Acid–base behavior of a colloidal clays fraction extracted from natural quartz sand: Effect of permanent surface charge

The electrical double-layer model (EDLM), generally used in order to describe the behaviour of the solids bearing only amphoteric groups at the surface (i.e. oxides), can be extended to solid based dispersion presenting simultaneously amphoteric sites (pH-dependent charge) and permanent structural charges (pH-independent charge) at the surface (i.e. clays). In this study, the acid–base behaviours of a fraction of colloidal clays extracted from a natural quartz sand have been modelled within the EDLM frameworks. These colloidal clays also exhibit heterogeneous macroscopic surface charge distribution. We have considered the case where the colloidal fraction presents only amphoteric sites at the surface (case 1) and the case where both pH-depending and pH-independent charges are present simultaneously at the surface (case 2). The modelling results show a great difference between the theory and the experimental data for case 1 but a good corroboration for case 2. Therefore, in order to describe correctly the physical-chemistry properties of the colloidal fraction it is necessary to take into account the permanent charges (microscopic surface charge heterogeneity) to explain the increase in the zero protonic charge (ZPC) and in the protonation amount with decreasing ionic strength. In case 1, the double-layer model leads to a constant ZPC and to an increase in the protonation amount as a function of ionic strength, which is in disagreement with the experimental results.