Flocculation degree

The change of the flocculation degree is particularly considerable with increasing concentration of oil-soluble surfactants, such as Span-80 (see Fig. 6.10). While at a Span-80 concentration of 0.4 wt% the emulsion is practically completely flocculated, increasing concentration to 1.15 wt% leads to a flocculation degree less than 0.1 %. The same rule of the change of the flocculation degree is observed for emulsions stabilized by pentol. At 0.3 wt%... 

It is common to use mixtures of two surfactants for emulsion stabilisation. The rules of emulsion flocculation with binary mixtures of water-soluble and oil-soluble surfactants have been investigated at constant total surfactant concentration. The water-soluble component was introduced into the bidistilled water, and the oil-soluble component was introduced into the hydrocarbon phase. When using binary mixtures pentol-Tween-80 to stabilise water-heptane emulsions (V2 =0.1) (cf. Fig. 6.11), the minimum flocculation degree (F=0.05) is achieved at a pentol Tween ratio of 6 4 (total surfactant concentration is 1 wt%). Note that, at the given surfactant ratio, the emulsion droplet radius is 0.27 pm as compared with R=0.37 pm for... 

Note that the minimum flocculation degree value is observed at the same surfactant ratios at which the minimum interfacial tension value is achieved. This makes clear the previously known facts about the increase of the emulsion stability at low y. This occurs, first of all, due to an appreciable decrease in the flocculation degree up to its complete suppression. These phenomena result in an increase in the sedimentation stability both for non-flocculated emulsions (dispersity increase) and partly flocculated emulsions (decrease in flock size and flocculation degree). 

Fig. 6.11 Flocculation degree (a) for emulsions with V2 = 0.1 and interfacial tension y (b) as a function of the emulsion composition...

Fig. 5.9 Changes of the flocculation degree of particles in cement paste during the measurements in rotation viscometer at one cycle of increasing and decreasing shear rate, (according to [11])...

The flocculation is promoted by a decrease of the particle size caused either by a high impeller head (mixing) and/or by osmotic transfer. The flocculation degree increases with the number of submicron particles. So, the high-shear mixing devices, producing many particles of small diameter, lead to unstable products. 

There appear to be two stages in the collapse of emulsions flocculation, in which some clustering of emulsion droplets takes place, and coalescence, in which the number of distinct droplets decreases (see Refs. 31-33). Coalescence rates very likely depend primarily on the film-film surface chemical repulsion and on the degree of irreversibility of film desorption, as discussed. However, if emulsions are centrifuged, a compressed polyhedral structure similar to that of foams results [32-34]—see Section XIV-8—and coalescence may now take on mechanisms more related to those operative in the thinning of foams. 

The conventional one-pass clarifier is designed for the lowest specific overflow rate (flow per unit area of Hquid surface), which is usually 1—3 m/h depending on the degree of flocculation. These clarifiers can be started and stopped without difficulty. 

Before determining the degree of stabiUty of an emulsion and the reason for this stabiUty, the mechanisms of its destabilization should be considered. When an emulsion starts to separate, an oil layer appears on top, and an aqueous layer appears on the bottom. This separation is the final state of the destabilization of the emulsion the initial two processes are called flocculation and coalescence (Fig. 5). In flocculation, two droplets become attached to each other but are stiU separated by a thin film of the Hquid. When more droplets are added, an aggregate is formed, ia which the iadividual droplets cluster but retain the thin Hquid films between them, as ia Figure 5a. The emulsifier molecules remain at the surface of the iadividual droplets duiing this process, as iadicated ia Figure 6. 

In a gas stream cariying dust or fume, some degree of particle flocculation will exist, so that both discrete particles and clusters of adhering particles will be present. The discrete particles composing the clusters may be only loosely attached to each other, as by van der Waals forces [Lapple, Chem. Eng., 75(11), 149 (1968)]. Flocculation tends to increase with increases in particle concentration and may strongly influence collector performance. 

---