Brewer Nicholas J, Leggett Graham J
Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, UK.
Langmuir. 2004 May 11;20(10):4109-15. doi: 10.1021/la036301e.
Mixed self-assembled monolayers formed by the coadsorption of hydroxyl- and methyl-terminated alkanethiols with similar chain lengths have been characterized by friction force microscopy. Friction coefficients have been determined by assuming a fit to Amonton's law. The friction coefficients vary linearly with the fraction of polar-terminated adsorbates in the self-assembled monolayer (SAM). With carboxylic acid-terminated tips, the coefficient of friction increases with the fraction of hydroxyl-terminated thiols, while with methyl-terminated tips it decreases. Similar trends are observed for pull-off forces, which increase and decrease as a function of the fraction of polar-terminated adsorbates for carboxylic acid- and methyl-terminated adsorbates, respectively. Analysis of histograms of adhesion forces has yielded insights into the phase structure of mixed SAMs. Single-component monolayers yield histograms that may be fitted to symmetric Gaussian distributions, irrespective of the nature of the terminal group on either the tip or the SAM. However, mixed monolayers yield broad, asymmetric distributions that could not be fitted with a Gaussian distribution. The best explanation for these data is that mixed SAMs of hydroxyl- and methyl-terminated alkanethiols of similar chain length form phase-separated structures.
通过摩擦力显微镜对由链长相似的羟基封端和甲基封端的烷硫醇共吸附形成的混合自组装单分子层进行了表征。通过假设符合阿蒙顿定律来确定摩擦系数。摩擦系数随自组装单分子层（SAM）中极性封端吸附质的比例呈线性变化。使用羧酸封端的探针时，摩擦系数随羟基封端硫醇的比例增加，而使用甲基封端的探针时则降低。对于拉脱力也观察到类似趋势，拉脱力分别随羧酸封端和甲基封端吸附质中极性封端吸附质的比例增加和降低。对粘附力直方图的分析揭示了混合SAM的相结构。无论探针或SAM上末端基团的性质如何，单组分单分子层产生的直方图都可以拟合为对称高斯分布。然而，混合单分子层产生的是宽的、不对称的分布，无法用高斯分布拟合。对这些数据的最佳解释是，链长相似的羟基封端和甲基封端的烷硫醇混合SAM形成了相分离结构。
