We discuss the charging behavior of clays and clay minerals in aqueous electrolyte solutions. Clay platelets exhibit different charging mechanisms on the various surfaces they expose to the solution. Thus, the basal planes have a permanent charge that is typically considered to be independent of pH, whereas the edge surfaces exhibit the amphoteric behavior and pH-dependent charge that is typical of oxide minerals. Background electrolyte concentration and composition may affect these two different mechanisms of charging in different ways. To guide and to make use of these unique properties in technical application, it is necessary to understand the effects of the various master variables (i.e. pH and background salt composition and concentration). However, how to disentangle the various contributions to the charge that is macroscopically measurable via conventional approaches (i.e. electrokinetics, potentiometric titrations, etc.) remains a challenge. The problem is depicted by discussing in detail the literature data on kaolinite obtained with crystal face specificity. Some results from similar experiments on related substrates are also discussed. As an illustration of the complexity, we have carried out extensive potentiometric mass and electrolyte titrations on artificial clay samples (Na-, Ca-, and Mg-montmorillonite). A wide variety of salts was used, and it was found that the different electrolytes had different effects on the end point of mass titrations. In the case of a purified sample (i.e. no acid-base impurities), the end point of a mass titration (the plateau of pH achieved for the highest concentrations of solid), in principle, corresponds to the point of zero net proton and hydroxide consumption, at which in ideal systems, such as oxide minerals, the net proton surface charge density is zero. To such concentrated (dense) suspensions of clay particles, aliquots of salts can be added and the resulting pH indicates the specificity of a given salt for a given clay particle system. In the experimental data, some ambiguity remains, which calls for further detailed and comprehensive studies involving the application of all the available techniques to one system. Although, right now, the overall picture appears to be clear from a generic point of view (i.e. concerning the trends), clearly, in a quantitative sense, huge differences occur for nominally identical systems and only such a comprehensive study will allow to proof the current phenomenological picture and allow the next step to be taken to understand the fine details of the complex clay-electrolyte solution interfaces.
Part of the book: Clays, Clay Minerals and Ceramic Materials Based on Clay Minerals