Surfactant-Induced Phases in Water-Supported Alkane Monolayers: II. Structure

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• 作者：Shai Yefet ; Eli Sloutskin ; Lilach Tamam ; Zvi Sapir ; 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：8010-8019
• 全文大小：524K
• ISSN：1520-5827

文摘

The structure of the Langmuir鈥揋ibbs films of normal alkanes C_n of length n = 12鈥?1 formed at the surface of aqueous solutions of C_mTAB surfactants, m = 14, 16, and 18, was studied by surface-specific synchrotron X-ray methods. At high temperatures, a laterally disordered monolayer of mixed alkane molecules and surface-adsorbed surfactant tails is found, having thicknesses well below those of the alkanes鈥?and surfactant tails鈥?extended length. The mixed monolayer undergoes a freezing transition at a temperature T_s(n,m), which forms, for n 鈮?m + 1, a crystalline monolayer of mixed alkane molecules and surfactant tails. For n 鈮?m + 2, a bilayer forms, consisting of an upper pure-alkane, crystalline monolayer and a lower liquidlike monolayer. The crystalline monolayer in both cases consists of hexagonally packed extended, surface-normal-aligned chains. The hexagonal lattice constant is found to decrease with increasing n. The films鈥?structure is discussed in conjunction with their thermodynamic properties presented in an accompanying paper.