Yefet Shai, Sloutskin Eli, Tamam Lilach, Sapir Zvi, Cohen Asaf, Deutsch Moshe, Ocko Benjamin M
Physics Department and Institute of Nanotechnology, Bar-Ilan University , Ramat-Gan 52900, Israel.
Langmuir. 2014 Jul 15;30(27):8000-9. doi: 10.1021/la501567s. Epub 2014 Jul 1.
Alkanes longer than n = 6 carbons do not spread on the water surface, but condense in a macroscopic lens. However, adding trimethylammonium-based surfactants, C(m)TAB, in submillimolar concentrations causes the alkanes to spread and form a single Langmuir-Gibbs (LG) monolayer of mixed alkanes and surfactant tails, which coexists with the alkane lenses. Upon cooling, this LG film surface-freezes at a temperature T(s) above the bulk freezing temperature T(b). The thermodynamics of surface freezing (SF) of these LG films is studied by surface tension measurements for a range of alkanes (n = 12-21) and surfactant alkyl lengths (m = 14, 16, 18), at several concentrations c. The surface freezing range T(s)-T(b) observed is up to 25 °C, an order of magnitude larger than the temperature range of SF monolayers on the surface of pure alkane melts. The measured (n,T) surface phase diagram is accounted for well by a model based on mixtures' theory, which includes an interchange energy term ω. ω is found to be negative, implying attraction between unlike species, rather than the repulsion found for SF of binary alkane mixtures. Thus, the surfactant/alkane mixing is a necessary condition for the occurrence of SF in these LG films. The X-ray derived structure of the films is presented in an accompanying paper.
碳原子数多于6个的烷烃不会在水面上铺展，而是会凝聚成一个宏观的透镜状。然而，添加亚毫摩尔浓度的基于三甲铵的表面活性剂十六烷基三甲基溴化铵（C(m)TAB）会使烷烃铺展，并形成由混合烷烃和表面活性剂尾部组成的单一朗缪尔 - 吉布斯（LG）单分子层，该单分子层与烷烃透镜共存。冷却时，这种LG膜在高于本体凝固温度T(b)的温度T(s)下发生表面冻结。通过测量一系列烷烃（n = 12 - 21）和表面活性剂烷基链长度（m = 14、16、18）在几种浓度c下的表面张力，研究了这些LG膜的表面冻结（SF）热力学。观察到的表面冻结温度范围T(s) - T(b)高达25℃，比纯烷烃熔体表面上的SF单分子层的温度范围大一个数量级。基于混合物理论的模型很好地解释了测量得到的（n，T）表面相图，该模型包括一个交换能项ω。发现ω为负，这意味着不同种类之间存在吸引力，而不是二元烷烃混合物SF中发现的排斥力。因此，表面活性剂/烷烃混合是这些LG膜中发生SF的必要条件。薄膜的X射线衍生结构在随附的论文中给出。
