COLLOID STABILITY OF ELECTROSTATICALLY STABILIZED SOLS - PART-III - THE ROLE OF PH IN HYDRATION COAGULATION AND PEPTIZATION OF SIO2-SOLS AND AL2O3-SOLS

The dependence of the stability of SiO2 and Al2O3 sols on the pH of th eir medium has been studied. Vapor adsorption isotherms on powders wer e prepared from acidic and basic sols the immersion enthalpy (heat) of samples containing preadsorbed water have been determined and, also, the reversibility of the sol reversible arrow=xerogel transformation, i.e. the peptizability of the powders, has been investigated, Based on the flocculation values determined with KCI. the sols have been class ified into three groups. The stability of highly hydrophilic sols (aci dic SiO2- and Al2O3-sols) is ensured by a thick continuous diffuse lyo sphere formed around the particles. as the continuity principle by Ost wald-Buzagh suggests. In this case, no electric charge is needed for e nsuring stability, These sols are thermodynamically stable (group 1). Sols with medium stability are stabilized by the electrical double lay er around the particles and by 1-2 layers of adsorbed water. The flocc ulation value of these sols is determined by the electrostatic interac tion, whereas the peptizability of the flocs is related to formation o f water layers. Such sols are the basic SiO2- and Al2O3-sols (group 2) . Sols of low stability are of hydrophobic nature, Their flocculation value with 1:1 electrolytes is smaller than 0.1 mol kg(-1). The transf ormation process sol double right arrow floccule double right arrow xe rogel is mostly irreversible. There are a lot of such sols (group 3). The existence of a hydrosphere is proved by the almost identical value of the hydration energy for both the acidic and the basic SiO2 sols, in spite of an order of magnitude difference in the flocculation value . The remnants of adsorbed water after drying hinders sintering of the particles and ensures the peptizability of powders. The highly hydrop hilic sols (sequence of hydrophilicity: SiO2 > Al2O3 > FeO OH . 0.5 H2 O) are all oxides which are formed in acidic media.