Kunze Julia, Leitch Jay, Schwan Adrian L, Faragher Robert J, Naumann Renate, Schiller Stefan, Knoll Wolfgang, Dutcher John R, Lipkowski Jacek
Department of Chemistry, University of Guelph, Guelph, Ontario, N1G2W1 Canada.
Langmuir. 2006 Jun 6;22(12):5509-19. doi: 10.1021/la0535274.
For a monolayer of 2,3-di-phytanyl-sn-glycerol-1-tetraethylene glycol-D,L-a-lipoic acid ester lipid (DPTL) self-assembled (SAM) at a gold electrode surface we propose a new method to determine the charge number per adsorbed molecule and the packing density (area per molecule) in the monolayer. The method relies on chronocoulometry to measure the charge density at the SAM covered gold electrode surface. Two series of measurements have to be performed. In the first series, charge densities are measured for a monolayer transferred from the air-solution to the metal-solution interface using the Langmuir-Blodgett (LB) technique. This series of measurements allows one to determine charge numbers per adsorbed DPTL molecule. The second series is performed using a gold electrode covered with a self-assembled monolayer. The charge densities obtained in this series are then used to calculate the packing density with the help of charge numbers per adsorbed DPTL determined in the first series. The area per adsorbed molecule determined by the new method was compared to the area per molecule determined by the popular reductive desorption method. The molecular area determined with the new method is about 20% larger than the area calculated from the van der Waals model, which is a physically reasonable result. In contrast, the popular reductive desorption method gives an area per molecule 20% lower than the minimum estimated based on a van der Waals model. This is a physically unreasonable result. It is also shown that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electrons participating in the reductive desorption step. An explanation of the origin of the "partial charge numbers" is provided. We recommend the new method be used in future studies of thiol adsorption at metal surfaces.
对于在金电极表面自组装的单层2,3 - 二植烷基 - sn - 甘油 - 1 - 四乙二醇 - D,L - α - 硫辛酸酯脂质（DPTL），我们提出了一种新方法来确定单层中每个吸附分子的电荷数和堆积密度（每个分子的面积）。该方法依靠计时库仑法来测量自组装单层覆盖的金电极表面的电荷密度。必须进行两组测量。在第一组测量中，使用朗缪尔 - 布洛杰特（LB）技术测量从气 - 溶液转移到金属 - 溶液界面的单层的电荷密度。这组测量可以确定每个吸附的DPTL分子的电荷数。第二组测量是使用覆盖有自组装单层的金电极进行的。然后利用在第一组测量中确定的每个吸附DPTL的电荷数，根据这组测量得到的电荷密度来计算堆积密度。将通过新方法确定的每个吸附分子的面积与通过常用的还原脱附法确定的每个分子的面积进行了比较。用新方法确定的分子面积比根据范德华模型计算出的面积大约大20%，这是一个物理上合理的结果。相比之下，常用的还原脱附法得到的每个分子的面积比基于范德华模型估计的最小值低20%。这是一个物理上不合理的结果。还表明，每个吸附分子的电荷数取决于电极电位，并且可能取小于参与还原脱附步骤的电子数的值。对“部分电荷数”的起源提供了一种解释。我们建议在未来关于金属表面硫醇吸附的研究中使用这种新方法。
