Institute of Chemistry, Martin-Luther-University Halle-Wittenberg, von-Dankelmann-Platz 4, D-06120 Halle (Saale), Germany.
J Phys Chem B. 2012 Oct 11;116(40):12245-56. doi: 10.1021/jp306612k. Epub 2012 Sep 28.
We studied the behavior of monolayers at the air/water interface of T-shaped facial amphiphiles which show liquid-crystalline mesophases in the bulk. The compounds are composed of a rigid p-terphenyl core (TP) with two terminal hydrophobic ether linked alkyl chains of equal length and one facial hydrophilic tri(ethylene oxide) chain with a carboxylic acid end group. Due to their amphiphilic nature they form stable Langmuir films at the air/water interface. Depending on the alkyl chain length they show markedly different compression isotherms. We used infrared reflection absorption spectroscopy (IRRAS) to study the changes in molecular organization of the TP films upon compression. We could retrieve information on layer thickness, alkyl chain crystallization, and the orientation of the TP cores within the films. Films of TPs with long (16 carbon atoms: TP 16/3) and short (10 carbon atoms: TP 10/3) alkyl chains were compared. Compression of TP 16/3 leads to crystallization of the terminal alkyl chains, whereas the alkyl chains of TP 10/3 stay fluid over the complete compression range. TP 10/3 shows an extended plateau in the compression isotherm which is due to a layering transition. The mechanism of this layering transition is discussed. Special attention was paid to the question of whether a so-called roll-over collapse occurs during compression. From the beginning to the end of the plateau, the layer thickness is increased from 15 to 38 Å and the orientation of the TP cores changes from parallel to the water surface to isotropic. We conclude that the plateau in the compression isotherm reflects the transition of a TP monolayer to a TP multilayer. The monolayer consists of a sublayer of well-organized TP cores underneath a sublayer of fluid alkyl chains whereas the multilayer consists of a well oriented bottom layer and a disordered top layer. Our findings do not support the model of a roll-over collapse. This study demonstrates how the IRRA band intensity of OH or OD stretching vibrations can be used to retrieve information about layer thickness and refractive indices of the film and how multicomponent IRRA bands can be fitted to retrieve information about the orientation of molecules within the monolayer.
我们研究了在具有液晶中间相的 T 形两亲分子的空气/水界面上单分子层的行为，这些分子在本体中由刚性对三联苯核心（TP）组成，具有两个相等长度的末端疏水性醚连接的烷基链和一个带有羧酸末端基团的面状亲水性三（氧化乙烯）链。由于它们的两亲性质，它们在空气/水界面上形成稳定的 Langmuir 膜。根据烷基链的长度，它们显示出明显不同的压缩等温线。我们使用红外反射吸收光谱（IRRAS）研究了 TP 薄膜在压缩时分子组织的变化。我们可以获取有关层厚度、烷基链结晶和薄膜中 TP 核取向的信息。比较了具有长（16 个碳原子：TP16/3）和短（10 个碳原子：TP10/3）烷基链的 TP 薄膜的压缩情况。TP16/3 的压缩导致末端烷基链结晶，而 TP10/3 的烷基链在整个压缩范围内保持液态。TP10/3 在压缩等温线上显示出扩展的平台，这是由于分层转变。讨论了这种分层转变的机制。特别关注在压缩过程中是否发生所谓的“翻滚崩溃”。从平台的开始到结束，层厚度从 15 增加到 38 Å，TP 核的取向从平行于水面变为各向同性。我们得出结论，压缩等温线上的平台反映了 TP 单层向 TP 多层的转变。单层由下面有序的 TP 核亚层和上面的流体烷基链亚层组成，而多层由取向良好的底层和无序的顶层组成。我们的发现不支持翻滚崩溃的模型。这项研究表明，如何使用 OH 或 OD 伸缩振动的 IRRA 带强度来获取有关膜的层厚度和折射率的信息，以及如何拟合多组分 IRRA 带以获取有关单层中分子取向的信息。
