Department of Chemistry, University of Massachusetts Lowell, 1 University Avenue, Lowell, Massachusetts 01854, USA.
Langmuir. 2009 Oct 20;25(20):12151-9. doi: 10.1021/la901678c.
We have used friction force microscopy to study the effects of adhesion on the boundary friction of self-assembled monolayers of the aromatic compounds thiophenol, p-phenylthiophenol, p-terphenyl thiol, 2-naphthalenethiol, and benzyl mercaptan on gold. To control the adhesion between the monolayer-covered tip and substrate, the friction measurements were made in dry N(2) gas or in ethanol. At low loads, low adhesion (in ethanol) resulted in a linear dependence of the friction force on load (i.e., F = muL) whereas higher adhesion between the same monolayers (in N(2)) gave an apparent area-dependent friction. The friction in the adhesive systems was well described by F = S(c)A with the contact area, A, calculated for a thin, linearly elastic film confined between rigid substrates using the thin-coating contact mechanics (TCCM) model in a transition regime between its DMT- and JKR-like limits. With increasing packing density of the monolayers, a systematic decrease was found in the friction coefficient (mu) obtained in ethanol and the critical shear stress (S(c)) obtained in N(2). To describe these aromatic monolayers with the extended TCCM model, a higher Young's modulus was neeeded than for fatty acid monolayers of similar packing density.
我们使用摩擦力显微镜研究了粘附对自组装芳香族化合物巯基苯、对苯基苯硫酚、对三联苯硫醇、2-萘硫醇和苯甲硫醇在金上的单层边界摩擦的影响。为了控制单层覆盖尖端和基底之间的粘附力，在干燥的氮气或乙醇中进行了摩擦测量。在低负载下，低粘附力（在乙醇中）导致摩擦力与负载呈线性关系（即 F = μL），而相同单层之间的高粘附力（在氮气中）则导致明显的面积相关摩擦。在粘性系统中，摩擦力可以很好地用 F = S(c)A 来描述，其中接触面积 A 是根据薄弹性膜在刚性基底之间的薄涂层接触力学 (TCCM) 模型计算得出的，该模型处于其 DMT 和 JKR 类似极限之间的过渡区。随着单层堆积密度的增加，在乙醇中获得的摩擦系数（μ）和在氮气中获得的临界剪切应力（S(c)）呈系统性下降。为了用扩展的 TCCM 模型描述这些芳香族单层，需要比具有相似堆积密度的脂肪酸单层更高的杨氏模量。
