Surface Chemistry and Physicochemistry of Oxides

Oxide particles resulting from precipitation have at least one dimension less than a few nanometers. Therefore, as their specific area (surface-to-mass ratio) may reach several hundred square meters per gram, the behavior of these particles is closely related to their surface physical-chemical characteristics. Thus, the dispersion state of particles in solution is dependent on attractive and repulsive forces between surfaces. The balance control of these forces limits the aggregation of particles and promotes the formation of sols or gels, or, contrariwise, flocculates the particles and separates them from a suspension. The divison state of solids resulting from precipitation is ruled by forces that exert themselves onto the surface (interfacial—or surface—tension). They determine the extent of the surface area and, therefore, the particle size. Adsorption of ions or molecules within the dispersion depends on forces exerting between soluble species and the surface. These forces may be due to electrostatic charges on the surface. They may also be due to the ability of the surface cations to be coordinated by soluble species and/or the ability of surface oxygenated groups to coordinate cations from solution. The attachment of various species on the surface of oxide particles plays a major role in various fields—for instance, the transport of matter in natural or industrial waters, catalysis and corrosion phenomena, formation of stable and homogeneous dispersions. It is somewhat difficult to characterize the surface of nanometer-sized objects from structural as well as chemical standpoints. The geometry of such small particles is not easily defined with precision, and the surface often includes defects such as steps, truncations, and stacking faults. These sites are difficult to recognize but exhibit largely variable chemical reactivities. In addition, the study of the oxide– solution interface is complicated because few of its physical quantities are experimentally accessible. These quantities are treated as fitting parameters in more or less complex modelings. The current state of the art, however, allows suit­able interpretation of experimental data.