Gelation: After a period of time the sol experiences a transition from liquid solution to a cross-linked gel state where it can support an elastic stress. This period of time is known as gel time or gelation time, and during this time the viscosity of the solution undergoes a rapid increase corresponding to the transition from a viscous fluid to an elastic gel. At the end of gelation there is a continuous phase containing a structure that reflects formation and branching of particles under specific growth conditions. The formation of a network results in entrapping of the solution. The gel structure is determined by the ionic character of the M-O bond and the relationship between the hydrolysis and condensation rate.

Fig. 4 : Double layer structure for particles

Effect of zeta potential on gelation: The particle surface may be positively or negatively charged depending on the pH. With an acidic solution, that is low pH, the equilibrium is driven towards positive surfaces. As pH increases, the surfaces become less positively charged and finally negatively charged. However, the effective charge on the surface is partially neutralized by the counter- ions in the solution that may originate from the bases used during precipitation or electrolytes added during aging. Theses counter- ions form a space charge, part of which is held sufficiently strongly to be carried along as the particles move with Brownian motion (Fig. 4). The result is an effective charge called Zeta potential. Both the original charge and the neutralizing counter- ions respond to pH changes. This zeta potential determines the rate of gelation. If the charge is high, particles effectively repel one another and avoid contact. If it is low, then thermal motion leads to collision and coalescence. These rates are highest at the isoelectric point where the zeta potential is zero. The variation of zetapotential with pH for alumina is shown in Fig. 5.

Fig. 5. Effect of pH on zeta potential of alumina

Two other important parameters are temperature and solvent. Varying temperature is most effective when it can alter the relative rate of the competing reactions. Solvent can change the nature of an alkoxide through solvent exchange or affect the condensation reaction directly.