Effect of Electrolyte Addition on the Colloidal Stability of Aqueous Zeolite Sols

Abstract

Introduction. ‐ Handling of aqueous sols or suspensions of zeolite crystals is a frequently recurring operation in the manufacturing of zeolite-based catalysts or adsorbents. Starting with the zeolite synthesis in aqueous medium, the typical workup comprises repeated ion exchange in aqueous solutions and, finally, a wet shaping process during which bodies of suitable size, shape, strength, pore texture, and site distribution must be formed. Large agglomerates of crystals are beneficial, when zeolites must be recovered from their mother liquor or from ion-exchange solutions, whereas isolated zeolite single crystals, dispersed in the porous matrix of a shaped particle, are highly desirable when catalytic properties are to be optimized. These opposing requirements for efficient solid-liquid separations, on one hand, and optimum product quality, on the other hand, demand for means to control the particle size in a reversible manner. It is well-known that the stability of particles with respect to aggregation depends on the balance between attractive London-Van der Waals and repulsive electrostatic forces. The magnitude of the electrostatic repulsion as a function of the distance from the particle surface can be influenced. It depends on the ionic strength in the diffusive layer and on the surface potential (Nernst potential), which, in turn, can be altered by adjusting the pH value. While the Nernst potential is not experimentally accessible, the electrokinetic potential at the shear plane, the zeta (z) potential, can be monitored. Dispersions can be regarded as stable, when the zeta potential is higher than ca. j 30 mVj , whereas the particles tend to form aggregates near the isoelectric point (IEP), which is defined as the pH at which the zeta potential is zero. The measurement of the zeta potential, therefore, is a widely used tool to characterize the stability of disperse systems. Electrokinetic data of numerous materials, in particular, of inorganics are documented in the literature.