Subnanometer-Scale Mobile Structures Localized at an Interface Between One-Dimensionally Aligned Sulfonate Groups and Water Investigated by Three-Dimensional Scanning AFM

The sulfonate groups play critical roles in various interfacial phenomena, such as proton conduction and ion exchange. In proton-conductive materials such as Nafion, control of the network structure formed by the coexistence of hydrophobic chains and sulfonate groups leads to efficient proton conduction. Understanding the spatial distribution of water molecules and ions around sulfonate groups at the single functional group scale is required to elucidate the proton conduction mechanism. For this purpose, a technique for directly measuring the interface between sulfonate groups and water at the nanoscale is required. Although direct measurements of local interfacial waters and ions are difficult even with well-established methods, a three-dimensional scanning atomic force microscope (3D-AFM) can visualize the local interfacial structures at subnanometer resolution by combining frequency modulation AFM (FM-AFM).<br/>In this study, we directly measured the interfacial structure formed on one-dimensionally arranged sulfonate groups in a self-assembled monolayer (SAM) with the coexistence of hydrophobic alkyl chains by 3D-AFM. Prepared sulfonate-terminated alkanethiols SAMs (SO₃-SAM) showed stripe-like structures. Owing to the lying-down orientations of sulfonate-terminated alkanethiol molecules in the stripe-like structures, one-dimensionally aligned structures of the sulfonate groups were formed. As a result, the 1D-aligned sulfonate groups were surrounded by hydrophobic alkyl chains. The lying-down SO₃-SAM structures can be considered a structural model for a controlled nano-environment where hydrophobic groups and sulfonate groups coexist. Our results suggested that the growth and collapse of mobile interfacial structures exist at the SO₃-SAM surfaces based on analysis of the successive AFM images. In addition, the mobile interfacial structures were localized in the hydrophilic region with the 1D-aligned sulfonate groups. The 3D-AFM measurements were performed to understand the spatial distribution of the mobile interfacial structures. By analyzing the 3D-AFM images obtained at the interfacial nanospace, the mobile interfacial structures were visualized as localized regions with higher repulsive forces than the surrounding areas. In the presentation, we will discuss the detailed analysis of FM-AFM and 3D-AFM images and the origin of mobile interfacial structures.