Surfactant-Induced Phases in Water-Supported Alkane Monolayers: I. Thermodynamics

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• 作者：Shai Yefet ; Eli Sloutskin ; Lilach Tamam ; Zvi Sapir ; Asaf Cohen ; Moshe Deutsch ; Benjamin M. Ocko
• 刊名：Langmuir
• 出版年：2014
• 出版时间：July 15, 2014
• 年：2014
• 卷：30
• 期：27
• 页码：8000-8009
• 全文大小：429K
• ISSN：1520-5827

文摘

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_mTAB, in submillimolar concentrations causes the alkanes to spread and form a single Langmuir鈥揋ibbs (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鈥?1) and surfactant alkyl lengths (m = 14, 16, 18), at several concentrations c. The surface freezing range T_s鈥?i>T_b observed is up to 25 掳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鈥?theory, which includes an interchange energy term 蠅. 蠅 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.