The dependence of the stability of SiO2 and Al2O3 sols on the pH of their medium has been studied. Vapor adsorption isotherms on powders were 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⇄xerogel transformation, i.e. the peptizability of the powders, has been investigated. Based on the flocculation values determined with KCl, the sols have been classified into three groups. The stability of highly hydrophilic sols (acidic SiO2- and Al2O3-Sols) is ensured by a thick continuous diffuse lyosphere formed around the particles, as the continuity principle by Ostwald-Buzágh suggests. In this case, no electric charge is needed for ensuring stability. These sols are thermodynamically stable (group 1). Sols with medium stability are stabilized by the electrical double layer around the particles and by 1-2 layers of adsorbed water. The flocculation value of these sols is determined by the electrostatic interaction, whereas the peptizability of the floes is related to formation of 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 transformation process sol ⇄ floccule ⇄ xerogel 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 hydrophilic sols (sequence of hydrophilicity: SiO2 > Al2O3 > FeO OH 0.5 H2O) are all oxides which are formed in acidic media.
- Colloid stability
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Polymers and Plastics
- Colloid and Surface Chemistry
- Materials Chemistry