Leng Yongsheng, Jiang Shaoyi
Department of Chemical Engineering, University of Washington, Box 351750, Seattle, WA 98195-1750, USA.
J Am Chem Soc. 2002 Oct 2;124(39):11764-70. doi: 10.1021/ja026274z.
A hybrid molecular simulation approach has been applied to investigate dynamic adhesion and friction between a chemical force microscope (CFM) tip and a substrate, both modified by self-assembled monolayers (SAMs) with hydrophobic methyl (CH(3)) or hydrophilic hydroxyl (OH) terminal groups. The method combines a dynamic model for the CFM tip-cantilever system and a molecular dynamics (MD) relaxation technique for SAMs on Au(111) at room temperature. The hybrid simulation method allows one to simulate force-distance curves (or adhesion) and friction loops (or friction coefficient) in the CFM on the experimental time scale for the first time. The simulation results also provide valuable molecular information at the interface that is not accessible in CFM experiments, such as the actual tip position with respect to the cantilever support position, molecular and hydrogen-bonding structures at the interface, and load distributions among different molecular chains (or single-molecule forces). Results show that the adhesion force and friction coefficient for the OH/OH contact pair are much larger than those for the CH(3)/CH(3) pair due to the formation of hydrogen bonds. During the retraction of a CFM tip from a surface, the CFM tip is away from the sample surface slightly while the spring undergoes dramatic elongation in the normal direction before rupture occurs. Single-molecule forces are distributed unevenly at the contact area. Surface energies calculated for functionalized surfaces compare well with those determined by experiments.
一种混合分子模拟方法已被用于研究化学力显微镜（CFM）探针与基底之间的动态粘附和摩擦，二者均由具有疏水甲基（CH(3)）或亲水羟基（OH）端基的自组装单分子层（SAMs）修饰。该方法结合了CFM探针 - 悬臂梁系统的动力学模型和室温下Au(111)上SAMs的分子动力学（MD）弛豫技术。这种混合模拟方法首次使人们能够在实验时间尺度上模拟CFM中的力 - 距离曲线（或粘附力）和摩擦环（或摩擦系数）。模拟结果还提供了CFM实验中无法获得的界面处有价值的分子信息，例如相对于悬臂梁支撑位置的实际探针位置、界面处的分子和氢键结构以及不同分子链之间的负载分布（或单分子力）。结果表明，由于氢键的形成，OH/OH接触对的粘附力和摩擦系数远大于CH(3)/CH(3)对。在CFM探针从表面缩回期间，在破裂发生之前，CFM探针会稍微远离样品表面，同时弹簧在法线方向上会发生剧烈伸长。单分子力在接触区域分布不均匀。功能化表面计算得到的表面能与实验测定的结果吻合良好。
