Structural Chemical Study of Nonionic Surfactant-Water Systems by Vibrational Spectroscopy

为了阐明烷基氧乙烯非离子表面活性剂与水二元体系的各种特性，亲水氧乙烯链的性质起着重要的作用。特别是，氧乙烯链的构象行为是理解体系结构的重要因素之一。因此，在1992年，对非环和环氧乙烯链的构象行为进行了详细的研究。所使用的模型化合物分别是-甲基-omega-甲氧基己基（氧乙烯）和18-冠-6。利用红外光谱法结合正坐标计算，在较宽的浓度范围内研究了氧乙烯链的水合和构象状态。根据水化结构的变化，阐明了OC-CO基团的构象。在纯液体中加水后，OC-CO基团的间扭构象更加稳定，氧乙烯单元的醚氧与水分子之间形成了桥式氢键。间扭OC-CO基团的氧之间的距离约为0.28 nm。然而，对于单个水分子来说，这个距离太短，无法形成稳定的桥接氢键，但可能足以通过质子-孤对相互作用来稳定间扭式结构。本年度还对典型的烷基氧乙烯非离子表面活性剂C_<16>E_3的多态行为进行了研究。用拉曼光谱分析了结晶性和构象随相变的变化。这种多晶现象与非离子表面活性剂在水溶液中形成的各种中间相的结构密切相关。

For elucidation of various characteristics of binary systems of alkyloxyethylene nonionic surfactants and water, properties of the hydrophilic oxyethylene chain play an important role. In particular, the conformational behavior of the oxyethylene chain is one of the important factors for understanding the structure of the system. Accordingly, in this year of 1992, conformational behavior of noncyclic and cyclic oxyethylene chains was studied in detail. The model compounds used are alpha-methyl-omega-methoxyhexakis(oxyethylene) and 18-crown-6, respectively. By utilizing infrared spectroscopy in combination with normal coordinate calculations, hydration and conformational states of the oxyethylene chain were examined in an extensive concentration range. Conformation of the OC-CO group was elucidated on the basis of changes of the hydration structure. With addition of water to neat liquid, the gauche conformation of the OC-CO group is more stabilized with the formation of bridged hydrogen bonds between the ether oxygens of the oxyethylene unit and water molecules. The distance between the oxygens of the gauche OC-CO group is about 0.28 nm. This distance is, however, too short for a single water molecule to form a stable bridged hydrogen bond, but may be adequate to stabilize the gauche structure through proton-lone pair interactions.In this year, another study was also performed on polymorphic behavior of C_<16>E_3, which is one of the typical alkyloxyethylene nonionic surfactants. Changes in crystallinity and conformation with phase transformations were elucidated by Raman spectroscopy. This polymorphic phenomenon is closely related to the structure of various mesophases formed by nonionic surfactants in aqueous solution.