Ionic distribution at water – air, solid, liquid interfaces : ionic specificity
L. Belloni, F. Malloggi, J. Daillant logo_tutelle logo_tutelle 

Hg-water interfaces. left: reflectivity measurements; right: corresponding electron density profiles.

Within the ANR project ANR-06-BLAN-0276 "SISCI" (Spécificité Ionique dans les Systèmes Colloïdaux et Interfaciaux), we have explored the specific local ionic distributions in water in the vicinity of interfaces between aqueous electrolytes and external media.

Three different X-ray reflectivity techniques have been developed[1]. (i) The air-water interface has been investigated using grazing incidence x-ray fluorescence for different alkali-halide solutions[2]. The measured integrated amount of desorbed ions follows a Hofmeister series, in agreement with the surface tension behavior, and is interpreted in terms of ion-surface interaction potentials. (ii) The spatial ion distribution has been resolved at the sub-nanometer level using x-ray standing waves at the interface with a multilayer silica substrate. (iii) x-ray reflectivity analysis of polarized liquid mercury surface in contact with aqueous electrolyte has shown how the Hg-surface layering and the local ionic profiles depend on both applied potential and ion nature[3].

 

From the theoretical point of view, we have intensively developed the integral equations   approach of liquid physics at the discrete, molecular solvent level of description[4]. Thanks to powerful home-made algorithms and codes, the (approximate) HNC closure is numerically solved in terms of angular-dependent pair distribution functions. This approach can be used as an efficient alternative to the very time-consuming numerical simulations. Its degree of validity can be improved by adding the so-called bridge functions (those missing in the HNC approximation) extracted from exact Monte Carlo simulations[5].

 

[1] Padmanabhan V, Girard L, Daillant J, Belloni L, Spalla O, Novikov D, in Specific Ion Effects, ed. W. Kunz, World Scientific (2009)

[2] Padmanabhan V, Daillant J, Belloni L, Mora S, Alba M, Konovalov O, Phys. Rev. Lett. 99, 86105 (2007)

[3] Duval J, Bera S, Michot L, Daillant J, Belloni L, Konovalov O, Pontoni D, Phys. Rev. Lett. 108, 206102 (2012)

[4] Belloni L, Chikina I, in Specific Ion Effects, ed. W. Kunz, World Scientific (2009)

[5] Puibasset J, Belloni L, J. Chem. Phys. 136, 154503 (2012)

 

Maj : 13/01/2016 (2243)

 

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