Temperature Dependence of the Structure of Langmuir Films of Normal-Alkanes on Liquid Mercury

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Original Entry by Xu Zhang for AP225, Fall 2009

Reference

Temperature dependence of the structure of Langmuir films of normal-alkanes on liquid mercury, H. Kraack, B.M. Ocko, P.S.

Pershan, L. Tamam, and M. Deutsch. J. Chem. Phys. 121(16), 8003-8009 (2004).

Key Words

Langmuir film, n-alkanes, phase diagram, transition temperature, SL(single layer), DL(double layer), TL(triple layer)

Summary

On water

The behaviors of Langmuir films of normal-alkanes <math>[CH_3(CH_2)_{n-2} CH_3] </math> on the surface of water have been

studied, where molecules invariably orient roughly surface normal.

On Mercury

Different from those on water, alkanes on mercury are oriented surface parallel and show no long-range in-plane order at any surface pressure. Moreover, by controlloing n and T we can get different structures: SL(single layer), DL(double layer) and TL (triple layer).

Measurement Methods

A. surface pressure-molecular area isotherms

The isotherm was 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.

B. X-ray measurements

X-ray reflectivity(XR) measurements yielded the surface-normal electron density profile and the surface roughness, while grazing incidence diffraction(GID) probed the in-plane order.

Results

1.Isotherms

The isotherms of C22 was measured for <math>29 \leq T \leq 43.6^ \circ </math>

As we can see in the figure below, at <math>T=23^\circ</math> there are three distinct steps in the isotherm indicating the

single, double and triple layer of lying-down molecules.All phases of the C22 film were found to be disordered in-plane at

this temperature.

The isotherm at <math>T=29^\circ</math> is very similar to that at <math>T=23^\circ</math> and the change of the shape

occurs at <math>T=34^\circ</math> but most features remain unaltered. The phase transition occurs at

<math>T=35.4^\circ</math>, where the third plateau disappears. The second transition occurs at <math>T=40.6^\circ</math>

Langmuir-1.png

2.X-ray reflectivity

The measurement of temperature dependent X-ray reflectivity was also conducted to confirm the structural hints provided by the

measurement of isotherms.

Langmuir 3.png Langmuir3-2.png

The cross comparison of the isotherms and the XR data makes our conclusions concerning the the structure of the high-T phases

reasonable and reliable.

Molecular length dependence

The figure below shows the transition temperatures for the Langmuir films and in the bulk for all alkanes addressed in this study.

Langmuir-5.png

The bulk transition temperatures (solid lines) show the well-known linear behavior for the rotator-liquid melting transition, and the odd-even effect for the rotator-crystal transition. A plausible explanation for the identical slopes and close transition temperatures of the bulk and film transitions may be obtained by assuming that the melting of the bulk and the collapse of the molecular layers of the surface film are due to a common effect:the proliferation of gauche conformations in the molecular chains with increasing temperature. This effect distorts the regular,cigarlike, shape of the molecule. The distortions render a close packing of these molecules increasingly more difficult with increasing T, causing the bulk solid to melt and the layers in the Langmuir film to collapse at some temperature.