Langmuir Films of Normal-Alkanes on the Surface of Liquid Mercury

Abstract

The coverage dependent phase behavior of molecular films of n-alkanes (\(CH_{3}CH_{n−2}CH_3\), denote Cn) on mercury was studied for lengths 10 \(\leq\) n \(\leq\) 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 \(\leq\) 18), a sequence of single and double layers (19 \(\leq\) n \(\leq\) 20; n \(\geq\) 26), or a sequence of single, double, and triple layers (22 \(\leq\) n \(\leq\) 24). The thermodynamical and structural properties of these layers, as determined from the measurements, are discussed.