Reversed Ostwald ripening

For a sufficiently reducing reactant like ethanol both reversible and irreversible deactivation can be neglected. For MGP however unable to keep the catalyst in a low oxidation state particle growth occurs most probably via an Ostwald-ripening mechanism resulting in an irreversible decrease of the platinum surface area exposed. 

Stability is an essential condition for PFC emulsions to be of practical use. The principal mechanism for irreversible droplet growth in submicronic PFC emulsions during storage is molecular diffusion (also known as Ostwald ripening or isothermal distillationj.P Coalescence may contribute to instability when mechanical stress is applied and at higher temperatures, as during heat sterilization. Sedimentation and flocculation are fully reversible and pose no problem. 

As emulsions are inherently unstable, they eventually revert to the original state of two separate liquids, that is, will break or crack. In the presence of an emulsifier and other additives, this state is approached via several distinct processes, some of which are reversible such as creaming and flocculation and others irreversible such as coalescence and Ostwald ripening. Phase inversion when an oil-in-water emulsion inverts to form a water-in-oil emulsion or visa versa is a special case of irreversible instability that occurs only under well-defined conditions such as a change in emulsifier solubility due to specific interactions with additives or to a change in temperature (Fig. 3). 

The crystal size generally increases as a result of Ostwald ripening small particles dissolve faster than the large ones and dissolved small particles dynamically and reversibly contribute to growing of the large ones. The process proceeds until the solubilities of the large and the small particles are very close to each other. 

In most cases AAyi2 TAS , which means that AG ° is positive, i.e. the formation of emulsions is nonspontaneous and the system is thermodynamically unstable. In the absence of any stabilization mechanism, the emulsion will break by flocculation, coalescence, Ostwald ripening or a combination of all these processes. This is illustrated in Fig. 3.29 which shows several paths for emulsion breakdown processes. In the presence of a stabilizer (surfactant and/or polymer), an energy barrier is created between the droplets and therefore the reversal from state II to state I becomes non-continuous as a result of the presence of these energy barriers this is illustrated in Fig. 3.30. In the presence of the above energy barriers, the system becomes kinetically stable [85]. As discussed before, the energy barrier can be created by electrostatic and/or steric repulsion which will overcome the everlasting van der Waals attraction. 

Based on these facts, there are several possibilities to control the crystallite size the crystallite size can be decreased by increasing the current density for a constant the crystallite size can be increased for long due to Ostwald ripening the use of organic additives (grain refiners) enables the control of the crystallization process during the time because these molecules are adsorbed on the electrode surface in a reversible way and hinder the surface diffusion of the adatoms and changing the temperature influences all diffusion processes (ion diffusion in the electrolyte, surface diffusion of the nuclei) - if small crystallite sizes are desired, the deposition should be performed at lower temperatures. 

Nano-emulsions, as nonequilibrium systems, tend to phase separation by some of the four mechanisms of disperse systems destabilization sedimentation or creaming and flocculation, as reversible mechanisms, and coalescence and Ostwald ripening, as irreversible ones. Nano-emulsions, due to the small characteristic size are stable against sedimentation or creaming. An adequate selection of surfactant molecules can protect nano-emulsions from flocculation and coalescence. In addition, the greater curvature as a result of the smaller droplet size does not favor flocculation or coalescence phenomena. These considerations leave the Ostwald ripening as the main destabilization mechanism of nano-emulsions. This fact has been experimentally confirmed in numerous studies and discussed in several reviews [63,1]. 

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