Temperature dependence of the structure of Langmuir films of normal-alkanes on liquid mercury

• Published in: The Journal of chemical physics 2004-10, Vol.121 (16), p.8003-8009
• Main Authors: Kraack, H, Ocko, B M, Pershan, P S, Tamam, L, Deutsch, M
• Format: Article
• Language: English
• Subjects: Alkanes - chemistry; Mercury - chemistry; Phase Transition; Surface Properties; Thermodynamics; Transition Temperature; X-Ray Diffraction
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.1799993

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.