Institute of Physical and Theoretical Chemistry, University of Regensburg, Regensburg, Germany.
Langmuir. 2013 Apr 16;29(15):4726-36. doi: 10.1021/la304439t. Epub 2013 Apr 2.
Natural nonionic amphiphiles forming monolayers, bilayers, micelles, or biomembranes create a positive dipole potential at the boundary with water. In a series of papers we have reported on Langmuir monolayers with CF3 terminals of the polar heads, which show a negative surface dipole potential ΔV (Petrov , J. G.; Andreeva, T. D.; Kurt, D. K.; Möhwald, H. J. Phys. Chem. B 2005, 109, 14102). Here we use vibrational sum frequency spectroscopy (SF) to study the origin of the opposite ΔV signs of Langmuir films of CH3(CH2)20COCH2CH3 (ethyl ether, EE) and CH3(CH2)20COCH2CF3 (fluorinated ethyl ether, FEE). The vibrational sum frequency spectra are recorded at the same film density of the S-phase of the EE and FEE monolayers and analyzed in the spectral regions of OH, COC, CH3, and CF3 stretching vibrations because these functional groups could be responsible for the different dipole potentials. We compare the rearrangement of the pure water surface by EE and FEE monolayers and the conformations of EE and FEE polar heads. The analysis is performed according to the three-capacitor model of the dipole potential of Langmuir monolayers (Demchak, R. T.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191). The results show that reversal of the ΔV sign caused by fluorination of the polar heads originates from the upward-oriented CF3 terminals of the FEE heads, whose negative normal dipole moment component determines the negative dipole potential of the FEE monolayer.
形成单层膜、双层膜、胶束或生物膜的天然非离子两亲物在与水的边界处产生正偶极势。在一系列论文中，我们报告了具有 CF3 端基的极性头的 Langmuir 单层膜，它们显示出负的表面偶极势ΔV（Petrov，J. G.；Andreeva，T. D.；Kurt，D. K.；Möhwald，H. J. Phys. Chem. B 2005，109，14102）。在这里，我们使用振动和频光谱（SF）来研究 CH3（CH2）20COCH2CH3（乙基醚，EE）和 CH3（CH2）20COCH2CF3（氟化乙基醚，FEE）Langmuir 薄膜相反的ΔV 符号的起源。在 EE 和 FEE 单层 S 相的相同膜密度下记录振动和频光谱，并在 OH、COC、CH3 和 CF3 伸缩振动的光谱区域进行分析，因为这些官能团可能负责不同的偶极势。我们比较了 EE 和 FEE 单层对纯水表面的重新排列以及 EE 和 FEE 极性头的构象。根据 Langmuir 单层偶极势的三电容模型（Demchak，R. T.；Fort，T.，Jr. J. Colloid Interface Sci. 1974，46，191）进行分析。结果表明，氟原子取代极性头引起的ΔV 符号反转源自 FEE 头向上取向的 CF3 端基，其负的正常偶极矩分量决定了 FEE 单层的负偶极势。
