Thermodynamic assessment of the variation of the surface areas of two synthetic swelling clays during adsorption of water

International audienceTwo synthetic smectites (montmorillonite and beidellite) are studied by a water adsorption technique in order to assess their specific surface areas under atmospheric conditions. A route recently proposed for extracting the thermodynamic data from experimental adsorption isotherms is used. The variation of the specific surface area during water adsorption is obtained, which can be linked to the enlargement of the interlayer space determined using X-ray diffraction. This variation is compared to an idealized specific surface area obtained from TEM and X-ray measurements in agreement with crystallographic models. All these results are also compared with those obtained previously for a natural montmorillonite. A simple view of swelling is proposed

Mesure et modélisation de l'hétérogénéité énergétique à l'interface oxyde/électrolyte/métaux

Oxides surfaces are energetically heterogeneous towards adsorption. At solid/gas interface, argon or nitrogen have already been used to evidence surface heterogeneity (morphology or reactivity). In this study, protons or hydroxyls are used as surface probes of topochemical and energetic heterogeneity at solid/liquid interface. The goal was to work with raw experimental data since smoothing procedures may smear out valuable information on local heterogeneity. Highresolution potentiometric titration (up to 50 data points per pH unit) are treated as accurate H+ or OH- adsorption isotherms. The derivative form of isotherms is recomposed by using local adsorption isotherms (TitriDIS procedure (Titration Derivative Isotherm Summation)) to identify various energetic domains and also affinity distributions. Local isotherms are basically derived from 1-pK charging equation, which can be written as a Langmuir-Ternkin isotherm. Bach domain is described by a specifie local dissociation/complexation equilibrium, characterized by one pK, a pK distribution and/or an effect of surrounding sites. Energetic domains show various reactivity of various surface sites and evidence the proton affinity distribution of the surface. In this work, three synthetic ( oxy-hydr)oxides, among the most studied in literature, were used: anatase, goethite and amorphous silica. Electrophoretic mobility, adsorption edges and surface charge have been measured, witout or with metallic cations (Cu2+, Cd2+ and Pb2+). A set of different pKs (which represents the affinity for the cation) was determined for the three samples (quasicontinuous distribution for silica, 2 main domains for anatase, and 4 kinds of reactives sites for goethite ). The energetic distributions reveal the various effects of crystallographic faces, defects, impurities, or dissolution. Such an analytical procedure represent an exciting tool to map (nature, localization and density) smface active sites of importance in interfacial phenomena.Outre la caractérisation de l'interface solide/liquide par les méthodes classiques (charge de surface, seuil d'adsorption, mobilité électrophorétique), l'objectif de ce travail est d'obtenir, au moyen d'une approche nouvelle de type "distribution d'affinité", une description continue des équilibres dans les systèmes complexes. Les systèmes étudiés comportent des solides représentatifs des milieux naturels (goethite, silice amorphe), et un solide de référence stable en solution (anatase). Les métaux utilisés sont CU2+, Cd2+ et Pb2+. Afin de mettre en évidence l'hétérogénéité énergétique superficielle, nous avons développé une méthodologie, appelée TitriDIS pour Titration Derivative Isotherm Summation, de détermination des distributions d'affinité protonique (ou hydroxylique en l'occurrence), fondée sur la recomposition des isothermes dérivées d'adsorption d'OH-. Les titrages potentiométriques sont considérés comme des isothermes d'adsorption d'hydroxyles ou de protons, pour lesquelles le nombre élevé de points expérimentaux (50 points par unité de pH) permet de travailler sur la dérivée numérique des courbes, sans recourir à l'interpolation des données. La démarche de recomposition des isothermes dérivées d'adsorption par des isothermes dérivées théoriques locales (de type Langmuir-Temkin), validée à l'interface solide/gaz (DIS), a été appliquée à l'interface solide/électrolyte. Sur les trois solides étudiés, des distributions d'affinité de proton ont été mises en évidence, en lien avec les caractéristiques cristallochimiques des surfaces. Nous avons ainsi pu identifier : sur la silice, une quasi-continuité de la distribution; sur l'anatase, 2 domaines majeurs, sur la goethite, 4 familles de sites réactifs. Chaque domaine énergétique est alors caractérisé par une constante thermodynamique (le pK moyen), une abondance relative des sites, et un terme traduisant les interactions latérales en même temps que l'hétérogénéité locale

