Langmuir films of normal-alkanes on the surface of liquid mercury

The coverage dependent phase behavior of molecular films of n-alkanes (CH3CHn−2CH3, denote Cn) on mercury was studied for lengths 10⩽n⩽50, using surface tensiometry and surface x-ray diffraction methods. In contrast with Langmuir films on water, where roughly surface-normal molecular orientation is invariably found, alkanes on mercury are always oriented surface-parallel, and show no long-range in-plane order at any surface pressure. At a low coverage a two-dimensional gas phase is found, followed, upon increasing the coverage, by a single condensed layer (n⩽18), a sequence of single and double layers (19⩽n⩽20; n⩾26), or a sequence of single, double, and triple layers (22⩽n⩽24). The thermodynamical and structural properties of these layers, as determined from the measurements, are discussed.