Grazing Incidence X-ray Diffraction Studies of Thin Films at the air-liquid Interface
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Grazing Incidence X-ray Diffraction Studies of Thin Films at The air-liquid Interface Coralie Alonso (1), Anne Renault (2), Meir Lahav (1), Leslie Leiserowitz (1) Weizmann Institute of Science, ‘Materials and Interfaces’ Dpt, Rehovot 76100, Israel (2) Universite Rennes I, ‘Matiere Condensee et Materiaux’ Dpt, Rennes 35042, France (1)
ABSTRACT Monomolecular films at the air-water interface can be investigated on the subnanometer scale with grazing incidence X-ray diffraction (GIXD) using synchrotron radiation. This surface semsitive technique utilizes the property of total external reflection of X-rays from a water surface : an evanescent wave generated within the film diffracts in the surface plane giving an image of the film reciprocal lattice. Three applications of GIXD are presented ranging from poorly to highly crystalline thin films. (i) Cholesteryl-L-glutamate forms a crystalline monolayer at the air-water interface within which the glutamate moieties are not closely packed. This system specifically incorporates hydrophobic amino acids from the subphase. (ii) Long-chain cholesteryl esters deposited on the water surface spontaneously self assemble, forming crystalline interdigitated bilayers. The molecular structure, solved at the atomic resolution, was found to be similar to the 3D counterpart. (iii) According to 2-D diffraction theory, the shape of Bragg peaks is related to the mechanical constants of the film. Rigidity of the film can be deduced from a detailed peak analysis for secondary short chain alcohols showing a softening of the monolayer close to melting. INTRODUCTION Surface studies are relevant to present many fields in science ranging from material scince to biology. But the direct information from a surface is hard to get because of experimental difficulties. Langmuir films, composed of molecules spread at the air-liquid interface, have proven to be useful models for elucidating structure and function of biological membranes [1, 2]. They may also be engineered to induce nucleation of three-dimensional crystals of various solutes dissolved in the subphase and thus give means to understand mechanisms of crystal nucleation and growth [3]. Such a broad field of applications explains the constant interest in finding and exploiting sensitive methods for characterizing Langmuir films. Currently used techniques are mainly surface pressure or surface potential measurements [4], IR spectroscopy [5], epifluorescence microscopy [6] and more recently Brewster angle microscopy [7]. But none of them give direct information on the lateral molecular arrangement. The two-dimensional organization is Langmuir films maybe extrapolated from two-dimensional packing within the three-dimensional crystal assuring layer structure. In order to obtain direct stuctural information, diffraction from the film itself is necessary but the signal from the surface would generally be swamped by the contribution from the bulk. This drawback can be circumvented by limiting the penetration depth to a few nanometers making use of highl
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