Ripening, coalescence

Scheme 1 Illustration of the general synthetic method followed in our group for the synthesis of metal nanoparticles i decomposition of the precimsor, nucleation ii first growth process in ripening or coalescence leading to size and shape controlled objects through addition of stabilizers which prevent the full precipitation of the metal (iv)...

Pt particle coalescence is due to migration. This mechanism is supported by observations that, upon cycling, Pt particle size distributions are shifted toward larger sizes, indicating that smaller particles are more mobile. It is noted that this observation could also result from the effects of Ostwald ripening. 

It has been well established that Pt can dissolve under oxidizing conditions, although the exact manner of how the species formed is a matter of debate at present. The formation of Pt crystallites in the membrane (or at the anode if no H2 is present) would indicate that micrometer transport of soluble Pt occurs. However, careful analysis of the Pt particle size distributions in the cathode after testing suggested that purely Ostwald ripening could not explain the observed distributions. Therefore, at present, it is concluded that a mixture of Pt dissolution/reprecipitation and Pt particle coalescence is responsible for Pt ECA loss. 

An analogy may be drawn between the phase behavior of weakly attractive monodisperse dispersions and that of conventional molecular systems provided coalescence and Ostwald ripening do not occur. The similarity arises from the common form of the pair potential, whose dominant feature in both cases is the presence of a shallow minimum. The equilibrium statistical mechanics of such systems have been extensively explored. As previously explained, the primary difficulty in predicting equilibrium phase behavior lies in the many-body interactions intrinsic to any condensed phase. Fortunately, the synthesis of several methods (integral equation approaches, perturbation theories, virial expansions, and computer simulations) now provides accurate predictions of thermodynamic properties and phase behavior of dense molecular fluids or colloidal fluids [1]. 

In this chapter, we review some results in the field of emulsion metastability, emphasizing the destruction of concentrated emulsions (droplet volume fraction

70%) through coalescence. The review concerning Oswald ripening (Section 5.2) is more concise, as this mechanism is fairly well understood and has been extensively documented in the literature. So far, the destruction of concentrated emulsions through coalescence is much less understood and has motivated many recent studies and developments that we summarize (Sections 5.3 to 5.6). 

K. Pays Double Emulsions Coalescence and Compositional Ripening. Ph.D thesis, Bordeaux I University (2000). 

Note 2 Representative mechanisms for coarsening at the late stage of phase separation are (1) material flow in domains driven by interfacial tension (observed in a co-continuous morphology), (2) the growth of domain size by evaporation from smaller droplets and condensation into larger droplets, and (3) coalescence (fusion) of more than two droplets. The mechanisms are usually called (1) Siggia s mechanism, (2) Ostwald ripening (or the Lifshitz-Slyozov mechanism), and (3) coalescence. 

Although the above derivation is for crystals, the theory is also applicable to bubble size distribution. In addition to the above four assumptions, the other conditions for its application include (v) no Ostwald ripening, which would modify CSD, and (vi) no coalescence of bubbles. 

Zeolite crystallization can be interpreted in terms of a ripening mechanism. The initially formed gel consists of amorphous dispersed particles of the order of 100-300 A in size. Growth of these particles to approximately 1000 A occurs during the induction period after which zeolite crystals appear imbedded in the amorphous gel matrix. This is especially evident in electron microscopic studies of gel solids (66,88). Ciric comments on the observation of growing crystals imbedded in gel particles which, as the crystals grow, tend to shrink together, resulting in coalescence (74)-... 

Because of the high surface free energy at the liquid-solid interface, it is suggested that the stages of nucleation, transport of species by surface diffusion, and crystallization occur at the interface in the boundary layer. Culfaz and Sand in this volume (48) propose a mechanism with nucleation at the solid-liquid interface. This mechanism should be most evident in more concentrated gel systems where interparticle contact is maximized for aggregation, coalescence, or ripening processes. The epitaxy observed by Kerr et al. (84) in cocrystallization of zeolites L, offretite, and erionite further supports a surface nucleation mechanism. 

In the pharmaceutical industry, it is common to immediately suspend a portion of sample in solutions of a small-molecule surfactant. The surfactant is expected to rapidly adsorb at incompletely covered droplet surfaces to prevent droplet coalescence between sample withdrawal and analysis of droplet size or concentration. However, the addition of small surfactant molecules can result in a displacement of the original emulsifier from the droplet interface and profoundly alter droplet-droplet interactions. Changes in system composition may therefore lead to greater errors than those generated by the lag between sample withdrawal and analysis (see Background Information, discussion ofOstwald ripening). 

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