The Structure of Organic Langmuir Films on Liquid Metal Surfaces

Abstract

Langmuir films (LFs) on water have long been studied for their interest for basic science and their numerous applications in chemistry, physics, materials science and biology. We present here Å-resolution synchrotron X-ray studies of the structure of stearic acid LFs on a liquid mercury surface. At low coverage, \(\geq\) 110 Å\(^2\)/mol, a 2D gas phase of flat-lying molecules is observed. At high coverage, \(\leq\) 23 Å\(^2\)/mol, two different hexatic phases of standing-up molecules are observed. At intermediate coverage, 52 \(\leq\) A \(\leq\) 110 Å\(^2\)/mol, novel single- and double-layered phases of flat-lying molecular dimers are found, exhibiting a 1D in-layer order. Such flat-lying phases were not hitherto observed in any LF. Measurements on LFs of fatty acids of other chain lengths indicate that this structure is generic to chain molecules on mercury, although the existence of some of the flat-lying phases, and the observed phase sequence, depend on the chain length. Organic LFs on Hg, and in particular the new flat-lying phases, should provide a broader nano-structural tunability range for molecular electronic device construction than most solid-supported self-assembled monolayers used at present.