Fatty acid Langmuir films on liquid mercury: X-ray and surface tension studies

The structure and phase behavior of liquid-mercury-supported molecular films of fatty acids (CH3(CH2)n-2COOH, denoted CnOOH) were studied for molecular lengths 7 < or = n < or = 24, by surface tensiometry and X-ray methods. Two qualitatively different film structures were found, depending on coverage. For high coverage, the film consists of a monolayer of roughly surface-normal molecules, showing a pressure-dependent sequence of structures similar, though not identical, to that of the corresponding water-supported Langmuir films. At low coverage, phases consisting of surface-parallel molecules are found, not observed on the aqueous subphases employed to date. In this range, a two-dimensional (2D) gas followed by a single and, for 14 < or = n < or = 24, also by a double layer of surface-parallel molecules is found as coverage is increased. Depending on chain length, the flat-lying phases have a crystalline 2D-ordered, a smectic-like 1D-ordered, or a disordered in-plane structure consisting of molecular dimers. The structure and thermodynamics of the films are discussed.