Ostwald ripening

  Ostwald ripening is an observed phenomenon in solid (or liquid) solutions which describes the evolution of an inhomogenous structure over time. The phenomenon was first described by Wilhelm Ostwald in 1896.^[1] When a phase precipitates out of a solid, energetic factors will cause large precipitates to grow, drawing material from the smaller precipitates, which shrink.

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Causes

This thermodynamically-driven spontaneous process occurs because larger particles are more energetically favored than smaller particles.^[2] This stems from the fact that molecules on the surface of a particle are energetically less stable than the ones already well ordered and packed in the interior. Large particles, with their greater volume to surface area ratio, represent a lower energy state (and have a lower surface energy). As the system tries to lower its overall energy, molecules on the surface of a small (energetically unfavorable) particle will tend to diffuse through solution and add to the surface of larger particle^[2]. Therefore, the smaller particles continue to shrink, while larger particles continue to grow.

Ostwald ripening is also observed in liquid-liquid systems. For example, in an oil-in-water emulsion polymerization,^[3] Ostwald ripening causes the diffusion of monomers from smaller to larger droplets due to greater solubility of the single monomer molecules in the larger monomer droplets. The rate of this diffusion process is linked to the solubility of the monomer in the continuous (water) phase of the emulsion. This can lead to the destabilization of emulsions (for example, by creaming and sedimentation).^[4]

Specific Examples

An everyday example of Ostwald ripening is the re-crystallization of water within ice cream which gives old ice cream a gritty, crunchy texture. Larger ice crystals grow at the expense of smaller ones within the ice cream, thereby creating a coarser texture.^[5]

In geology, it is the textural coarsening, aging or growth of phenocrysts and crystals in solid rock which is below the solidus temperature. It is often ascribed as a process in the formation of orthoclase megacrysts,^[6] as an alternative to the physical processes governing crystal growth from nucleation and growth rate thermochemical limitations.

In chemistry, the term refers to the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones. In the process, many small crystals formed initially slowly disappear, except for a few that grow larger, at the expense of the small crystals. The smaller crystals act as fuel for the growth of bigger crystals. The process of Ostwald ripening is fundamental in modern technology for the solution synthesis of quantum dots.^[7] Ostwald ripening is also the key process in the digestion of precipitates, an important step in gravimetric analysis. The digested precipitate is generally purer, and easier to wash and filter.

References

See also

• Rock microstructure
• Kirkendall effect
• Critical radius