Tao Feng, Bernasek Steven L
Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009, USA.
Langmuir. 2007 Mar 27;23(7):3513-22. doi: 10.1021/la0613631. Epub 2007 Mar 1.
Self-assembled monolayers of bifunctional molecules HOOC(CH2)nCOOH (n = 20, 18, 16, 14, 12, 10), HOOC(CH2)nCH2OH (n = 13, 14), and HOCH2(CH2)14CH2OH dissolved in octanoic acid were investigated using scanning tunneling microscopy, to understand the self-assembly of bifunctional molecules and the influence of a carboxylic acid solvent on the formation of self-assembled structures on HOPG. In the series of di-acids (HOOC(CH2)nCOOH), only HOOC(CH2)20COOH forms stable coadsorption structures with the solvent octanoic acid. The remaining di-acids form stable single-component monolayers and do not coadsorb with solvent octanoic acid. Coadsorption structures involving mixtures of di-acids were observed. This result suggests that coadsorption with acid solvent or with other di-acids occurs to maximize hydrogen-bond density in the overlayer. A quantitative model based on this concept is proposed. For hetero-bifunctional molecules HOOC(CH2)nCH2OH (n = 13, 14), the coadsorption of HOOC(CH2)14CH2OH and octanoic acid at the molecular level produces a microscopic mesh made of homogeneously arranged openings with a dimension of approximately 12.5 A x approximately 5.0 A x approximately 1.8 A. For the hetero-bifunctional molecule HOOC(CH2)13CH2OH, hydroxyl groups of two adjacent lamellae assemble to form a herringbone geometry, and the two carboxylic acid groups assemble with a straight head-to-head configuration. In addition, a new mixed hydrogen-bonding network of COOH...O-H was observed in another self-assembled structure of this molecule. The bifunctional molecule HOCH2(CH2)14CH2OH exhibits multiple packing patterns on HOPG via different hydrogen-bonding networks. HOCH2(CH2)14CH2OH self-assembles using the H-O...O-H network typical of the n-alcohol herringbone structure, forming an asymmetric adsorbate on HOPG. It also forms domains with another hydrogen-bonding network, in which molecules in adjacent lamellae are parallel to each other. This investigation demonstrates the complexity and diversity of self-assembled structures formed from bifunctional molecules on solid surfaces. It also indicates that a solvent with the same functional group as the solute can significantly impact the formation of the self-assembled structures of these bifunctional molecules.
使用扫描隧道显微镜研究了溶解在辛酸中的双功能分子HOOC(CH2)nCOOH（n = 20、18、16、14、12、10）、HOOC(CH2)nCH2OH（n = 13、14）和HOCH2(CH2)14CH2OH的自组装单分子层，以了解双功能分子的自组装以及羧酸溶剂对在高定向热解石墨（HOPG）上形成自组装结构的影响。在二酸系列（HOOC(CH2)nCOOH）中，只有HOOC(CH2)20COOH与溶剂辛酸形成稳定的共吸附结构。其余二酸形成稳定的单组分单分子层，不与溶剂辛酸共吸附。观察到涉及二酸混合物的共吸附结构。该结果表明，与酸性溶剂或其他二酸发生共吸附是为了使覆盖层中的氢键密度最大化。基于这一概念提出了一个定量模型。对于异双功能分子HOOC(CH2)nCH2OH（n = 13、14），HOOC(CH2)14CH2OH和辛酸在分子水平上的共吸附产生了一个微观网格，该网格由均匀排列的开口组成，尺寸约为12.5 Å×约5.0 Å×约1.8 Å。对于异双功能分子HOOC(CH2)13CH2OH，两个相邻薄片的羟基组装形成人字形几何结构，两个羧酸基团以直的头对头构型组装。此外，在该分子的另一种自组装结构中观察到了一种新的COOH...O-H混合氢键网络。双功能分子HOCH2(CH2)14CH2OH通过不同的氢键网络在HOPG上呈现多种堆积模式。HOCH2(CH2)14CH2OH利用正构醇人字形结构典型的H-O...O-H网络进行自组装，在HOPG上形成不对称吸附质。它还形成了具有另一种氢键网络的畴，其中相邻薄片中的分子相互平行。这项研究证明了双功能分子在固体表面形成的自组装结构的复杂性和多样性。它还表明，与溶质具有相同官能团的溶剂会显著影响这些双功能分子自组装结构的形成。
