Coalescence of emulsion droplets

Fig. 9 Schematic presentation of flocculation and coalescence of emulsion droplets. (From Ref. 144.)...

It seems that increasing the surfactant concentration causes thinning of the films between adjacent droplets of dispersed phase. Above a certain level, the films become so thin that on polymerisation, holes appear in the material at the points of closest droplet contact. A satisfactory explanation for this phenomenon has not yet been postulated [132], It is evident, however, that the films must be intact until polymerisation has occurred to such an extent as to lend some structural stability to the monomer phase if not, large-scale coalescence of emulsion droplets would occur yielding a poor quality foam. In general, vinyl monomers undergo a volume contraction on polymerisation (i.e. the bulk density increases) and in the limits of a thin film, this effect may play a role in hole formation, especially at higher conversions in the polymerisation process. 

Surfactants also reduce the coalescence of emulsion droplets. The latter process occurs as a result of thinning and disruption of the liquid film between the droplets on their close approach. The latter causes surface fluctuations, which may increase in amplitude and the film may collapse at the thinnest part. This process is prevented by the presence of surfactants at the O/W interface, which reduce the fluctuations as a result of the Gibbs elasticity and/or interfacial viscosity. In addition, the strong repulsion between the surfactant layers (which could be electrostatic and/or steric) prevents close approach of the droplets, and this reduces any film fluctuations. In addition, surfactants may form multilayers at the O/W interface (lamellar liquid crystalline structures), and this prevents coalescence of the droplets. 

Coalescence of emulsion droplets tends to occur more readily for larger droplets (Section 13.4). 

Preventing aggregation of the particles. See Chapter 12 for causes of aggregation, hence for measures to prevent it. Also coalescence of emulsion droplets should be prevented see Section 13.4. 

TABLE 13.1 Role of Weber Number [Eq. (13.31)] in Coalescence of Emulsion Droplets (Calculated Examples)... 

For coalescence of emulsion droplets, an important variable is whether a flattened film between the droplets is formed. This is governed by the ratio of the external stress over the Laplace pressure. The external stress can be due to colloidal attraction (e.g., van der Waals forces), a shear stress, or gravitational forces in a sediment layer. Small protein-stabilized droplets will not deform, except in a sediment layer in a centrifuge, and they are very stable to coalescence. If the drops are large, the interfacial tension is low, and the external stress is high, droplets will deform and coalescence can readily occur. Water-in-oil emulsions cannot be made with protein as the surfactant, and it is often difficult to stabilize them against coalescence, except by a layer of small hydrophobic particles (Pickering stabilization). 

As we shall see, this is the case in the destruction of emulsions by the coalescence of emulsion droplets. In more complex systems, which we shall not discuss, colloid stability is controlled by changes in both surface tension and area ... 

At low surfactant concentrations it is observed that an attraction dominates at short separations. The attraction becomes important at separations below about 12 nm when the surfactant concentration is 0.01 mM, and below about 6 nm when the concentration is increased to 0.1 mM. Once the force barrier has been overcome the surfaces are pulled into direct contact between the hydrophobic surfaces at D = 0, demonstrating that the surfactants leave the gap between the surfaces. The solid surfaces have been flocculated. However, at higher surfactant concentrations (1 mM) the surfactants remain on the surfaces even when the separation between the surfaces is small. The force is now purely repulsive and the surfaces are prevented from flocculating. Emulsion droplets interacting in the same way would coalesce at low surfactant concentrations once they have come close enough to overcome the repulsive barrier, but remain stable at higher surfactant concentrations. Note, however, that the surfactant concentration needed to prevent coalescence of emulsion droplets cannot be accurately determined from surface-force measurements using solid surfaces. 

Depending on the type of crude oil, the adsorbed film at the interface can be either fluid or very viscoelastic and able to form a skin. Depending on the properties of the crude oil (e.g. API gravity (2), sulfur, salt and metals content, viscosity, pour point, etc.), the structure of the film can vary significantly. Therefore, the molecular packing, surface viscosity, surface elasticity and surface charge of the adsorbed film are very important parameters that determine various phenomena such as coalescence of emulsion droplets, as well as oil drop migration in porous media. 

FIGURE 18.3 Interfacial tension gradients causing a liquid flow that opposes coalescence of emulsion droplets or foam bubbles. 

Another repulsive force between the lipid bilayers in water is the hydration force which is a short range force with exponential falloff. This is related to repulsion between dipoles and induced dipoles. It is quite obvious that the hydration force will tend to inhibit coalescence of emulsion droplets with a multilayer structure as schematically shown in Fig. 5.12. 

An important aspect of the stabilization of emulsions by adsorbed films is that of the role played by the film in resisting the coalescence of two droplets of inner phase. Such coalescence involves a local mechanical compression at the point of encounter that would be resisted (much as in the approach of two boundary lubricated surfaces discussed in Section XII-7B) and then, if coalescence is to occur, the discharge from the surface region of some of the surfactant material. 

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. 

Two Liquids Plus a Solid. SoHd particles may be used to stabilize an emulsion, avoiding the problem of simultaneous stabilization of both the oil drops of the emulsion and the soHd particles of the suspension. The key factor for the use of particles as stabilizers is their location. If they are located at the iaterface between the two Hquids, they will stabilize the emulsion, serving as a mechanical barrier to prevent the coalescence of the droplets (Fig. 17). 

