The structure of organic Languir films on liquid metal surfaces
Soft Matter Keywords
Langmuir film, Insoluble monolayers, Liquid metals, Hard Sphere Gas Model, Excluded Volume
This paper reports a new flat-lying structure observed at certain conditions for Langmuir films consisting of stearic acid on liquid mercury. We can regard a Langmuir film as a two-dimensional structure, which means that depending on whether it is flat-lying or standing-up the stearic acid molecules will have very different effective molecular areas, as evident from the image of stearic acid below. The effects of this difference in molecular area were observed using X-ray measurements.
The graph below shows the surface pressure versus molecular area measurements for a stearic acid Langmuir film on liquid mercury. The solid line is for a stearic acid Langmuir film on water, the circles and solid line are for a stearic acid Langmuir film on liquid mercury. The dashed line is the isotherm of a hard-sphere gas with 112Å2 exclusion area. At low coverage (the higher the area per molecular, the lower the coverage of the film because the molecules are spaced farther apart), the 112Å2 exclusion area model matches the data very well. Since the exclusion area of a flat-lying stearic acid molecule is very close to this number, the authors concluded that for coverage above 110Å2/mol, the Langmuir film consists of flat-lying stearic acid molecules.
To test whether this flat-lying structure was unique to stearic acid or a generic phenomenon, the authors observed Langmuir films composed of other organic molecules. The graph below shows data of surface pressure versus molecular area for Langmuir films composed of fatty acids of varying lengths. All of them show a similar behavior to the stearic acid, exhibiting a flat-lying structure at low molecular coverage. The inset graph plots circles corresponding to the best fit exclusion areas corresponding to the low coverage data. The solid line in the inset is the calculated molecular area of a flat-lying molecule corresponding to carbon number (n) in the fatty acid.
It appears that the behavior of the flat-lying structures and the conditions for their formation depends on the characteristics of the headgroup of the material (e.g. for stearic acid, the headgroup is carboxylic acid, -COOH). Furthermore, the reason that the flat-lying structure is observed on liquid mercury, but not water, is that the carboxylic-Hg bond strength is about ten times less than that of carboxylic acid-water. This permits the stearic acid molecules more mobility and allows them to reach thermal equilibrium at all degrees of coverage because they can move about more easily.
The authors proceeded to determine that at high coverages (molecular area less than 23Å2/mol) two varieties of standing-up molecular structures are observed in the Langmuir films and that at intermediate coverages, flay-lying molecular dimers form.
Relevance to Soft Matter and Applications
Langmuir films and surfactants are an important area of study in soft matter. The impact of understanding Langmuir films can extend into various applications and other fields. In studies of biological systems, Langmuir films are sometimes used as models for cell membranes, which also comprise of organic surfactants among other molecules. In chemistry, they are used for crystal nucleation. In physics, Langmuir films resemble 2D gases, and indeed, the hard sphere models used to determine exclusion area aboves can be derived from statistical mechanical treatment of ideal gases in two dimensions. The authors also cite molecular electronics as a potential application of this research. In particular, this ability to tune the structure of organic Langmuir films can be exploited in designs.
Additionally, as a side note, as per "X-ray study of the liquid potassium surface:structure and capillary wave excitations" by Shpyrko, et. al., I wonder if layering in the liquid metal itself has any relation with the structure of the Langmuir films. Moreover, since liquid metals layer, would a mixture of multiple liquid metals yield the same properties for Langmuir films as a homogeneous liquid of only one of the metals, and if so which one? (The one on top of course, but what determines that?)
H. Kraack, M. Deutsch, B.M. Ocko and P.S. Pershan, "The structure of organic Languir films on liquid metal surfaces", Nuclear Inst. and Methods in Phys. Res. B 200, 363 (2003).