*Open Post-doctoral Position at LIONS, CEA Saclay*

*Measurement of interfacial ionic profiles in water with sub-nm resolution *


There is a broad range of phenomena in biology, environmental, and physical sciences where ions of the same valency like Cl^- and Br^- have a dramatically different effect. Such ‘‘ion specific’’ effects have been illustrated by many examples ranging from enzymatic activity and amyloidosis to humics stability and halide heterogeneous chemistry in the atmosphere. In many of these examples interfacial effects appear to play a key role. However, there is presently no first principles theory or general agreement on the mechanisms involved even for the apparently simple case of surface tension and surface potential at the air solution interface.

We have recently investigated the surface composition of alkali-halide aqueous solutions using grazing incidence x-ray fluorescence. Using mixtures of salts as a means to enhance the short-range effects, small differences in concentration could be resolved, with, for example I or Br > Cl. In order to explain our data, we need to include an effective potential accounting for the short-range solvent mediated couplings, responsible for specific effects together with dispersion forces [1]. We are now developing a theory taking into account the molecular nature of the solvent (water) and the polarizability of the ions in order to understand these short-range effects.

The next step will be to test the theory against experimental ionic profiles at the solid / aqueous solution interface with sub-nm resolution. The aim of the present project is to develop x-ray standing wave (XSW) technique, a high-resolution technique able to obtain elemental profiles of materials across buried interfaces, in order to measure such profiles. The standing waves are formed due to the interference between transmitted and reflected/diffracted electric fields and can be varied by changing the angle of incident X-rays. An atomic species in a fixed location is thus subjected to the electric field which can then excite a suitable fluorescence line. The fluorescence intensity profile as a function of incident angle of the X-ray beam can then be modeled to obtain the density profile of only the particular atomic species whose fluorescence emission has been collected. Using multilayers or crystals as substrate, nm or sub-nm resolution can be obtained. Preliminary experiments at DESY (Hamburg, FRG) have demonstrated the feasibility of such experiments for a large number of ions. A challenge in such experiments is to limit the contribution from the bulk aqueous solution in contact with the interface. Part of the work will therefore be devoted to the design of appropriate experimental cells using microfabrication techniques.

The expected output of this work, i.e. ionic profiles with sub-nm resolution will provide a stringent test of theories and should help understanding the important and long-standing issue of ionic specificity.

The position is available at the Laboratoire Interdisciplinaire sur l'Organisation Nanométrique et Supramoléculaire (LIONS) at CEA Saclay in the stimulating environment of the South-Paris area with several research centers and universities. The project is funded by the French National Research Agency (ANR) and will be done in collaboration with LEM (Nancy) for mineral surfaces and DESY (Hamburg) for synchrotron radiation experiments.

The position is for one year with possible extension of a second year. The candidate will preferentially have a first experience in x-ray scattering and/or microfabrication and an interest in statistical physics.

[1] Padmanabhan V, *Daillant J*, Belloni L, et al. _Specific ion adsorption and short-range interactions at the air aqueous solution interface <http://dx.doi.org/10.1103/PhysRevLett.99.086105>_ PHYSICAL REVIEW LETTERS 99 (8): Art. No. 086105 AUG 24 2007


/Contact: / Jean Daillant
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Laboratoire Interdisciplinaire sur l'Organisation Nanométrique et Supramoléculaire (LIONS)
SCM - bât. 125, CEA Saclay
F-91191 Gif-sur Yvette, France
tel: 33 1 69 08 81 57
F ax: 33 1 69 08 66 40