Temperature Dependence of the Structure of Langmuir Films of Normal-Alkanes on Liquid Mercury
Ocko, B. M.
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CitationKraack, H., B. M. Ocko, Peter S. Pershan, L. Tamam, and M. Deutsch. 2004. Temperature dependence of the structure of Langmuir films of normal-alkanes on liquid mercury. Journal of Chemical Physics 121(16): 8003-8009.
AbstractThe temperature dependent phase behavior of Langmuir films of n-alkanes [CH3(CH2)n−2CH3, denote Cn] on mercury was studied for chain lengths 19 ⩽ n ⩽ 22 and temperatures 15 ⩽ T ⩽ 44 °C, using surface tensiometry and surface x-ray diffraction methods. In contrast with Langmuir films on water, where molecules invariably orient roughly surface normal, alkanes on mercury are always oriented surface parallel and show no long-range in-plane order at any surface pressure. A gas and several condensed phases of single, double, and triple layers of lying-down molecules are found, depending on n and T. At high coverages, the alkanes studied here show transitions from a triple to a double to a single layer with increasing temperature. The transition temperature from a double to a single layer is found to be ∼ 5 °C, lower than the bulk rotator-to-liquid melting temperature, while the transition from a triple to a double layer is about as much below the double-to-single layer transition. Both monolayer and bulk transition temperatures show a linear increase with n with identical slopes of ∼ 4.5 °C/CH2 within the range of n values addressed here. It is suggested that the film and bulk transitions are both driven by a common cause: the proliferation of gauche defects in the chain with increasing temperature.
Citable link to this pagehttp://nrs.harvard.edu/urn-3:HUL.InstRepos:5028195
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