School of Chemical, Biological and Materials Engineering and Institute of Applied Surfactant Research , University of Oklahoma , Norman , Oklahoma 73019 , United States.
School of Chemistry , Monash University , Wellington Road , Clayton , Victoria 3800 , Australia.
Langmuir. 2018 Jun 26;34(25):7223-7239. doi: 10.1021/acs.langmuir.8b00574. Epub 2018 Jun 15.
Force curves collected using an atomic force microscope (AFM) in the presence of adsorbed surfactants are often used to draw conclusions about adsorbed film packing, rigidity, and thickness. However, some noteworthy features of such force curve characteristics have yet to be thoroughly investigated and explained. In this work, we collected force curves from tetradecyltrimethylammonium bromide films adsorbed on highly oriented pyrolytic graphite (HOPG), silica, and silica that had been hydrophobized by functionalization with dichlorodimethyl silane. Breakthrough events in the force curves from several different trials were compared to show that the breakthrough distance, often reported as the adsorbed film thickness, increased with concentration below the critical micelle concentration (CMC) but was approximately 3.5 nm on all surfaces between 2× and 10× CMC; an unexpected result because of the different surface chemistries for the three surfaces. We employed an AFM probe with a different force constant ( k) value as well as a colloidal probe and the breakthrough distance remained approximately 3.5 nm in all cases. Gradient mapping, a variant of force mapping, was also implemented on the three surfaces and resulted in a new technique for visualizing adsorbed surfactant in situ. The resulting maps showed patches of adsorbed surfactant below the CMC and revealed that with increasing concentration, the size of the patches increased resulting in full coverage near and above the CMC. These results are, to our knowledge, the first time force mapping has been used to spatially track patches of adsorbed surfactant. Finally, layers of surfactants on an AFM tip were investigated by collecting a force map on a single AFM tip using the tip of a separate AFM probe. A breakthrough event was observed between the tips, indicating that a layer of surfactant was present on at least one, if not both tips.
使用原子力显微镜（AFM）在吸附的表面活性剂存在下收集的力曲线通常用于得出关于吸附膜堆积、刚性和厚度的结论。然而，这种力曲线特征的一些显著特征尚未得到彻底研究和解释。在这项工作中，我们从吸附在高取向热解石墨（HOPG）、二氧化硅和用二氯二甲基硅烷官能化的疏水性二氧化硅上的十四烷基三甲基溴化铵薄膜上收集力曲线。比较了来自几个不同试验的力曲线中的突破事件，以表明突破距离，通常报告为吸附膜厚度，在低于临界胶束浓度（CMC）的浓度下随浓度增加而增加，但在 2×至 10×CMC 之间，所有表面上的距离约为 3.5nm；这是一个意外的结果，因为三个表面的表面化学性质不同。我们还使用了具有不同力常数（k）值的 AFM 探针以及胶体探针，在所有情况下突破距离都约为 3.5nm。梯度映射，力映射的一种变体，也在三个表面上实施，并产生了一种新的原位可视化吸附表面活性剂的技术。得到的图谱显示了 CMC 以下吸附表面活性剂的斑块，并表明随着浓度的增加，斑块的大小增加，在接近和高于 CMC 时导致完全覆盖。据我们所知，这些结果是首次使用力映射来空间跟踪吸附表面活性剂的斑块。最后，通过使用单独的 AFM 探针的尖端在单个 AFM 探针上收集力图来研究 AFM 探针尖端上的表面活性剂层。在尖端之间观察到突破事件，表明至少在一个尖端上存在一层表面活性剂，如果不是两个尖端上都存在的话。
