School of Chemical Engineering, Northwest University, Xi'an 710069, Shaanxi, China.
State Key Laboratory of Eco-Hydraulics in Northwest Arid Region of China, Xi'an University of Technology, Xi'an 710048, Shaanxi, China.
Molecules. 2018 Mar 22;23(4):733. doi: 10.3390/molecules23040733.
The effect of solvent polarity on the quality of self-assembled -octadecanethiol (CSH) on Cu surfaces was systematically analyzed using first-principles calculations. The results indicate that the adsorption energy for CSH 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 CSH makes the monolayer self-assembly easier on a Cu (111) surface through competitive adsorption. Furthermore, the adsorption energy per unit area for CSH decreases from -3.24 eV·Å to -3.37 eV·Å 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 CSH. The decreased kinetic energy is mainly due to the generated push force in the polar solvent, which limits the mobility of CSH. 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 CSH on Cu in a high-polarity ethanol solution is significantly larger than that in a low-polarity -hexane solution at the stabilized stage.
使用第一性原理计算系统地分析了溶剂极性对自组装十八硫醇（CSH）在 Cu 表面质量的影响。结果表明，CSH 在 Cu 表面的吸附能为-3.37 eV，高于溶剂分子的吸附能。解离 CSH 的较高吸附能通过竞争吸附使单层自组装在 Cu（111）表面更容易进行。此外，CSH 的单位面积吸附能从 1 到 78.54 的介电常数增加的溶剂中的-3.24 eV·Å 降至-3.37 eV·Å。详细的能量分析表明，静电能逐渐增加，而动能随介电常数的增加而降低。增加的静电能主要归因于 CSH 硫端静电相互作用的消失。动能的降低主要是由于极性溶剂中产生的推斥力限制了 CSH 的迁移率。分子动力学模拟也证实-CH₃ 位点与 CH₃(CH₂)₄CH₃ 分子具有很强的相互作用，与 CH₃CH₂OH 分子的相互作用很弱。不同类型的相互作用有助于解释为什么在稳定阶段，CSH 在 Cu 上的表面覆盖率在高极性乙醇溶液中明显大于低极性正己烷溶液中的表面覆盖率。
