Chen Hua, Gan Wei, Lu Rong, Guo Yuan, Wang Hong-fei
State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, PR China 100080.
J Phys Chem B. 2005 Apr 28;109(16):8064-75. doi: 10.1021/jp0501906.
Vapor/methanol and vapor/methanol-water mixture interfaces have been among the benchmark liquid interfaces under extensive experimental and theoretical investigation. In this report, we studied the orientation, structure and energetics of the vapor/methanol-water interface with newly developed techniques in sum frequency generation vibrational spectroscopy (SFG-VS). Different from the interpretations in previous SFG-VS studies for a more disordered interface at higher bulk methanol concentrations, we found that the methanol-water mixture interface is well ordered in the whole concentration region. We are able to do so because direct polarization null angle (PNA) measurement allowed us to accurately determine the CH3 orientation at the interface and to separate the orientational and interface density contributions to the SFG-VS signal. We found that the CH3 groups at the interface pointed out almost perpendicularly from the interface. We further found that this well-ordered vapor/methanol-water mixture interface has an antiparallel structure. With the double layer adsorption model (DAM) and Langmuir isotherm, the adsorption free energies for the first and second layer are obtained as -1.7 +/- 0.1 kcal/mol and 0.5 +/- 0.4 kcal/mol, respectively. Therefore, the second layer adsorption is slightly negative, and this means that replacement of the second layer water molecule with methanol molecule is energetically unfavorable. Comparing this interface with the vapor/acetone-water mixture interface reported previously, we are able to correlate the second layer adsorption free energy with the work of self-association using the pairwise self- and mutual interaction energies between the water and solute molecules. These results provided detailed microscopic structural evidences for understanding of liquid interfaces.
蒸汽/甲醇以及蒸汽/甲醇 - 水混合物界面一直是在广泛的实验和理论研究中的基准液体界面。在本报告中，我们使用和频振动光谱（SFG - VS）中的新开发技术研究了蒸汽/甲醇 - 水界面的取向、结构和能量学。与之前SFG - VS研究中对于较高本体甲醇浓度下更无序界面的解释不同，我们发现甲醇 - 水混合物界面在整个浓度区域内都是有序的。我们之所以能够做到这一点，是因为直接极化零角（PNA）测量使我们能够准确确定界面处CH₃的取向，并区分取向和界面密度对SFG - VS信号的贡献。我们发现界面处的CH₃基团几乎垂直于界面指向。我们进一步发现这个有序的蒸汽/甲醇 - 水混合物界面具有反平行结构。通过双层吸附模型（DAM）和朗缪尔等温线，得到第一层和第二层的吸附自由能分别为 -1.7±0.1 kcal/mol和0.5±0.4 kcal/mol。因此，第二层吸附略显负值，这意味着用甲醇分子取代第二层水分子在能量上是不利的。将该界面与先前报道的蒸汽/丙酮 - 水混合物界面进行比较，我们能够利用水和溶质分子之间的成对自相互作用能和相互作用能将第二层吸附自由能与自缔合功联系起来。这些结果为理解液体界面提供了详细的微观结构证据。
