol solution is significantly larger than that in a low-polarity n-hexane solution at the stabilized stage.