Irreversible process coalescence

The coagulation theory of colloidal particles is based on a balance of repulsive and attractive forces between particles. When attractive forces dominate, particle aggregation (coagulation or flocculation) can take place. Flocculation is reversible aggregation, whereas coagulation and coalescence are irreversible processes. 

Further flocculation of bitumen globules, attributed to the weight of the mass of globules formed and the dominant hydrostatic pressure, causes fusion of the globules and the formation of a unified bituminous mass. This phenomenon is called coalescence. Coalescence is an irreversible process. Thus, after coalescence begins, the bitumen content of the emulsion is drastically reduced and the emulsion is unsuitable for use. 

Sintering is the processing step that aims to confer mechanical strength to ceramic or metal powders, shaped by pressing or deposited as films. The process occurs by coalescence of the particles in solid or liquid phase to form a more dense mass. The sintering is an irreversible process and results in decrease of the total free energy of the system. Mathematically, the equation related to total energy of the system is... 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

Transport control of flocculation is realized in an especially direct way in the process known as diffusion-limited cluster-cluster aggregation5 (aggregation as used in this term means flocculation in the present chapter). In this process, which is straightforward to simulate and visualize on a computer, particles undergo Brownian motion (i.e., diffusion) until they come together in close proximity, after which they coalesce instantaneously and irreversibly to form floccules (or clusters ). The clusters then diffuse until they contact one another and combine to form larger clusters, and so on, until gravitational... 

The exact mechanism of inversion remains unclear, although obviously some processes of coalescence and dispersion are involved. In the region of the inversion point multiple emulsions may be encountered. The process is also not always exactly reversible. That is, hysteresis may occur if the inversion point is approached from different sides of the composition scale. Figure 18 shows the irreversible inversion of a diluted bitumen-in-water emulsion brought about by the application of shear (60). 

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). 

Coalescence being the secondary process, the number of distinct droplets decreases leading to a stage of irreversibility and finally complete demulsification takes place. Coalescence rate very likely depends primarily on the film-film repulsion, film drainage and on the degree of kinetics of desorption. Kinetically, coalescence is a unimolecular process and the probability of merging of two droplets in an aggregate is assumed not to affect the stability at other point of contact (32). 

There are three processes by which the number of oil drops in an emulsion is decreased. These are Brownian flocculation, sedimentation flocculation and creaming. But it should be noted that if the absorbed film strength is quite high, flocculation may not necessarily result in coalescence. It is also important to note that flocculation which may be due to any of above three reasons is reversible, but coalescence which follows flocculation is irreversible. 

Two processes take place simultaneously in the emulsion formation a) dispersion of one liquid in another b) coalescence, i.e. irreversible fusion of two or more droplets and formation of a larger one. The coalescence proceeds in several subsequent stages [54] formation of a thin film as result of droplet contact, thinning of the film, and its rupture when achieving a critical thickness. The role of the surfactant concentration in the formation of emulsions can be demonstrated using as an example sunflower oil emulsions stabilized by potassium monododecyl phosphate (see Fig. 6.8). 

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