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 19less than or equal tonless than or equal to22 and temperatures 15less than or equal toTless than or equal to44 degreesC, 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 similar to5 degreesC, 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 similar to4.5 degreesC/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.(C) 2004 American Institute of Physics.