Pr. Michel Goldmann
Institut des Nanosciences de Paris, UMR CNRS, Université Pierre et Marie Curie
Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin, BP48 91192 Gif sur Yvette CEDEX
“Playing with x-rays and liquid surface, semifluorinated alkanes Langmuir monolayer and surface radiolysis”
Langmuir monolayer formed by amphiphilic moieties adsorbed at the air/liquid interface can be considered as good 2D systems. They present the advantage that they can be probed at various scales, macroscopic by surface-pressure versus surface density isotherms, mesoscopic by Brewster Angle Microscopy (BAM) and microscopic by x-rays surface scattering. Among the wide variety of Langmuir films, I will present two systems which present a particular aspect:
1) SEMI FLUORINATED ALKANES MONOLAYER:
Fluorinated chains present specific interest. Indeed, they have various potential application, in example as blood substitute in medicine. Perfluoroalkylalkanes (SFA) are molecules made of a perfluorinated chain chemically linked to a hydrogenated chain. Given their mutual phobicity, the forced coexistence of alkyl and perfluoroalkyl segments gives SFA a marked surfactant character, despite the absence of strong polar interactions and has earned SFA the epithet of Primitive Surfactants. Although they are completely hydrophobic, SFA for Langmuir monolayer when deposited at the air-water interface. We studied these films by the above-mentioned experiments associated with GISAXS on liquid subphase. Our results show that the molecules aggregate into monodispersed domains (hemi-micelles) of several thousand molecules. Under pressure, these domains organize into a large hexagonal lattice with a parameter of 30-40 nm but the monolayer never becomes homogeneous, even for surface pressures above collapse. We were able to recover the self-assembled process leading to the hemi-micelles formation by M.D. simulations.
2) X-RAY SURFACE RADIOLYSIS: FORMATION OF METAL-ORGANIC INTERFACE
The formation of metal–organic interface is an open question in material science with applications from colloïds (microcatalysis) to thin layers (microelectronics). In the radiolysis method, the irradiation of an aqueous solution of metal ions provokes the radiolysis of water and induces the reduction into metal atoms which latter aggregates. We develop an original approach by applying this process to Langmuir monolayer deposited on metal ions solution, expecting that the aggregation will place around the polar head to create a metallic layer which thickness is defined by the x-ray irradiation. Indeed, irradiating with an x-ray beam at grazing incidence a Langmuir monolayer of behenic acid deposited onto a silver solution, we obtain a thin silver layer anchored below the organic film. We followed the formation and structure of this metal layer by diffuse scattering, GIXD and surface x-ray fluorescence. After transfer onto a solid substrate. XPS measurements confirm the presence of silver metallic atoms as AFM experiment evidence a thickness of about 4.5 nm for the metallic layer, in agreement with the penetration of the x-ray evanescent wave.