MUSIC Speciation of γ-Al 2 O 3 at the Solid Liquid Interface: How DFT Calculations Can Help with Amorphous and Poorly Crystalline Materials

International audienceTransition aluminum oxides, such as γ-Al2O3 or alumina, are widely used in many different technical applications that rely on the surface reactivity of this material at the solid liquid interface. The speciation of surface sites of this material confronts several obstacles. On the one hand, alumina is a poorly crystalline oxide, thus allowing for a limited amount of empirical structural information for an important number of surface sites with different trends in reactivity and, on the other hand, it is a metastable material. In this work, we show several ways in which the multisite complexation model, combined with atomistic information from density functional theory and ab initio molecular dynamics (AIMD), can manage to perform speciation calculations of γ-Al2O3 surface sites at the solid liquid interface. Although the results are in good qualitative agreement with experimental titration curves, and they can serve as a guide for the interpretation of the reactivity of this material at the initial stages of an impregnation experiment and chemical weathering phenomena, this work highlights the need of more complex AIMD simulations to accurately model these phenomena in γ-Al2O3 surface/liquid interfaces

Structural–chemical disorder of manganese dioxides : 1. Influence on surface properties at the solid–electrolyte interface

International audienceRelationships between lattice parameters of manganese dioxides and their surface properties at the solid–aqueous solution interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered MD samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth) and Tw (rate of microtwinning). Water adsorption isotherms showed that the cross sectional area of water molecules adsorbed in the first monolayer is positively correlated to Pr. Titration of the surface charge of the MD series evidenced a positive linear relationship between the PZC and Pr (Pr=0, Tw=0, PZC=1 for ramsdellite; Pr=1, Tw=0, PZC=7.3 for pyrolusite; γ-MD with intermediate values of Pr (0.2 to 0.45) have increasing PZC values). The rate of microtwinning appeared as a secondary factor for the increase of the PZC. The above correlations are explained by the chemical defects at the origin of the structural disorder, respectively Mn3+/Mn4+ substitution for Pr and Mn vacancies for Tw, which result in proton affinity and thus in increased PZC. The experimental results are compared with data collected in the literature for manganese dioxides as well as for dioxides of transition elements with tetragonal structure

Manganese Dioxides Surface Properties Studied by XPS and Gas Adsorption

Structurally well defined g-MnO2 in terms of Pr and Tw were studied using X-ray photoelectron spectroscopy ~XPS! to complete surface investigations performed by high-resolution gas adsorption, water adsorption, and acid-base surface titration. The O 1s and Mn 2p spectra were deconvoluted into three components Mn-OH2 1 , Mn-OH, and Mn-O2. Analysis of the O 1s spectra in energy shift and relative intensity shows that 70 to 80% of the surface groups stay as neutral OH, 5% as Mn-OH2 and 20 to 30% as Mn-OH: their relative amount varies with Pr as well as PZC, water cross-sectional area and energetic constant C. Mn 2p spectra are far less sensitive than O 1s to the charge variation. Nevertheless a correlation is shown between the relative amount of surface species, the binding energy of Mn 5 O and O 5 Mn species and Tw

Influence of electrolyte ion adsorption on the derivative of potentiometric titration curve of oxide suspension – theoretical analysis

The theory based on the 1-pK triple layer model of the oxide/electrolyte interface is used to obtain the derivative of a proton adsorption isotherm. The present approach is much more rigorous and complex from a physical point of view compared to our previous paper [J. Phys. Chem. B. 106 (2002) 13280]. In the present work we take into account adsorption of inert electrolyte ions. Also, the present approach takes into consideration the local surface heterogeneity. The aim of this work is not to propose new surface model, but to develop a theoretical basis for obtaining of proton affinity distribution from the derivatives of a potentiometric titration curve. We demonstrate how including of electrolyte ion adsorption influences on the shape and behavior of the local derivatives of potentiometric titration curves. The influence of the other parameters is also thoroughly investigated. It was found that complete description of the derivative of surface charging curve requires a high number of parameters. Therefore, it is difficult to adopt the obtained peaks for unambiguous decomposition of experimental derivative of a titration curve onto the parts coming from different patches of the surface

Structural–chemical disorder of manganese dioxides: II. Influence on textural properties

