Coalescence, of emulsion

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

Parameters such as ageing. Influence of pigment concentration and Influence of coalescence of emulsion paints can also be studied using AC Impedance test methods. 

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. 

A variety of interaction behaviours can be observed between liquid/liquid interfaces based on the types of colloidal forces present. In general, they can be separated into static and dynamic forces. Static forces include electrostatic, steric, van der Waals and hydrophobic forces, relevant to stable shelf life and coalescence of emulsions or dispersions. Dynamic forces arise ftom flow in the system, for instance during shear of an emulsion or dispersion. EHrect force measurements tend to center on static force measurements, and while there is a large body of work on the study of film drainage between both liquid or solid interfaces, there are very few direct force measurements in the dynamic range between liquid interfaces. Below are general descriptions of some of the types of force observed and brief discussions of their origins. 

Let us assume that the total free energy of the emulsion can be separated into several independent contributions. Considering hypothetically the formation or coalescence of emulsion of two immiscible liquids (e.g. oil and water), such that external field forces are absent. The total free energy (Gg) of the system just before emulsification process can be expressed in the form (10)... 

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). 

Stabilization of emulsions by powders can be viewed as a simple example of structural- mechanical barrier, which is a strong factor of stabilization of colloid dispersions (see Chapter VIII, 5). The stabilization of relatively large droplets by microemulsions, which can be formed upon the transfer of surfactant molecules through the interface with low a (Fig. VII-10), is a phenomenon of similar nature. The surfactant adsorption layers, especially those of surface active polymers, are also capable of generating strong structural mechanical barrier at interfaces in emulsions. Many natural polymers, such as gelatin, proteins, saccharides and their derivatives, are all effective emulsifiers for direct emulsions. It was shown by Izmailova et al [49-52]. that the gel-alike structured layer that is formed by these substances at the surface of droplets may completely prevent coalescence of emulsion drops. 

Another demonstration of a critical phenomenon, the rate of coalescence of emulsions in dependence of surface elasticity and viscosity, based on the work of Boyd et al. (1972), can also be found in the review of Malhotra Wasan (1988) shown in Fig. 3.20. 

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. 

In Table 5.3 the glass transition temperatures and the softening temperatures of all the homopolymers referred to in this chapter are given. The glass transition temperature is a good guide to the hardness or softness of the polymer. It has particular importance to the coalescence of emulsion polymers, as we shall see in Chapter 11. 

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. 

Interfacial tension. The larger the interfacial tension, the more readily coalescence of emulsions will occur but the more difficult the dispersion of one liquid in the other will be. Coalescence is usually of greater importance, and interfacial tension should therefore be high. Interfacial tension between equilibrium phases in systems of the type shown in Fig. 10.3 falls to zero at the plait point. 

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