Interfaces entre liquides modèles non miscibles (étude parallèle par la dynamique moléculaire et par la méthode de la fonctionnelle de densité)

L'interface entre liquides partiellement ou non miscibles intervient dans divers domaines de physicochimie (électrochimie, extraction liquide/liquide, biologie). En se référant au cas d'un liquide au voisinage d'une paroi solide, on peut s'attendre à ce que la concentration à travers l'interface entre liquides non miscibles présente une structure en couches. L'objectif de ce travail est d'examiner dans quelles conditions de température et de densité en phase homogène, une telle structure en couches est effectivement obtenue. Nous présentons l'étude du profil de densité par simulation numérique par dynamique moléculaire et du calcul issu de la théorie de fonctionnelle de densité (DFT). Les systèmes sont modélisés par le potentiel de Lennard-Jones et rendus non miscibles par un potentiel croisé répulsif. Les profils de densité présentent une structure oscillante dans un domaine limité de diagramme de phase du liquide homogène. A l'approche de la coexistence liquide/vapeur, on observe simultanément une déplétion entre les deux liquides et la disparition de toute structure dans les profils de densité. La comparaison avec le profil liquide/vapeur et l'analyse de l'adsorption nous conduisent à interpréter ce comportement en terme de démouillage. L'étude de rôle jouée par les ondes capillaires pour modifier les oscillations a été aussi évoquée en étudiant l'évolution des profils intrinsèques.Due to its importance in many phenomena in chemistry and biology. We present a study of the structure in the interface between two immiscible liquids by density-functional theory and molecular-dynamics calculations. The liquids are modeled by Lennard-Jones potentials, which achieve immiscibility by supressing the attractive interaction between unlike particles. The density profiles of the liquids display oscillations only in a limited part of the simple liquid-phase diagram (density, temperature). When approaching the liquid-vapor coexistence, a significant depletion appears while the layering behavior of the density profile vanishes. By analogy with the liquid-vapor interface and the analysis of the adsorption this behavior is suggested to be strongly related to the drying transition. We present also a study of intrinsic profils by molecular dynamics to schow capillary waves effects to damp oscillatory structure in the interface between immiscible liquids.PARIS12-Bib. électronique (940280011) / SudocSudocFranceF

Exploration of soil micromorphology to identify coarse-sized OM assemblages in X-ray CT images of undisturbed cultivated soil cores

We proposed to use micromorphology to help identify coarse-sized organic matter (OM) in X-ray computed tomography (CT) images of fresh undisturbed soil cores. We sampled three soil columns (5-cm diameter, 5-cm height) in the interfurrows that mainly contain macroporosity and straw residues of the ploughed horizon of a silt loamy Albeluvisol. Two of the samples were X-ray scanned at 68 and 88 μm voxel-resolution, and three thin-sections were performed in one of the scanned columns. Retention curve was measured on the third soil column. Micromorphology observations showed macropores from very fine to coarse size and numerous mesopores were heterogeneously distributed in the soil matrix and concentrations of OM of increasing stages of decomposition within and around the macroporosity. CT images easily detected coarse-sized OM concentrations and recovered part of the discontinuous pattern of the coarse-sized OM fragments in the macroporosity. We defined a table of correspondence between the structural features (porosity) and organo-mineral features of size down to about 40 μm and the grey level values of the scanned image. The values of the thresholds were adjusted from the micromorphological observations and the porosities of the 3D segmented images compared well with the measured porosity of the third soil column. The main limitation of the method was in choosing the threshold values separating pores and coarse-sized OM in order to recover the porosity of the zones containing the coarse-sized OM features. The proposed method is a preliminary work that showed to be well suited for undisturbed soil samples containing large pieces of straw residues incompletely mixed with soil matrix but cannot be applied to the identification of fine-sized OM