Wilkinson Katherine M, Qunfang Lei, Bain Colin D
Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, UKOX1 3TA.
Soft Matter. 2006 Dec 12;2(1):66-76. doi: 10.1039/b511561b.
Mixed monolayers at the air-water interface of the cationic surfactant, hexadecyltrimethylammonium bromide (CTAB), with alkanes show a first-order freezing transition as a function of temperature. Sum-frequency spectra and ellipsometric measurements are consistent with a structure in which the high-temperature phase is liquid-like and the low-temperature phase has all-, upright chains. There are strong structural similarities between the low-temperature phase in the mixed monolayers and frozen monolayers at the alkane-air and alkane-CTAB solution interfaces. The difference between the surface freezing point and the freezing point of the bulk alkane ranges from 1 °C for alkane chain length = 17, to 28 °C for = 11. Surface freezing is more favourable in mixed monolayers at the air-water interface than at the bulk alkane-water interface for the same surfactant concentration. Long-chain alkanes do not wet water, but it is postulated that if they did, they would also show surface freezing analogously to alkanes on silica. The surfactant plays the dual role of enhancing wetting and surface freezing of the alkane on water.
阳离子表面活性剂十六烷基三甲基溴化铵（CTAB）与烷烃在气-水界面形成的混合单分子层呈现出随温度变化的一级冻结转变。和频光谱和椭偏测量结果与这样一种结构相符：高温相呈类液态，低温相具有全直立链。混合单分子层中的低温相与烷烃-空气和烷烃-CTAB溶液界面处的冻结单分子层之间存在很强的结构相似性。表面冰点与本体烷烃冰点之间的差异范围从烷烃链长为17时的1℃到链长为11时的28℃。在相同表面活性剂浓度下，气-水界面的混合单分子层中的表面冻结比本体烷烃-水界面更有利。长链烷烃不与水浸润，但据推测，如果它们能与水浸润，也会类似于二氧化硅上的烷烃那样表现出表面冻结。表面活性剂在增强烷烃在水上的浸润和表面冻结方面起到了双重作用。
