Ostwald ripening and coalescence

Surfactants are used for stabilization of emulsions and suspensions against flocculation, Ostwald ripening, and coalescence. Flocculation of emulsions and suspensions may occur as a result of van der Waals attraction, unless a repulsive energy is created to prevent the close approach of droplets or particles. The van der Waals attraction Ga between two spherical droplets or particles with radius R and surface-to-surface separation h is given by the Hamaker equation,... 

Measurement of the droplet size distribution, rates of flocculation, Ostwald ripening and coalescence are also described in Chapter 6. These methods should be applied for agrochemical emulsions to ensure their long-term physical stability. In addition, the bulk rheology of the system should be investigated after storage at various temperatures. 

Strongly adsorbs at the interface, giving high elasticity that reduces both Ostwald ripening and coalescence. 

Emulsions are understood as dispersed systems with liquid droplets (dispersed phase) in another, non-miscible liquid (continuous phase). Either molecular diffusion degradation (Ostwald ripening) or coalescence may lead to destabilization and breaking of emulsions. In order to create a stable emulsion of very small droplets, which is, for historical reasons, called a miniemulsion (as proposed by Chou et al. [2]), the droplets must be stabilized against molecular diffusion degradation (Ostwald ripening, a unimolecular process or r, mechanism) and... 

Schubert 1999 Nilsson and Bergenstahl 2006 van der Graaf et al. 2004), whereas long-term stabihty can go up to years and is governed by processes such as oil droplet coalescence, Ostwald ripening and flocculation (Radford and Dickinson 2004 Dickinson et al. 2003 Dickinson 2003b Dickinson 2001). 

If flocculation occurs on storage (without any Ostwald ripening or coalescence), the values of //(o) and may show a gradual increase with increase in storage time. As discussed in the previous section on steady-state measurements, the trend becomes complicated if Ostwald ripening and/or coalescence occur simultaneously, as both have the effect of reducing //(o) and cr. ... 

The above measurements should be supplemented by particle size distribution measurements of the diluted dispersion (ensuring that no floes are present after dilution) to assess the extent of Ostwald ripening and/or coalescence. Another complication may arise from the nature of the flocculation, however. If the latter occurs in an irregular fashion, so as to produce strong and tight floes, //(o) may... 

The relaxation time may be used as a guide for the state of the dispersion. For a colloidally stable dispersion (at a given particle size distribution), t increases with increases in the volume fraction of the disperse phase, (p. In other words, the cross-over point shifts to lower frequency with an increase in (p. For a given dispersion, t increases with increase in flocculation, provided that the particle size distribution remains the same (i.e., no Ostwald ripening and/or coalescence). 

Especially Chapters 1 by B. P. Binks (introduction), 7 by A. S. Kabalnov (coalescence), and 9 by J. G. Weers (Ostwald ripening and related phenomena) are recommended. 

Macro- and miniemulsions are thermodynamically unstable. If not stabilized, the droplets tend to fiocculate, coalesce, sediment or cream [2-4]. Other instabilities, such as Ostwald ripening and phase inversion, are also known. At worst, an emulsion will break, i.e. the two phases will separate completely. A product becoming unstable will lose its quality within a short period of time and thus cannot be commercialized. Therefore, even in natural emulsion-based products, amphiphilic molecules are found (e.g. lecithin and proteins in egg yolk and milk and artificial surfactants and emulsifiers in cosmetics and chemical products. They adsorb at the droplets interfaces and stabilize them against flocculation and coalescence. Adsorption and stabilization mechanisms depend on the molecular structure of a surfactant or an emulsifier as depicted in Figure 20.1. Stabilization mechanisms are summarized in... 

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