Professor Ulf Olsson, Physical Chemistry, Lund University, Sweden

On the Ripening of Solid Nanoparticles¹

Ostwald ripening is a common coarsening mechanism for phases fragmented to small submicron dimensions. Driven by the interfacial tension, larger particles grow at the expense of smaller ones, that dissolve.^2,3 This instability of the particle size distribution, can be a serious issue for emulsion or nanoparticle formulations, for example in pharmaceutical nanoparticle formulations of weakly soluble drugs.^{4, 6, 7} For emulsions, this coarsening mechanism is well established. The tension can be made relatively small, allowing for accurate measurements of the ripening rate using time resolved scattering experiments.⁵ Amorphous solid nanoparticles undergo Ostwald ripening similar to emulsions.⁴ Crystalline nanoparticles, on the other hand, seem not. This difference appears to be a consequence of activated crystal growth^6,7 in combination with the fact that Ostwald ripening occurs very close to equilibrium. This  will all be discussed in the light of recent experimental data. In addition, we will describe a direct link between Wilhelm Ostwald and Jan Skov Pedersen. 

1. Behrens, M. A., Lindfors, L., Skantze U., Olsson U. (to be published)
2. Lifshitz, I.M. and Slyozov, V.V. The kinetics of precipitation from supersaturated solid solutions. Journal of Physics and Chemistry of Solids, 1961. 19(1–2): p. 35-50.
3. Wagner, C., Zeitschrift für Elektrochemie, 1961. 65: p. 581
4. Lindfors, L. et al., Amorphous Drug Nanosuspensions. 1. Inhibition of Ostwald Ripening. Langmuir, 2005, 22(3): p. 906-910.
5. Egelhaaf, S., Olsson, U., Schurtenberger, P., Morris, J., Wennerström, H. Quantitative measurements of Ostwald ripening using time-resolved small-angle neutron scattering. Physical Review E 1999, 60 (5), 5681-5684.
6. Lindfors, L.; Skantze, P.; Skantze, U.; Westergren, J.; Olsson, U. Amorphous Drug Nanosuspensions. 3. Particle Dissolution and Crystal Growth. Langmuir 2007, 23 (19), 9866-9874.
7. Lindfors, L., et al., Nucleation and crystal growth in supersaturated solutions of a model drug. Journal of Colloid and Interface Science, 2008. 325(2): p. 404-413.