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

Two 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.

Thermodynamic analysis of the immersion of a smectite substituted with Na or Ca: heat effect due to the cation.

The enthalpy produced during immersion in water of a smectite, identified as a montmorillonite, substituted by Ca or Na is analyzed using three different approaches. The heat of immersion is measured using the classical calorimetric method. It is compared to an estimate using adsorption techniques. And it is calculated from a theoretical estimate of the surface energy of a crystal model of montmorillonite. The comparison of the different values allows the enthalpy linked to the swelling phenomenon to be estimated. The Ca form appears in every case to interact more strongly with water molecules than the Na form.

Evaluation of Surface Enthalpy of Porous Aluminosilicates of the MCM-41 Type Using Immersional Calorimetry: Effect of the Pore Size and Framework Si:Al Ratio.

Surface properties of porous aluminosilicates of the MCM-41 type have been tested by immersional calorimetry. Two series of materials, referred to as SiAlxCn, where x is the Si : Al mole ratio and n the chain length of the surfactant template, having (1) x=32 and n=8, 12, 14, 16, 18 and (2) n=14 and x=8, 32, infinity, were used. The results of thermogravimetric analysis on these samples served to evaluate the surface density of hydroxyl groups. This parameter is rather sensitive to the pore size than to the aluminium content in the solid matrix. Based on the experimentally measured enthalpies of immersion in n-heptane, water, and formamide per unit BET specific surface area, estimates could be made of the apolar, Lewis acid, and Lewis base contributions to the total surface enthalpy of MCM-41 materials. The samples studied have a predominant surface acidic character, which is markedly enhanced by incorporating aluminium into the silica matrix. Surface acidity is also modified by changes in the porous structure, although the trends are less noticeable here. Nevertheless, the total surface enthalpy of MCM-41 aluminosilicates appears to be small in comparison with typical inorganic oxides, such as silica or alumina. Copyright 2001 Academic Press.

Surface Heterogeneity of Passively Oxidized Silicon Carbide Particles: Hydrophobic-Hydrophilic Partition.

The surface of silicon carbide (SiC) particles previously subjected to passive oxidation has been characterized using various techniques such as adsorption of positively and negatively charged surfactants in aqueous solution, immersion microcalorimetry in three probe liquids, and flow microcalorimetry in organic media. Adsorption data show that around one quarter of the surface appears negatively charged and thus hydrophilic, while three quarters appear uncharged and hydrophobic. This is attributed to dissociation of silanols groups. Immersion calorimetry in liquids having well-defined polar and nonpolar components of surface energy shows that the Lifshitz-van der Waals component of SiC is very high and that the acid and basic components are weak. The experimental results appear to be consistent with both computations of surface energy using Lifshitz theory and experimental data previously obtained with other minerals. The three indexes are discussed and it is argued that they represent different terms of the solid surface energy. Copyright 2000 Academic Press.

A New Interpretation of Contact Angle Variations in View of a Recent Analysis of Immersion Calorimetry.

The values of the contact angles of different liquids on the same solid are classically interpreted by theories assuming that the vapor adsorption is negligible on solids presenting contact angles. This implies that the solid may be defined by a certain tension, called "critical tension", which is not clearly thermodynamically defined. These assumptions are in disagreement with modern theories of vapor adsorption science. In this paper, it is shown that it is possible to understand the contact angle values using a rigorous derivation of the Young equation, which links the contact angle, the liquid-vapor surface tension of the drop, and the surface tension of the solid in equilibrium with its own vapor. Numerical approximations allow the contact angle variation to be predicted, without use of ad hoc definitions. It is then possible to link vapor adsorption results and contact angle experiments. It is also possible to deduce the surface tension of solids simply and to compute contact angles on powders without using the Washburn method. Copyright 2000 Academic Press.

Analysis of Thermally Stimulated Depolarization Currents (TSDC) Measured on Exchanged Clays.

This work shows that hydration of clays can be studied by means of a new interpretation of thermally stimulated depolarization currents technique. These measurements have been performed on four exchanged natural clays: Na-montmorillonite, Ca-montmorillonite, kaolinite, and illite. The results are analyzed using both the recently developed analysis of relaxation times distribution and the electronegativity equalization method. They provide evidence of the surface heterogeneity of clays. It is established that sites, identical from a crystallographic point of view, are different when the energy of the phenomenon is considered. The main interest of this work is to give for the first time a value of the hydration energy of cation onto clay surfaces. Copyright 2000 Academic Press.

Interactions between Talc Particles and Water and Organic Solvents

Three talc samples have been studied by adsorption and immersion methods after a classical characterization of their properties. The combination of adsorption isotherms and of immersion measurements allows the calculation of enthalpies and entropies of adhesion. The studied talcs are characterized as "middle energy" solids. The differences between the particle shapes of the different samples are shown to be of great importance, indicating a linkage between cristallinity and surface properties. The whole results are explained by the influence of intermolecular forces such as acid-base interactions in the interfacial layer. Copyright 1997 Academic Press. Copyright 1997Academic Press