Surface heterogeneity of passively oxidized silicon carbide particles: vapor adsorption isotherms.

The surfaces of silicon carbide particles subjected to two different passive oxidation treatments have been characterized by immersion calorimetry and vapor adsorption techniques. Surface enthalpies and surface free energies have been computed using semiempirical models and are compared to theoretical estimations. The surface entropy term appears higher than in the case of other solids studied with the same analysis. The definition of the surface entropy term is discussed in order to explain the discrepancy between calculation and experiment. An explanation of results is proposed, which is related to the constitution of silicon oxide layers at the surface of silicon carbide, a fact demonstrated by previous XPS measurements.

A Simple Experimental Way of Measuring the Hamaker Constant A(11) of Divided Solids by Immersion Calorimetry in Apolar Liquids.

Recent progress in interpretation of immersion calorimetry has shown the link between the immersion enthalpy (Delta(imm)H) value in apolar liquid and the Lifshitz-van der Waals contribution to surface energy (gamma(LW)(S)) of divided solids. In this work we show that a simple relationship can be established between the Hamaker constant (A(11)) of a solid and its immersion enthalpy in a pure apolar solvent. The results are satisfying compared to those already available in the scientific literature. The main interest of this method is the measurement of the Hamaker constants of solids for which contact angle measurements are impossible to carry out and for those for which the complexity of the solid structure hampers the ab initio calculation. Values are given for some clay minerals. Copyright 2000 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.