Kataoka Sho, Cremer Paul S
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77842-3012, USA.
J Am Chem Soc. 2006 Apr 26;128(16):5516-22. doi: 10.1021/ja060156k.
The orientation of the isopropyl group at the liquid/vapor interface in 2-propanol/water binary mixtures was studied by vibrational sum frequency spectroscopy. The CH(3) stretch modes of the two methyl groups were used to determine the molecule's orientation by employing a novel united atom approach to model the (CH(3))(2)X moiety. For this purpose, the changes in the molecular susceptibility of the isopropyl group stretches were derived in the laboratory frame as a function of the tilt and twist angles. The results indicated that the methyl groups lay down on the surface at low alcohol mole fraction and gradually twisted with increasing mole fraction. At the azeotrope, x(iso) = 0.68, one of the methyl groups aligned approximately parallel to the surface normal, whereas the other was nearly parallel with the liquid/vapor interface. When the mole fraction of 2-propanol was higher than 0.68, the orientation of 2-propanol remained almost constant. The change in the alcohol's orientation with 2-propanol mole fraction closely tracked changes in its bulk activity coefficient. Such results lead to a picture in which the surface structure and bulk properties of the system are closely linked.
通过振动和频光谱研究了2-丙醇/水二元混合物中异丙基在液/气界面的取向。利用两个甲基的CH(3)伸缩振动模式，采用一种新颖的联合原子方法对(CH(3))(2)X部分进行建模，以确定分子的取向。为此，在实验室坐标系中得出异丙基伸缩振动的分子极化率变化与倾斜角和扭转角的函数关系。结果表明，在低醇摩尔分数下甲基平躺于表面，且随着摩尔分数增加逐渐扭转。在共沸物处，x(iso)=0.68，其中一个甲基近似平行于表面法线排列，而另一个几乎平行于液/气界面。当2-丙醇的摩尔分数高于0.68时，2-丙醇的取向几乎保持不变。醇的取向随2-丙醇摩尔分数的变化紧密跟踪其本体活度系数的变化。这些结果描绘出一幅该体系表面结构与本体性质紧密相连的图景。
