Flocculation emulsions

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. 

There are two approaches to the kinetics of emulsion flocculation. The first stems from a relationship due to Smoluchowski [52] for the rate of diffusional encounters, or flux ... 

Four major phenomena are associated with the-physical instability of emulsions flocculation, creaming, coalescence, and breaking (Fig. 8) [144]. 

Dickinson, E., Semenova, M.G., Belyakova, L.E., Antipova, A.S., Il in, M.M., Tsapkina, E.N., Ritzoulis, C. (2001). Analysis of light scattering data on the calcium ion sensitivity of caseinate solution thermodynamics relationship to emulsion flocculation. Journal of Colloid and Interface Science, 239, 87-97. 

Divalent ions are four times as effective as monovalent ions in decreasing v /. Thus 0.25 M zinc sulfate, for example, is as effective as 1 M sodium chloride in promoting emulsion flocculation or coalescence. If gravity (creaming) is the only force bringing the droplets together, they will approach to a distance where repulsion... 

Figure 3. Plots of individual drops vs. time for sulfapyridine-stabilized emulsion flocculated by RNA and DNA.

Occurrence of flocculation may be explained if secondary minima aggregation is assumed. This is confirmed by the V/KT vs (H (A)) curves, which indicate that at higher values of H, repulsion becomes negligible and attraction predominates and emulsion flocculates. It is also observed that the depth of secondary minimum is more at higher concentration, 5-20 KT, which is deep enough for reversible aggregation, while at low concentrations, the depth of the secondary minima is too shallow to trap the particles. 

I 70 Formulation of Liquid/Uquid Dispersions (Emulsions) Table 10.4 Half-life of emulsion flocculation. 

Leal Calderon F, Biais J, Bibette J. Asphalt emulsions flocculation induced by a cationic surfactant and application to size partitioning. Colloids Surfaces A Physicochem Eng Aspects 1993 74 303-309. 

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... 

W emulsions are formed at low electrolyte concentrations, < 0.3M NaCl, and W/0 emulsions are formed at high electrolyte concentrations, > 0.3M NaCl. The continuous phase at 0.3M NaCl was indeterminable, by the simple method previously discussed. This emulsion phase collapsed to a value of V /Vm of 0.1 between 10,000 minutes (limit of time scale of Figure 11) and 15,000 minutes (last observation point) all the other emulsion phases formed with univalent alkalis at high pH s flocculated to V /Vrj. = 0.5 over the observation period. The flocculation rate of the W/O emulsions is faster than the 0/V7 emulsions because of double layer effects (25). The W/O emulsions flocculated but did not coalesce because of the presence of interfacial films discussed by Wasan et al. (17). The relative rates of flocculation of either the O/W or the W/O emulsions appear to depend on the concentration of electrolyte however, the data are insufficient to make a more definitive statement at this time. 

Coalescence requires that the molecules of liquid within two or more emulsion droplets coming into direct contact. Droplets therefore need to be in close proximity, which is for example the case in highly concentrated emulsions, flocculated emulsions, or creamed layers. In a subsequent step, a disruption of the interfacial membrane must occur to allow the liquid molecules to come into direct contact. The rate at which coalescence proceeds, and the physical mechanism by which it occurs, is thus highly dependent on the nature of the emulsifier used to stabilize the system. Improving the stability of an emulsion to coalescence may... 

Salts as photographic emulsion flocculant Ag halide emulsion flocculant... 

Generally, emulsions flocculate, coalesce, or do both on aging. These phenomena change the phase stability and the viscosity of the emulsion. 

---