International audienceRelationships between structural parameters of MnO2 and their surface properties at the solid–gas interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered γ-MnO2 samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth in the ramsdellite network) and Tw (rate of microtwinning). Analysis of the N2 adsorption isotherm evidenced positive correlations between specific surface area and Tw for γ-MnO2 only and between the energetic constant C and (1-Pr). No microporosity is evidenced. Water adsorption isotherms evidenced the dependence of the H2O monolayer volume on Tw and showed a positive correlation between the cross-section area of water molecules adsorbed in the first monolayer and Pr, ranging from 13.5 Å2 for Pr=1 to 6.3 Å2 for Pr=0.2 (12 sites/nm2). Energetic heterogeneity is quantified from Ar and N2 low-pressure adsorption isotherms with the DIS procedure and correlated with H2O adsorption. High-energy adsorption domains are quantified and assigned to the different crystal faces: (110) faces with a common 1×1 octahedra layer of pyrolusite and ramsdellite and the (001) face of ramsdellite with 2×2 octahedra on which channels and plateaus are differentiated. The specific surface area ratio of ramsdellite high-energy sites to total ramsdellite content is shown to depend on Tw. The dependence on microtwinning of low cross-sectional area of N2 and much lower cross-sectional of residual H2O molecules leads us to assume that their adsorption sites on grain boundaries are represented by the twin planes between the structured nanocrystals generated by oxygen evolution during MD synthesis

Simple and Straightforward Synthesis of Porous Ionosilica for Efficient Chromate Adsorption

International audienceWe report a very simple synthesis strategy for the formation of moderately porous ionosilica materials containing ammonium groups. We synthesized an ionosilica xerogel and an ionosilica aerogel, either via conventional or super-critical work-up. The synthesis scheme of both precursor and ionosilica materials could easily be up-scaled to a 100 g scale. All syntheses and work-up procedures are sustainable as no additional agent and very low quantities of solvents were used, and no additional purifications steps were necessary. Although both materials show excellent anion exchange properties for chromate adsorption, noticeable differences were found regarding the thermodynamics of the exchange process. We attribute these differences to different surface chemistries of the materials, induced by the different work-up procedures. Due to the easy availability of high quantities of material, ionosilicas can be implemented as functional ion exchange materials in larger scale processes, thus opening new fields of applications

Driving force for the hydration of the swelling clays: Case of montmorillonites saturated with alkaline-earth cations

International audienceImportant structural modifications occur in swelling clays upon water adsorption. The multi-scale evolution of the swelling clay structure is usually evidenced by various experimental techniques. However, the driving force behind such phenomena is still not thoroughly understood. It appears strongly dependent on the nature of the interlayer cation. In the case of montmorillonites saturated with alkaline cations, it was inferred that the compensating cation or the layer surface could control the hydration process and thus the opening of the interlayer space, depending on the nature of the interlayer cation. In the present study, emphasis is put on the impact of divalent alkaline-earth cations compensating the layer charge in montmorillonites. Since no experimental technique offers the possibility of directly determining the hydration contributions related to interlayer cations and layer surfaces, an approach based on the combination of electrostatic calculations and immersion data is developed here, as already validated in the case of montmorillonites saturated by alkaline cations. This methodology allows to estimate the hydration energy for divalent interlayer cations and therefore to shed a new light on the driving force for hydration process occurring in montmorillonites saturated with alkaline-earth cations. Firstly, the surface energy values obtained from the electrostatic calculations based on the Electronegativity Equalization Method vary from 450 mJ m−2 for Mg-montmorillonite to 1100 mJ m−2 for Ba-montmorillonite. Secondly, considering both the hydration energy for cations and layer surfaces, the driving force for the hydration of alkaline-earth saturated montmorillonites can be attributed to the interlayer cation in the case of Mg-, Ca-, Sr-montmorillonites and to the interlayer surface in the case of Ba-montmorillonites. These results explain the differences in behaviour upon water adsorption as a function of the nature of the interlayer cation, thereby allowing the macroscopic swelling trends to be better understood. The knowledge of hydration processes occurring in homoionic montmorillonites saturated with both the alkaline and the alkaline-earth cations may be of great importance to explain the behaviour of natural clay samples where mixtures of the two types of interlayer cation are present and also provides valuable information on the cation exchange occurring in the swelling clays