Structure of a Langmuir Film on a Liquid Metal Surface

Abstract

The structure of organic monolayers on liquid surfaces depends sensitively on the details of the molecular interactions. The structure of a stearic acid film on a mercury surface was measured as a function of coverage with angstrom resolution. Unlike monolayers on water, the molecules were found here to undergo a transition from surface-parallel to surface-normal orientation with increasing coverage. At high coverage, two condensed hexatic phases of standing-up molecules were found. At low coverage, a two-dimensional (2D) gas phase and condensed single- and double-layered phases of flat-lying molecular dimers were revealed, exhibiting a 1D longitudinal positional order. This system should provide a broader tunability range for nanostructure construction than solid-supported self-assembled monolayers.

Checkerboard Self-Patterning of an Ionic Liquid Film on Mercury

“…First, the linear sloping plateau at A < A 0 resembles that observed at the same A-range for a mercury-supported stearic acid LF [15], which was shown to correspond to a continuous conversion with decreasing A of a monolayer to a bilayer of surface-parallel molecules. Second, no sharp rise, the sign of a transition from lying-down to standing-up molecules, is observed near film collapse at A ≤ 50Å 2 /molecule, in contrast with mercury-supported LFs of fatty acids, thiols, and alcohols [15,16], which show such transitions. These isotherm-derived inferences are confirmed by our x-ray measurements, discussed next.…”

“…Even these handful of studies address only interfaces between bulk phases. NoÅ-resolution structure of 2D Langmuir films (LFs) have been published to date for any RTIL, even though numerous studies of LFs on water- [12][13][14] and mercury [15][16][17] of non-RTIL compounds demonstrate the deep insights obtainable from such measurements. In particular, LF x-ray measurements can determine the surface-parallel structure and its coverage dependence.…”

“…1) yield areas of 36 and 51Å 2 [18]. The measured surface pressure(π)/molecular area(A) isotherm [15,16] (for method see [19]) is shown in Fig. 2.…”

“…2. The steep rise at ∼90Å 2 /molecule marks the point where the molecules of the A ≫ 90Å 2 /molecule 2D gas start touching, forming a densely packed monolayer [13,15]. Fits (dashed lines) by the Volmer [20] and van der Waals [21] state equations of an ideal 2D gas of finite-area (A 0 ) molecules yield exclusion areas A V 0 = 93Å 2 /molecule and A vdW 0 = 88Å 2 /molecule, respectively.…”

“…3(a) shows the measured (symbols) and box model fitted (line) R(q z )/R F (q z ). The model, successfully used for numerous LFs of organic molecules on mercury [15,16,25], uses 6 slabs to mimic the decaying layering of mercury at the interface [26][27][28]. As the measurements were done at 50Å 2 /molecule, half the area of a flat-lying molecule, a bilayer was expected, as discussed above.…”