PVA acted as a protective polymer by being absorbed at the oil-water interface of the droplets to produce a steric barrier which prevented the coalescence of the droplets. Therefore PVA formed a stable emulsion of methylene chloride in water, even when nifedipine was dissolved in the methylene chloride phase. However, nifedipine tended to crystallize spontaneously in the aqueous phase of the emulsion or on the surface of the microspheres when solvent evaporation approached completion. This nifedipine crystal formation was detected even at a low drug payload of 5%... 

Flocculation versus Coalescence. The breaking of an emulsion is a two step process requiring the coalescence of the droplets after they are in contact.(17) If the system flocculates but is resistant to coalescence, the system will not phase separate. Over a period... 

Coalescence. In the case of coalescence, the separating film of the continuous phase between the droplets breaks and an irreversible fusion of emulsion droplets occurs. 

Flocculation. Flocculation means an aggregation of emulsion droplets but, in contrast to coalescence, the films of the continuous phase between the droplets survive. Hence, the process may be partially reversible. Both processes, flocculation and coalescence, speed up the creaming of an emulsion due to the increase of the drop size. The process of flocculation is even more important for dispersions of solids than for emulsions because in this case a coalescence is not possible. 

In the interfacial tension theory, the adsorption of a surfactant lowers the interfacial tension between two liquids. A reduction in attractive forces of dispersed liquid for its own molecules lowers the interfacial free energy of the system and prevents the coalescence of the droplets or phase separation. Therefore the surfactant facilitates the stable emulsion system of the large interfacial area by breaking up the liquid into smaller droplets. However, the emulsions prepared with sodium dodecyl (lauryl) sulfate separate into two liquids upon standing even though the interfacial tension is reduced. The lowering of the interfacial tension in the stabilization of emulsions is not the only factor we should consider. 

Sherman (1973) suggested that a strong interfacial film of emulsifier is also necessary to prevent the coalescence of water droplets in oil-continuous emulsions. 

Stability of emulsions refers to the resistance to the formation of two separate phases [13,14]. Coalescence of the droplets is responsible for the phase separation. Ostwald ripening constitutes an additional mechanism by which the large droplets grow in size at the expense of the smaller ones, which decrease in size. 

As the mixed micelles are very small (10-20 nm) the initial swelling must be very limited. To be able to absorb more of Z, the initial small droplets must be furnished with more emulsifier, and even more importantly, with more fatty alcohol (Z,). This may be achieved by coalescence of initial droplets or by absorbtion of mixed micelles from the surroundings. The assumption that the emulsification takes place by a diffusion process seems to be supported by experiments with mixed systems of ionic emulsifier and fatty alcohol and various dispersed phases, showing that a necessary condition for a rapid emulsification is that the compound to be emulsified have slight water solubility. Furthermore, it has been observed that if even small amounts of Zj are added to Z, before addition to the water-mixed emulsifier system, the extent of emulsification is reduced and the resulting emulsion becomes less stable. 

A second theory considers the relative ease with which the two types of droplets can coalesce. Upon shaking, drops of both phases are formed. Sodium stearate ionizes, and the electrical potential hinders approach and coalescence of oil droplets water droplets, on the other hand, experience no such hindrance and readily touch and coalesce. Zinc distearate, being un-ionized, does not interfere with the mutual approach of oil droplets, whereas van der Waal s forces favor subsequent coalescence. Thus the type of emulsion formed depends on the relative kinetics of oil-oil and water-water coalescence. 

Surface-active agents. Surface-active agents such as emulsifiers and surfactants play a very significant role in the stability of emulsions. They greatly extend the time of coalescence, and thus they stabilize the emulsions. Mechanisms by which the surface-active agents stabilize the emulsion are discussed in detail by Becher (19) and Coskuner 14). They form mechanically strong films at the oil-water interface that act as barriers to coalescence. The emulsion droplets are sterically stabilized by the asphaltene and resin fractions of the crude oil, and these can reduce interfacial tension in some systems even at very low concentrations (i7, 20). In situ emulsifiers are formed from the asphaltic and resinous materials found in crude oils combined with ions in the brine and insoluble dispersed fines that exist in the oil-brine system. Certain oil-soluble organic acids such as naphthenic, fatty, and aromatic acids contribute to emulsification 21). 

Each developer of transport emulsion technology selects specific surfactant formulations for particular applications. The primary functions of the surfactant are to reduce the interfacial tension between the crude oil and aqueous phases, to provide stability to the individual oil droplets formed during the shearing process, and to prevent subsequent coalescence of the droplets. The surfactant molecules collect at the phase boundaries and provide resistance to coalescence of the oil droplets by establishing mechanical, steric, and electrical barriers (5). 

The DLVO theory, which was developed independently by Derjaguin and Landau and by Verwey and Overbeek to analyze quantitatively the influence of electrostatic forces on the stability of lyophobic colloidal particles, has been adapted to describe the influence of similar forces on the flocculation and stability of simple model emulsions stabilized by ionic emulsifiers. The charge on the surface of emulsion droplets arises from ionization of the hydrophilic part of the adsorbed surfactant and gives rise to electrical double layers. Theoretical equations, which were originally developed to deal with monodispersed inorganic solids of diameters less than 1 pm, have to be extensively modified when applied to even the simplest of emulsions, because the adsorbed emulsifier is of finite thickness and droplets, unlike solids, can deform and coalesce. Washington has pointed out that in lipid emulsions, an additional repulsive force not considered by the theory due to the solvent at close distances is also important. 

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