Kaluarachchi Chathuri P, Lee Hansol D, Lan Yiling, Lansakara Thiranjeewa I, Tivanski Alexei V
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States.
Langmuir. 2021 Feb 23;37(7):2457-2465. doi: 10.1021/acs.langmuir.0c03507. Epub 2021 Feb 12.
To elucidate the intricate role that the sea surface microlayer (SML) and sea spray aerosols (SSAs) play in climate, understanding the chemical complexity of the SML and how it affects the physical-chemical properties of the microlayer and SSA are important to investigate. While the surface tension of the SML has been studied previously using conventional experimental tools, accurate measurements must be localized to the thickness of the air-liquid interface of the SML. Here we explore the atomic force microscopy (AFM) capabilities to quantify the surface tension of aqueous solution droplets with (sub)micrometer indentation depths into the interface. Sample droplets of hexanoic acid at molar concentrations ranging from 0.1 to 80 mM and SML from a recent wave flume study were investigated. A constant-radius AFM nanoneedle was used to probe ca. 200 μL droplets with 0.3-1.2 μm indentation depths. As a comparison, the surface tension of bulk samples was also measured using a conventional force tensiometer. The data for the hexanoic acid show an excellent overlap between the AFM and force tensiometer surface tension measurements. For the surface tension measurements of the SML, however, the measured values from the AFM were 2.5 mN/m lower than that from the force tensiometer, which was attributed to the structural and chemical complexity of the SML, differences in the probing depth for each method, and the time scale required for the surface film to restructure as the needle is retracted away from the liquid surface. Overall, the study confirmed the accuracy of the AFM method in quantifying the surface tension of aqueous solutions over a wide range of concentrations for surface-active organic compounds. The methodology can be further used to reveal small, yet important, differences in the surface tension of complex air-liquid interfaces such as liquid systems where the type and concentration of surfactants vary with the distance from the air-liquid interface. For such complex systems, AFM measurements of the surface tension as a function of the probing depth and pulling rate may reveal a sublayer film structure of the liquid interface.
为了阐明海表面微层（SML）和海飞沫气溶胶（SSA）在气候中所起的复杂作用，了解SML的化学复杂性及其如何影响微层和SSA的物理化学性质对于开展研究很重要。虽然此前已使用传统实验工具对SML的表面张力进行了研究，但精确测量必须局限于SML气液界面的厚度。在此，我们探索原子力显微镜（AFM）在量化水溶液液滴表面张力方面的能力，这些液滴在界面处有（亚）微米级的压痕深度。研究了摩尔浓度范围为0.1至80 mM的己酸样品液滴以及近期波浪水槽研究中的SML。使用恒定半径的AFM纳米针探测约200 μL、压痕深度为0.3 - 1.2 μm的液滴。作为对比，还使用传统力 tensiometer测量了本体样品的表面张力。己酸的数据显示AFM和力 tensiometer表面张力测量结果有很好的重叠。然而，对于SML的表面张力测量，AFM测量值比力 tensiometer低2.5 mN/m，这归因于SML的结构和化学复杂性、每种方法探测深度的差异以及当针从液体表面缩回时表面膜重新结构所需的时间尺度。总体而言，该研究证实了AFM方法在量化表面活性有机化合物广泛浓度范围内水溶液表面张力方面的准确性。该方法可进一步用于揭示复杂气液界面（如表面活性剂类型和浓度随距气液界面距离变化的液体系统）表面张力的微小但重要的差异。对于此类复杂系统，AFM作为探测深度和提拉速率函数的表面张力测量可能会揭示液体界面的亚层膜结构。
