Emulsion coalescence, surfactant molecules

Extensive dilution of a surfactant-stabilized emulsion may cause some of the surfactant molecules to move from the droplet surface into the continuous phase. This process can cause the droplets in a diluted emulsion to become unstable and coalesce. For this reason, it is often... 

Oil and water do not mix, but on addition of a suitable surfaetant a microemulsion can be formed depending on the relative concentrations of the three components. Microemulsions (i.e. surfactant/water/oil mixtures) can also be used as reaction media see references [859-862] for reviews. Microemulsions are isotropic and optically clear, thermodynamically stable, macroscopically homogeneous, but microscopically heterogeneous dispersions of oil-in-water (O/W) or water-in-oil (W/O), where oil is usually a hydrocarbon. The name microemulsion, introduced by Schulman et al. in 1959 [863], derives from the fact that oil droplets in O/W systems or water droplets in W/O systems are very small (ca. 10... 100 nm nanodroplets). Unlike conventional emulsions, microemulsion domains fluctuate in size and shape with spontaneous coalescence and breakup. The oil/water interface is covered with surfactant molecules and this area can amount to as much as 10 m per litre ( ) of microemulsion. 

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). 

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. 

The structures of the interfacial layers in emulsion droplets might be expected to be simple when small-molecule emulsifiers are used, but this is not necessarily the case, especially when not one but a mixture of surfactant molecules is present. Although simple inter-facial layers may be formed where the hydrophobic moieties of the surfactants are dissolved in the oil phase, and the hydrophilic head groups are dissolved in the aqueous phase, it is also possible to form multilayers and liquid crystals close to the interface (78). These, of coiuse, depend on the natiue and the concentrations of the different siufactants. Interactions between surfactants generally enhance the stability of the emulsion droplets, because more rigid and structured layers tend to inhibit coalescence. Also, mixtiues of different surfactants having different HLB numbers appears to provide structured interfacial layers, presumably because of the different affinities of the siufactants for the oil-water interface (79). 

The adsorbed surfactant molecules counteract flic drop coalescence in two ways (1,2). The presence of surfactant gives rise to repulsive surface forces (of either electrostatic, steric, or oscillatory structural origin) between the drops, thus providing a thermodynamic stabilization of the emulsion see also Refs 3 and 4. Moreover, the adsorbed surfae-... 

FIGURE 11.1. There are four primary mechanisms for the stabilization of emulsions (plus combinations, of course). Some emulsions may be weakly stabilized by the presence of adsorbed ions and nonsurface-active salts (a). The presence of colloidal sols partially wetted by both phases of the emulsion may form a mechanical barrier to drop contact and coalescence (b). Many emulsions are stabilized by adsorbed polymer molecules (c). Along with polymers, adsorbed surfactant molecules represent the most common stabilization mechanism (d). 

At the end of the emulsion polymerisation, polymer particles will exist in the dispersed water phase with surfactant molecules adsorbed on the particle surface. It is usually considered that it is the hydrophobic portion of the surfactant that is adsorbed onto the surface while the hydrophilic portion goes into the water phase. The role of the surfactant is now to keep the system stable by preventing coalescence of the polymer particles in the dispersion. If particle coalescence is not prevented the settling of the coagulated particles can take place, giving a non-redispersible sediment. 

The interactions between the hydrophobic parts of the surfactant molecules and the nonpolar liquid phases play an important role in controlling the stability of emulsions in these systems [13-20,56], While each particular system requires an individual approach, it can be concluded that the high stability of fluorocarbon annlsions against coalescence is related to a deficit in the adhesion in the HS/FL system resnlting in the sqneezing ont of hydrophobic chains from the nonpolar liquid phase. 

This depicts the conventional view of the arrangement of surfactant molecules at an interface between water and a less polar material, such as air, oil, or a hydrophobic solid (e.g. paraffin wax, graphite, polythene). Thus bubbles or oil droplets dispersed in a surfactant solution will gather an equilibrium coating of surfactant up to a maximum densi corresponding to a close-packed monolayer. It is this surface coating which imparts stability to foams (air/water) and to emulsions (oil/ water) by inhibiting bubble or droplet coalescence. 

