Molecular interactions at the solid–liquid interface have a profound impact on the macroscale wetting properties of surfaces. Surface-specific sum frequency generation (SFG) spectroscopy, one of a limited number of techniques having the capability to probe such interfaces, generates a surface vibrational spectrum that is sensitive to molecular structure and is used to determine the orientation of molecules at the interface. This study utilized SFG spectroscopy to study the interactions between the polar liquid acetonitrile and selectively fluorinated self-assembled monolayer (FSAM) interfaces generated by the adsorption of CF₃(CH₂)₁₇SH and CD₃(CF₂)₆(CH₂)₁₀SH on gold; these SAMs exhibit strong oriented molecular dipoles at their chain termini. We used SFG spectroscopy to examine the C–N and C–H stretching regions, 2000–2300 and 2800–3050 cm^–1, respectively, to probe the solid–liquid interface of the FSAMs in contact with acetonitrile. The appearance of positive- or negative-going peaks in the SFG spectrum of acetonitrile as a function of the variation in the terminal group of the FSAMs could be attributed, to a first-order approximation, to the orientation (CH₃ up or down) of the acetonitrile molecules with respect to changes in the direction of the FSAM dipole. Coherent molecular spectroscopy was used as a direct probe of this effect by examining the differences in the SFG spectra to determine the relative orientation of acetonitrile with respect to the underlying SAM and, therefore, provided direct evidence for the dipole-influenced wettability behavior of these unique model surfaces.

中文翻译：

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表面偶极子在接触极性液体时导致均匀的取向

固液界面上的分子相互作用对表面的宏观润湿性能产生了深远的影响。特定于表面的总频率生成（SFG）光谱是能够探测此类界面的有限数量的技术之一，它会生成对分子结构敏感的表面振动光谱，并用于确定分子在界面处的取向。这项研究利用SFG光谱研究了极性液体乙腈与CF ₃（CH ₂）₁₇ SH和CD ₃（CF ₂）₆（CH ₂）吸附产生的选择性氟化自组装单层（FSAM）界面之间的相互作用。黄金₁₀ SH；这些SAM在其链末端显示出强取向的分子偶极子。我们使用SFG光谱仪分别检查了2000–2300和2800–3050 cm ^–1的C–N和C–H拉伸区域，以探测FSAM与乙腈接触的固液界面。乙腈SFG谱中正向或负向峰的出现与FSAM端基变化的关系可以归因于一级近似（CH ₃乙腈分子相对于FSAM偶极子方向的变化）。通过检查SFG谱的差异来确定乙腈相对于底层SAM的相对方向，相干分子光谱法被用作此效应的直接探针，因此，为这些独特的偶极子影响的可湿性提供了直接证据。模型曲面。