Direct evidence for orientational flip-flop of water molecules at charged interfaces: A heterodyne-detected vibrational sum frequency generation study
Direct evidence for orientational flip-flop of water molecules at charged interfaces: A heterodyne-detected vibrational sum frequency generation study
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DOI：
10.1063/1.3135147
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发表时间：
2009-05-28
影响因子：
4.4
通讯作者：
Tahara, Tahei
中科院分区：
文献类型：
--
作者：
Nihonyanagi, Satoshi;Yamaguchi, Shoichi;Tahara, Tahei
本文首次用外差探测宽带振动和频产生光谱法测量了荷电表面活性剂存在下空气/水界面的复合chi（2）光谱。与纯净水表面相反，在带电表面活性剂存在下，两个宽OH带的chi（（2））符号相同。获得的chi（（2））光谱清楚地显示了界面水分子的翻转，这是由表面活性剂头部基团的相反电荷引起的。利用理论上获得的β（（2））的符号，绝对取向，即，水分子在带电水表面上的上/下取向角由χ（2）的符号和分子取向角之间的关系唯一地确定。水分子的取向是氢原子在带负电荷的水界面上，而氧原子在带正电荷的水界面上。
Complex chi((2)) spectra of air/water interfaces in the presence of charged surfactants were measured by heterodyne-detected broadband vibrational sum frequency generation spectroscopy for the first time. In contrast to the neat water surface, the signs of chi((2)) for two broad OH bands are the same in the presence of the charged surfactants. The obtained chi((2)) spectra clearly showed flip-flop of the interfacial water molecules which is induced by the opposite charge of the head group of the surfactants. With the sign of beta((2)) theoretically obtained, the absolute orientation, i.e., up/down orientation, of water molecules at the charged aqueous surfaces was uniquely determined by the relation between the sign of chi((2)) and the molecular orientation angle. Water molecules orient with their hydrogen up at the negatively charged aqueous interface whereas their oxygen up at the positively charged aqueous interface.