Aging Ostwald ripening

D. Aging Ostwald ripening and solution-mediated phase transformation... 

D. Aging Ostwald Ripening and Soiution-Mediated Phase Transformation... 

Interesting aging effects are frequently observed in these systems. If the precipitated particles are left in contact with the hydrolysis catalyst and water they appear to reorganize, so that their surfaces become better defined and their sizes become more uniform.15 The process seems quite analogous to the Ostwald ripening 33 much studied by colloid chemists. 

An important example of this phenomenon is to be found in the ageing of colloidal dispersions (often referred to as Ostwald ripening). In any dispersion there exists a dynamic equilibrium whereby the rates of dissolution and deposition of the dispersed phase balance in order that saturation solubility of the dispersed material in the dispersion medium be maintained. In a polydispersed sol the smaller particles will have a greater solubility than the larger particles and so will tend to dissolve, while the larger particles will tend to grow at their expense. In... 

The dispersion of the precipitate changes with the degree of supersaturation and its evolution during precipitation. Low supersaturation leads to poorly dispersed solids. Highly dispersed solids are thermodynamically unstable and tend to lose dispersion (Ostwald ripening). This takes place during the process of precipitation itself. If the effect is desired, a special maturation (or ageing) step is carried out at the end of the precipitation. 

Ostwald ripening, the disappearance of very small crystals because of an increase in solubility, has been discussed in Sections 4.2.1.2 and 4.3.1.7 Despite the fact that the concept applies most directly to particles <1 micron in size, it has considerable commercial signilicance because of its participation in the aging of seed to increase the size, narrow the size distribution, and improve the crystallinity of particles in a crystal growth operation. Mullin (2001, pp. 320-322) contains an excellent discussion of this phenomenon. 

SCHEME 23 Coalescence of silica gel through Ostwald ripening during alkaline aging, resulting in loss of surface area, expanding pore size (initially), and a change from convex to concave pore surface. 

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