Surface Heterogeneity at the Solid-Gas Interface of Hydrophilic Solids Modified by Water-Repellent Molecules

The surface heterogeneity of a hydrophilic calcium carbonate (calcite) at the solid-gas interface was studied before and after modification by the adsorption of different amounts of a water-repellent molecule (WRM). The surface heterogeneity was analysed using nitrogen, argon and water as molecular probes. Low-pressure adsorption techniques coupled with derivative isotherm summation (DIS) analysis of the experimental curves gave quantitative information on the decrease in the calcite surfaces and the increase in the WRM surfaces. The modelling results enabled the monolayer capacity for WRMs as measured by argon adsorption to be correlated with the disappearance of calcite surfaces and the disappearance of high-energy adsorption sites as measured by nitrogen adsorption. In addition, the argon results suggested that WRMs first adsorb on argon low-energy carbonate faces and then on low- and high-energy faces. With water, the adsorption energy distribution remained unchanged in shape, indicating that this molecule can diffuse into the adsorbed layer between and around the adsorbed hydrophilic heads of the WRMs and thereby interact directly with the carbonate surface

Evidence of a critical content in Fe(0) on FoCa7 bentonite reactivity at 80 °C

In order to assess the evolution of the confinement properties of clay engineered barriers (EBS) when in contact with metallic canisters containing radioactive wastes, Fe(0)-bentonite interactions need to be assessed. "45 days-80 °C" tests were performed using powdered FoCa7 bentonite and metallic iron. Since one fundamental parameter may be the available quantity of Fe(0), a wide range of Iron/Clay mass ratios (I/C) from 0 to 1/3 is used. The confinement power of clay material results from the swelling properties and the retention capacity. Thus, the major criterion which is chosen to assess the evolution of the confinement properties in this study is the variation of Cation Exchange Capacity (CEC). In parallel, the physico-chemical evolution of bentonite is studied using XRD and EDS-TEM microanalyses. The evolution of the distribution of iron environments is obtained by 57Fe Mössbauer spectroscopy. This study evidences that both kaolinite and smectite from the bentonite are altered into SiAlFe gels when in contact with Fe(0). These gels maturates into Fe-rich di-trioctahedral phyllosilicates, whose composition is bounded by the one of odinite and greenalite in a Fe-M+-4Si diagram when I/C = 1/3. Most of all, it is evidenced that the reaction depends on the available quantity of Fe(0). When the I/C ratio is between 1/30 and 1/7.5, the exchange capacity of FoCa7 bentonite starts decreasing, the consumption of Fe(0) becomes significant, the alteration of smectites occurs and secondary oxides are formed. The crystallization of Fe-rich phyllosilicates is observable when I/C ratio is higher, from a threshold between 1/7.5 and 1/5. Above I/C = 1/3.75, initial iron oxides are strongly consumed and participate in the incorporation of Fe2+ and Fe3+ in gels or new phyllosilicates octahedra. These experimental results were used as input data for the prediction of the long-term evolution of the EBS using Crunch reaction-transport model. © 2007 Elsevier B.V. All rights reserved

Intercalation of Al13-Polyethyleneoxide Complexes into Montmorillonite Clay

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