Particle Growth by Ripening

Although colloidal systems may be in a mctastable state in relation to adhesion between the particles, it is important to stress that, if they have any significant solubility or volatility, they arc in general thermodynamically unstable with respect to particle growth processes. 

However, there arc special circumstances under which such a colloid may be effectively stable, despite the fact that ihe free energy of a particle decreases as its particle size increases. Thus if the particle concentration is high, and the concentration of solute low, then growth of the particles may deplete the solution to below the solubility of the particles, and particle growth will he inhibited. An analogous process may well play a significant role in, for example, the formation of stable rain drops in a cloud. 

Ostwald ripening is of considerable importance in several technological situations. For example, in the preparation of photographic emulsions the silver halide particles are subjected to carefully controlled ripening. On the other hand, in agrochemical preparations it is important that ripening should not occur 

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