Temperature dependence of the structure of Langmuir films of normal-alkanes on liquid mercury.
The temperature dependent phase behavior of Langmuir films of n-alkanes [CH3(CH2)(n-2)CH3, denote Cn] on mercury was studied for chain lengths 19< or =n< or =22 and temperatures 15< or =T< or =44 degrees 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 approximately 5 degrees 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 approximately 4.5 degrees 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.
Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel.
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Deutsch
D000473:Alkanes / Q000737:chemistry*; D008628:Mercury / Q000737:chemistry*; D044367:Phase Transition; D013499:Surface Properties; D013816:Thermodynamics; D044366:Transition Temperature; D014961:X-Ray Diffraction
D000473:Alkanes; D008628:Mercury; C470023:docosane