Due to this structure they tend to adsorb at the interfaces between a polar and a nonpolar fluid, for example water and oil. Emulsifiers reduce the surface tension and stabilize the surface by steric, electrostatic or hydrodynamic (Gibbs-Mar-angoni) effects [14], Droplet coalescence (flowing of one or more droplets together) can thus be reduced or prevented. Some emulsifiers can be characterized by their hydrophilic/lipophilic balance (HLB) value that provides information on the ratio of hydrophilic to lipophilic character of the surfactant molecule. The HLB value helps to determine the phase in which the emulsifier is soluble. Usually, the emulsifier used is soluble in the continuous phase (Bancroft rule). Furthermore, the HLB value gives a first hint whether the emulsifier is suitable for the production of an o/w (HLB value 8-18) or a w/o emulsion (HLB value 4—6) [15]. 

Studies of W/O emulsions and thin aqueous surfactant films between oil phases using Span 80 and other low HLB surfactants by Sonntag and Klare [119] have drawn attention to the importance of the stability of the thin film between the water droplets when flocculation occurs. Addition of electrolyte causes dehydration of the surfactant molecules and promotes de-emulsification. As with O/W emulsions, addition of electrolyte causes a shift in the HLB of the surfactant molecules. Increase in temperature results in an increased rate of flocculation because of surfactant desorption and an increase in the rate of coalescence. Using water droplets in octane the temperature for coalescence increased with Span 80 concentration from 42°C at 0.1 gl" to 68°C at 1 gl and > 75 C at 5gl" The equilibrium non-aqueous black films which form between the water droplets are unaffected by temperature but their thickness is controlled by the nature of the oil phase. Span 80 in octane forms a film 3.9 nm thick, while in xylene the black film has a thickness of 28 nm [119]. Sonntag and Netzel have carried out similar measurements on this film relevant to O/W stability [120]. Because of the complexities of the real emulsion system such studies, discussed in some more detail in [4], have many uses in allowing the experimenter to isolate some of the variables in the system. The reader is referred to the literature on these films (see [121]) for further details and to Sonntag s paper in particular for information concerning non-ionic surfactant behaviour in aqueous and non-aqueous films (see [122-124]). 

How does the nature of a surfactant molecule aflFect the emulsion stability Which molecular parameters directly affect the emulsion stability and which are less important Why do some surfactants tend to stabilize OAV emulsions and others W/0 What surfactant concentration is sufficient to prevent coalescence Why does the addition of some surfactants to a stable emulsion cause rapid coalescence and demulsification These questions have been addressed since the beginning of this century. Already in these very early studies, correct experimental trends were established. However, only relatively recently, with the advances in the physics of surfactant monolayers, has a mechanistic picture of emulsion coalescence started to emerge. 

Figure 7.16 By contrast to the classical oriented wedge theory, in the Kabalnov-Wenner-strdm theory it is assumed that the spontaneous curvature affects not the free energy of the emulsion droplets but the free energy of the coalescence transition state the hole in the film. If the spontaneous curvature of the surfactant molecule fits the neck, the hole propagates without a significant barrier (a). In the opposite case, the nucleation is suppressed and the emulsions are stable (b)...

Nano-emulsions, as nonequilibrium systems, tend to phase separation by some of the four mechanisms of disperse systems destabilization sedimentation or creaming and flocculation, as reversible mechanisms, and coalescence and Ostwald ripening, as irreversible ones. Nano-emulsions, due to the small characteristic size are stable against sedimentation or creaming. An adequate selection of surfactant molecules can protect nano-emulsions from flocculation and coalescence. In addition, the greater curvature as a result of the smaller droplet size does not favor flocculation or coalescence phenomena. These considerations leave the Ostwald ripening as the main destabilization mechanism of nano-emulsions. This fact has been experimentally confirmed in numerous studies and discussed in several reviews [63,1]. 

Thus, it should be stressed that interactions between hydrophobic parts of surfactant molecules and nonpolar liquid phase play a critical role in controlling the emulsion stability (Davis [2]). Of course, each particular system needs an individual approach in the quantitative evaluation of such interactions. However, we see that in the case of fluorocarbon emulsions stabilization (with respect to coalescence), the high stability relates to some deficiency in the HS/FL adhesion, when some kind of squeezing out of hydrophobic radicals from the nonpolar hquid phase can take place. 

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