Ostwald Ripening and Related Phenomena

Abstract

Ostwald ripening in multicomponent systems is discussed in terms of thermodynamic stability and metastability. While the insoluble species trapped in the bulk of the drops can provide thermodynamic stability, only metastability is possible when the species is trapped at the interface between the phases. Similarly, a Helfrich-type dependence of the surface tension on curvature can produce metastability, but not thermodynamic stability. When the drops are encapsulated with a cross-linked permeable membrane, or with an insoluble surfactant monolayer, the membrane must be able to withstand the stress of the Laplace pressure and not collapse; this condition is in general hard to implement unless the emulsion drops are small, ∼10–100 nm. The effect of micelles on various mass transfer processes in surfactant systems, such as Ostwald ripening, composition ripening, adsorption from micellar solutions, and solubilization kinetics are discussed. Two mechanisms are possible: In the first, micelles fuse directly with the macroscopic interfaces; in the second, the fusion-fission is prohibited and micelles affect the mass transfer only by participating in the dynamic equilibrium between the micelles and monomers. It is argued that whether or not the direct fusion of the micelles with the interface is allowed, is controlled by the free energy of the transition state (e.g., for ripening, the energy of the neck joining a micelle and a drop). Because of this, direct adsorption of micelles at the air-water interface is strongly unfavorable. However, it is possible at the oil-water interface, if the system is close enough to the balanced point. This means that Ostwald ripening and composition ripening can be strongly affected by micelles only if the system is in the narrow range of spontaneous curvatures in the vicinity of the balanced point, where the fusion of micelles with the interface is significant, but the rate of coalescence remains small. The experimental data supporting and contradicting this point of view are discussed.