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 A A-resolution synchrotron X-ray studies of the structure of stearic acid LFs on a liquid mercury surface. At low coverage, P 110 A A 2 /mol, a 2D gas phase of flat-lying molecules is observed. At high coverage, 6 23 A A 2 /mol, two different hexatic phases of standing-up molecules are observed. At intermediate coverage, 52 6 A 6 110 A A 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.

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

“…While no phases comprising surface-normal molecules are found here, fatty acids on mercury do show such phases. 21,39 This fact highlights the importance of the head groups in determining the structure of Langmuir films on mercury. Another manifestation of the importance of the headgroup is the existence of long-range, smectic-like 1D long-range in-plane order in the flat-lying phases of C18 fatty acid, 21 and 2D long-range in-plane order in C24 fatty acid Langmuir films on mercury.…”

“…10 Similarly high collapse pressures were found for other organic molecules on mercury. 21,39,40 The fact that the collapse pressure on Hg is much higher than that on water may point towards film buckling as a possible collapse mechanism here. For such a mechanism the attraction of the monolayer to the surface is the dominant factor in determining the collapse pressure.…”

Temperature dependence of the structure of Langmuir films of normal-alkanes on liquid mercury

“…As in previous experiments, 8,9,12 the trough was cleaned carefully, and then its enclosure was flushed with a pure inert gas ͑He or N 2 ) for about 1 h. Mercury was introduced from a reservoir through a capillary under the inert atmosphere. The isotherm was then measured by stepwise deposition of measured amounts of the standard solution using a calibrated micropipette, waiting after each step until an equilibrium surface pressure was reached.…”

“…To investigate the role of the molecule's headgroup and tail interactions with the subphase in the determination of the Langmuir film's structure, we have studied the structure of Langmuir films of fatty acids, [8][9][10] alcohols, 11 and alkanes 12 on mercury, using x-ray and surface tension methods. In addition to phases with standing-up molecules, similar to those on aqueous surfaces, a variety of phases comprising surfaceparallel-oriented molecules were found, exhibiting different types of order ranging from a complete disorder to unidirectional liquid-crystal-like order to two-dimensional crystalline order.…”

Surface tension of liquid mercury: a comparison of density-dependent and density-independent force fields

“…Because of its seamless surface liquid mercury also enables the creation of high quality defectless self-assembled monolayers (SAMs) of various organic molecules [13][14][15]. Therefore the surface of liquid mercury serves as a model system for the experimental study of universal (free of the underlying crystal substrate) properties of numerous organic films [16][17][18][19][20][21][22][23][24][25][26]. Moreover, the unique properties of liquid mercury, especially its high surface tension, make Hg droplets superior to other metals for producing metal-SAM-metal [13,27] and metal-SAM-semiconductor [28,29] junctions.…”